# Optimizing extrusion parameters to develop strawberry by-products-enriched puffed snacks: impact on phenolic composition, textural and physical properties

Jeimmy Rocío Bonilla Méndez, Carine Le Bourvellec, Jeffrey León Pulido

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Jeimmy Rocío Bonilla Méndez, Carine Le Bourvellec, Jeffrey León Pulido. Optimizing extrusion parameters to develop strawberry by-products-enriched puffed snacks: impact on phenolic composition, textural and physical properties. LWT - Food Science and Technology, 2025, 237, pp.118752. 10.1016/j.lwt.2025.118752. hal-05364926

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# Optimizing extrusion parameters to develop strawberry by-products-enriched puffed snacks: impact on phenolic composition, textural and physical properties

Jeimmy Rocío Bonilla Méndez<sup>a, c</sup>, Carine Le Bourvellec<sup>b, *</sup> ID, Jeffrey León Pulido<sup>c</sup>

## ARTICLE INFO

**Keywords:**

Expansion index

Functional ingredient

Proanthocyanidin

Response surface methodology

Valorization

## ABSTRACT

Strawberry by-products (SBP), generated during industrial processing, are rich in phenolic compounds and dietary fiber. However, they remain underutilized. This study evaluated the feasibility of using SBP as functional ingredients in tapioca starch-based extrudates and optimized extrusion-cooking conditions to improve organoleptic and nutritional quality. A central composite design and response surface methodology were applied to assess the effects of die temperature (DT), feed moisture (FM), and SBP content on expansion index (EI), bulk density (BD), textural properties, color, specific mechanical energy, and phenolic compound contents (PCC). The goal was to maximize PCC and EI while minimizing BD and crispness-related hardness to obtain nutritionally enhanced and texturally appealing snacks.

Optimal conditions (DT: 192.01 °C, FM: 18.64 %, SBPC: 16.46 %) maximized PCC, including free (0.67 mg GAE/g Dry Matter) and bound (3.52 mg GAE/g DM) fractions while increased EI (2.15) and reduced BD (0.25 g/cm³) and fracture work (0.7 N mm). High SBP inclusion increased PCC, especially proanthocyanidins, while high FM promoted degradation through reduction of their mean degree of polymerization. These structural changes provide valuable insight into how extrusion conditions could influence proanthocyanidin characteristics. By optimizing extrusion conditions, the present work supports the sustainable valorization of fruit by-products into functional and nutritionally enhanced snacks.

## 1. Introduction

Strawberries (*Fragaria × ananassa*) are among the most widely cultivated berries globally, with high consumer demand driven by their well-known sensory and nutritional properties. However, due to their highly perishable nature, part of the production is processed into various products such as jam, puree, juice, wine, and frozen good. During such processing, significant by-products are generated. Pomace a by-product of juice production is composed of seeds, skins, and residual pulp and accounts for approximately 4–11 % of the total fruit weight (*Pukalskiéné et al., 2021*). These by-products are rich in valuable nutrients such as minerals, dietary fiber, and phenolic compounds (e.g., anthocyanins, proanthocyanidins, ellagic and other phenolic acids, ellagitannins) (*Cubero-Cardoso et al., 2021*; *Felix et al., 2018*; *Pukalskiéné et al., 2021*) known for their health benefits such as protection against coronary heart

disease, type II diabetes, and obesity (Bohn, 2014).

Despite their nutrient density and potential applications, strawberry by-products are often underutilized. Current uses are mainly limited to low-value applications such as animal feed or composting, or they are simply discarded as waste (Cubero-Cardoso et al., 2021). This highlights the need for innovative strategies to valorize these residues within the food industry such as their inclusion into value-added foods. One promising approach is their incorporation into snack-type products, enriching them with fiber and phenolic compounds to meet current consumer lifestyles, demand for convenience and portability, and a growing interest in natural and functional foods. When formulated with ingredients rich in dietary fiber and phenolic compounds, snacks could offer a healthier alternative to conventional ready-to-eat products and can improve nutritional intake and overall well-being (Hashemian et al., 2025).

<footnote><sup>a</sup> International Agroindustrial Biotechnology Center, Faculty of Agricultural Sciences, University of Cauca, Popayán, Cauca, 690001, Colombia</footnote>

<footnote><sup>b</sup> INRAE, Avignon University, UMR SQPOV, Avignon, F-84000, France</footnote>

<footnote><sup>c</sup> Faculty of Engineering, EAN University, Bogotá D.C., 111321, Colombia</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: carine.le-bourvellec@inrae.fr (C. Le Bourvellec).</footnote>
Extrusion is a versatile technology widely used for snack production allowing incorporation of various functional ingredients including fruit by-products (Hashemian et al., 2025; Natabirwa et al., 2018). However, the integration of by-products into food matrices is challenging due to the destructive processing conditions. In particular it is necessary to maintain component bioactive properties and to achieve final product targeted physical and sensory characteristics.

The high-temperature and shear conditions of extrusion process facilitate starch gelatinization and formation of expanded structures, which are crutial for producing snacks with targeted textures. However, these conditions can also alter the sensitive bioactive compounds present in food such as phenolic compounds (Sárka et al., 2021). Several studies have shown that while extrusion may degrade thermolabile phenolic compounds (Lohani & Muthukumarappan, 2017a; 2017b; Ortiz-Cruz et al., 2020), it could also enhance the release of compounds bound to cell walls, improving their functional availability (Wójtowicz et al., 2019). Consequently, recent researches have focused on identifying extrusion conditions that preserve or improve phenolic retention while maintaining targeted textural and structural qualities (Korkerd et al., 2016; Lohani & Muthukumarappan, 2017a, 2017b; Medina-Rendon et al., 2023; Rathod & Annapure, 2017). However, most of these approaches have emphasized the preservation of free phenolics without taking into account cell wall-bound phenolic compounds (Le Bourvellec et al., 2007; Le Bourvellec et al., 2012). These structurally diverse compounds may respond differently to the mechanical and thermal stresses of extrusion depending on the specific processing conditions applied (Khanal et al., 2009), thus requiring a more comprehensive evaluation.

In this context, an optimized approach is essential, not only to maximize phenolic compound retention but also to understand how distinct phenolic compound fractions behave under varying processing conditions and to access feasibility of using pomace as a functional ingredient. Hence, this study aims to investigate the effects of extrusion parameters (die temperature, feed moisture, inclusion level of by-product) on the physical, textural and nutritional properties (expansion index, bulk density, textural properties, color, free and bound phenolic compound contents) of tapioca starch-based extrudates enriched with strawberry by-products (SBP) using Response Surface Methodology (RSM) in order to optimize the processing conditions.

# 2. Materials and methods

## 2.1. Raw materials

Strawberry fruit from the Sabrina variety were sourced from local cultivars in the municipality of Sotará, Cauca and were used for juice extraction. Strawberry by-products (SBP) were then obtained from pulp extraction process provided by Monkeyfruit S.A. (Popayán, Colombia). The SBP were frozen at -80 °C for 12 h and freeze-dried (Freeze-drier YR05188, Kalstein, Paris, France) for 72 h. The dried SBP were grinded (MF 10 basic Molino IKA, Germany), passed through a 1 mm sieve, rapidly and the powder was vacuum sealed and stored at -20 °C until use. Tapioca (Manihot esculenta) starch was supplied by Ingredion Colombia S.A. (Cali, Colombia). The SBP had a moisture content of 6.62 g/100g, protein 1.91 g/100g Dry Matter (DM), fat 0.79 g/100g DM, ash 3.51 g/100g DM, and dietary fiber 29.56 g/100g DM. The free and bound phenolic compounds contents were 6.86 and 1.71 mg Gallic Acid Equivalent (GAE)/g DM, respectively. Tapioca starch had a moisture content of 13.48 g/100g, protein <0.1 g/100 g DM, fat 1.47 g/100g DM, ash 0.24 g/100 g DM, and dietary fiber <0.5 g/100g DM and total phenolic content was negligible.

## 2.2. Standards and chemicals

All reagents were of analytical grade, HPLC solvents were of chromatographic purity, and water was purified through deionization using

a Milli-Q system (Millipore, Bedford, MA, USA). Folin-Ciocalteu reagent and sodium carbonate were obtained from Panreac (Barcelona, Spain). Acetonitrile of HPLC grade was obtained from Fisher Scientific (Strasbourg, France). Acetone was supplied by Merck (Darmstadt, Germany). Methanol, formic acid, and standards of (+)-catechin, (-)-epicatechin, quercetin, kaempferol, pelargonidin-3-O-β-glucoside, cyanidin-3-O-β-glucoside, coumaroylhexoside acid, as well as other standards and reagents, were sourced from Sigma-Aldrich (Steinheim, Germany).

## 2.3. Preparation of mixtures

The mixtures were prepared with tapioca starch and SBP powder at varying inclusion levels and moisture contents, as specified in the experimental design (Table 1). The amount of water required to reach the target moisture content was calculated considering the raw material initial moisture and then added by spraying. Hydration was carried out at room temperature, followed by mixing for 10 min at medium speed in a KP26M1XER commercial mixer (KitchenAid, MI, USA). Finally, the mixtures were packed in polyethylene bags and stored overnight (12-14 h) at room temperature (20 ± 2 °C) to ensure homogeneity before processing.

## 2.4. Extrusion process

Extrusion cooking was carried out using a laboratory-scale co-rotating twin-screw extruder (HAAKE™ Rheomex OS, ThermoFisher Scientific, Germany) with a screw diameter of 11 mm, a length-to-diameter ratio of 40:1, and a die opening of 3 mm. The barrel temperatures (140/160/180 °C), screw speed (140 min⁻¹), and feed rate (20 g/min) were kept constant throughout the process and the strawberry by-products contents (SBPC), die temperature (DT) and feed moisture (FM) were established based on 20 experimental runs (Table 1), according to the experimental design described in Section 2.7. The extrudates were dried in a forced air oven at 60 °C for 30 min, packed in polyethylene bags, and stored in an airtight container until physical tests were performed. For chemical and color analysis, the samples were ground using a laboratory grinder (A 11 basic Molino IKA, Germany) and passed through a mesh-60 sieve.

<table><caption>**Table 1**
Experimental design for the extrusion process.</caption><thead><tr><th rowspan="2">Run</th><th rowspan="2">Coded levels</th><th colspan="3">Actual levels</th></tr><tr><th>SBPC (g/ 100g dry matter)</th><th>DT (°C)</th><th>FM (g/ 100 g)</th></tr></thead><tbody><tr><td>1</td><td>-1</td><td>-1</td><td>-1</td><td>15</td><td>180</td><td>20</td></tr><tr><td>2</td><td>1</td><td>-1</td><td>-1</td><td>25</td><td>180</td><td>20</td></tr><tr><td>3</td><td>-1</td><td>1</td><td>-1</td><td>15</td><td>200</td><td>20</td></tr><tr><td>4</td><td>1</td><td>1</td><td>-1</td><td>25</td><td>200</td><td>20</td></tr><tr><td>5</td><td>-1</td><td>-1</td><td>1</td><td>15</td><td>180</td><td>24</td></tr><tr><td>6</td><td>1</td><td>-1</td><td>1</td><td>25</td><td>180</td><td>24</td></tr><tr><td>7</td><td>-1</td><td>1</td><td>1</td><td>15</td><td>200</td><td>24</td></tr><tr><td>8</td><td>1</td><td>1</td><td>1</td><td>25</td><td>200</td><td>24</td></tr><tr><td>9</td><td>-1.68179</td><td>0</td><td>0</td><td>11.6*</td><td>190</td><td>22</td></tr><tr><td>10</td><td>1.68179</td><td>0</td><td>0</td><td>28.4*</td><td>190</td><td>22</td></tr><tr><td>11</td><td>0</td><td>-1.68179</td><td>0</td><td>20</td><td>173.2*</td><td>22</td></tr><tr><td>12</td><td>0</td><td>1.68179</td><td>0</td><td>20</td><td>206.8*</td><td>22</td></tr><tr><td>13</td><td>0</td><td>0</td><td>-1.68179</td><td>20</td><td>190</td><td>18.6*</td></tr><tr><td>14</td><td>0</td><td>0</td><td>1.68179</td><td>20</td><td>190</td><td>25.4*</td></tr><tr><td>15</td><td>0</td><td>0</td><td>0</td><td>20</td><td>190</td><td>22</td></tr><tr><td>16</td><td>0</td><td>0</td><td>0</td><td>20</td><td>190</td><td>22</td></tr><tr><td>17</td><td>0</td><td>0</td><td>0</td><td>20</td><td>190</td><td>22</td></tr><tr><td>18</td><td>0</td><td>0</td><td>0</td><td>20</td><td>190</td><td>22</td></tr><tr><td>19</td><td>0</td><td>0</td><td>0</td><td>20</td><td>190</td><td>22</td></tr><tr><td>20</td><td>0</td><td>0</td><td>0</td><td>20</td><td>190</td><td>22</td></tr></tbody></table>

SBPC: Strawberry by-products content; DT: Die temperature; FM: Feed moisture.
*Values were rounded up to the nearest whole number to set extrusion conditions.
## 2.5. Specific mechanical energy (SME)

The total mechanical energy input, commonly referred to as specific mechanical energy (SME), was calculated using Equation (1) (Reißner et al., 2022; Ribeiro et al., 2024). The average mass flow rate, Q (g/min), was determined based on the mass throughput measured over 30 s in quadruplicate. Energy input was calculated using the mean values of torque (T) and screw speed (SS), which were recorded every 15 s during extrusion. Each test condition included at least 15 recorded values.

$$SME = (2\pi*T*SS)/Q \qquad (1)$$

## 2.6. Determination of physical properties and system parameters

### 2.6.1. Expansion index (EI) and bulk density (BD)

The expansion index was determined by measuring the diameter of the extrudate using a Vernier caliper (Mitutoyo Co., Kawasaki, Japan) and dividing it by the internal diameter of the extruder die. Ten measurements were taken for each extrudate sample (Medina-Rendon et al., 2023).

The bulk density was calculated based on the extrudate's volume, derived from its dimensions, and its measured weight. The length and diameter of the extrudate were measured using a digital Vernier caliper (Mitutoyo Co., Kawasaki, Japan), while an analytical balance (KERN ABJ 220-4NM, Balingen, Germany) was used to determine its weight. The volume and weight of 10 extrudates were recorded, and the bulk density was calculated as mass per unit volume (g/cm³) (Mohamad Mazlan et al., 2020).

### 2.6.2. Textural properties

The textural properties of the extrudates were evaluated using a texture analyzer (EZ Test Texture Analyzer, Shimadzu Corporation, Japan) equipped with Trapezium X software (Shimadzu Corporation, Japan). Twenty-five extrudates from each treatment were equilibrated in a humidity chamber (30 % RH) for 94 h. The samples were compressed perpendicular to the extrusion direction to 50 % of their original diameter using a 1 mm compression punch at a test speed of 0.5 mm/min. The hardness (H) values were directly obtained from the software data and define as the force needed to completely crush the sample with the molars. A force-deformation curve was obtained using Origin software (version 2018, OriginLab Corporation, Northampton, MA, USA), and the number of peaks (n) above 1.5 N, integral of the curve (S) (or area below the curve), and distance of compression (x) were computed. Using n, S, and x values, spatial frequency of ruptures (NSr), average crushing force (FCr) and crispness or fracture work (WCr) were calculated using Equations (2)–(4) (Devi et al., 2013; Karkle et al., 2012).

$$NSr (mm^{-1}) = n/x \qquad (2)$$

$$FCr (N) = S/x \qquad (3)$$

$$WCr (N.mm) = FCr/NSr \qquad (4)$$

Spatial frequency of ruptures (NSr) describes the number of fractures events during puncture (Uribe-Wandurraga et al., 2020), refering to the frequency of fractures or breaks in the snack's structure during mastication, hence measuring crispness (Sahu, 2020). Crushing force (FCr) measures the average force required to crush the foam structure, analogous to the sensory parameter of hardness (H). WCr measures the average work required to fracture a cell or a group of cells simultaneously, similar to the sensory parameter of fracturability, which is defined as the force with which the sample fractures on the first bite down with the molars (Devi et al., 2013).

### 2.6.3. Color

The color parameters L*, a* and b* were measured using tristimulus colorimetry with a Konica Minolta CR-400 colorimeter (Minolta Co.,

Osaka, Japan). The samples were placed in 5 cm diameter Petri dishes, and 15 readings were recorded for each treatment. These parameters were then used to calculate the Color index (CI), Chroma (C*) and Hue (h) according to Equations (5)–(7) (Neder-Suárez et al., 2021; Téllez-Morales et al., 2022). CI provides an integrated value based on L*, a*, and b* to compare color changes; C* indicates color intensity or saturation; and h describes the basic shade or color tone of the sample.

$$CI = \frac{1000 \times a^*}{L^* \times b^*} \qquad (5)$$

$$C^* = \sqrt{(a^{*2} + b^{*2})} \qquad (6)$$

$$h = \arctan (b / a) \qquad (7)$$

## 2.7. Phenolic compounds

### 2.7.1. Free and bound phenolic compound

Phenolic compounds are categorized into Free Phenolic Compounds (FPC), which are extracted with aqueous-organic solvents, and Bound Phenolic Compounds (BPP) attached to the cell wall after processing which need an acid hydrolysis to be extracted because they are bound to macromolecules like proteins or polysaccharides from the cell wall.

The FPC extraction and analysis was based on the method described by Pico et al. (2020) with modifications. 2 g (±0.0500 g) of powder sample was weighed in a centrifuge tube and 8 mL of 0.01 g/mL formic acid in 0.8 g/L methanol were added for the first extraction during 15 min using a wrist shaker. The sample was centrifuged for 5 min at 3500 min⁻¹ and 20 °C and 40 µL of 0.02 g/mL EDTA were added to the supernatant for the stabilization of flavan-3-ols. The sample solution was kept on ice. 8 mL of 0.01 g/mL formic acid in 0.7 g/mL methanol were added to the pellet for a second extraction for 15 min using the wrist shaker. After centrifugation, the supernatant was combined with the methanolic extract from the first extraction and made up to 20 mL with deionized water. The extract was kept at -80 °C for the colorimetric determination by Folin-Ciocalteu reaction.

The pellet was also kept at -80 °C for further sequential determination of BPC. The extraction of this fraction was based on the method described by Pico et al. (2019). 0.8 g (±0.0050 g) of sample from the FPC pellet were weighed in a glass tube and 10 mL of methanol/sulfuric acid (90/10; v/v) were added. The hydrolysis was carried out for 22 h at 85 °C, with magnetic stirring. The sample was then centrifuged at 3500 min⁻¹ for 20 min and the supernatant was made up to 25 mL with deionized water. The extract was kept at -80 °C for the colorimetric determination by Folin-Ciocalteu reaction.

For the Folin-Ciocalteu reaction, 260 µL of Milli-Q water, 26 µL of 0.75 g/mL Na₂CO₃, 20 µL of extract and 20 µL of Folin-Ciocalteu reagent were mixed in a 96-well plate. The samples were incubated for 1 h at room temperature in the dark and the spectrophotometric determination was performed at 765 nm in a multimode microplate reader Varioskan™ LUX (ThermoFisher Scientific, Germany). For the blank, 20 µL of deionized water were used. The total content of FPC and BPC were expressed as mg of gallic acid equivalents (GAE)/g dry matter, based on a gallic acid calibration curve. All analyses were carried out in quadruplicate and the handle of the reagents was performed in conditions as dark and cold as possible.

### 2.7.2. Identification and quantification of individual phenolic compounds

Identification of phenolic compounds was performed using HPLC coupled to ElectroSpray Ionization Mass Spectrometry (HPLC/ESI-MS²) using an Acquity Ultra performance LC (UPLC) apparatus from Waters (Milford, MA, USA), equipped with a photodiode array detector coupled with a Bruker Daltonics (Bremen, Germany) HCT ultra ion trap mass spectrometer with an electrospray ionization source. Separations were achieved using a Luna Omega Polar C18 column (50 mm × 2.1 mm × 3 µm column, Phenomenex, Torrance, USA) operated at 30 °C. The mobile
phase consisted of water/formic acid (98,2 mL/mL) (eluent A) and acetonitrile (eluent B). The flow rate was 1 mL/min. The elution program was as follows: 3–9 % B (0–5 min); 9–16 % B (5–15 min); 16–50 % B (15–45 min); 50–90 % B (45–48 min); 90 % B (48–52 min). 10 µL of samples (“furanolysis” and “crude” extracts) were injected.

For phenolic compounds characterization, a capillary voltage of 2 kV was used in the negative ion mode. N₂ was used as drying and nebulizing gas with a flow rate of 12 L/min. The desolvation temperature was set at 365 °C and the nebulization pressure at 0.4 MPa. The ion trap was operated in Ultrascan mode from m/z 100 to 1000.

For anthocyanin characterization, a capillary voltage of 1.8 kV was used in the positive ion mode under the same conditions.

Phenolic compounds were quantified by HPLC-DAD (Prominence system, Shimadzu, Kyoto, Japan) and separations were achieved as described in Rincon et al. (2025). Analyses of phenolic compounds were carried out with or without the acidic depolymerization of proanthocyanidins (PACs) in the presence of menthofuran (named furanolysis). PACs were thus characterized by their subunit composition, average degree of polymerization (mDP) and contents. HPLC-DAD analyses of crude methanolic extracts (not submitted to furanolysis) were performed to titrate monomeric catechins.

Furanolysis and methanol extracts were obtained as described in Rincon et al. (2025). The most prevalent compounds were quantified in the UV-visible region by chromatographic analysis conducted at wavelengths of 280 nm (flavanols), 320 nm (phenolic acids), 360 nm (flavonols), and 520 nm (anthocyanins). Individual compounds were quantified in mg/100 g of fresh weight (FW) by comparison with external standards: (+)-catechin and (−)-epicatechin for flavanols, caffalic and para-coumaric acids for phenolic acids, quercetin and kaempferol for flavanols, and pelargonidin-3-O-β-glucoside and cyanidin-3-O-β-glucoside for anthocyanins.

The mean degree of polymerization (mDP) of proanthocyanidins was calculated as the molar ratio of all flavan-3-ol units (i.e., furan adducts plus terminal units, minus monomers from crude methanol extract) to the sum of terminal units, minus monomers from crude methanol extract (Rincon et al., 2025).

## 2.8. Experimental design, statistical analysis and optimization

An orthogonal and rotatable central composite design 2³ with three independent variables (SBPC, DT and FM) was used, resulting in a total of 20 experimental runs (Table 1). The dependent variables measured included expansion index, bulk density, crushing force, hardness, spatial frequency of ruptures, color, specific mechanical energy, free and bound phenolic compound contents. Response surface methodology (RSM) was applied to determine the optimal combination of conditions and to explore the relationships between the selected process variables for extrusion. Experimental data were evaluated through analysis of variance (ANOVA), Tukey's test, and multiple regression analysis at a significance level of 0.05 (p < 0.05). The data were modeled using Design-Expert® 13 (Stat-Ease, Inc., Minneapolis, MN, USA) and analyzed using IBM® SPSS® Statistics (Version 27, IBM Corp., Armonk, NY, USA). The behavior of each response variable concerning the independent parameters was fitted to a second order polynomial model. The quality and adequacy of the models were assessed by evaluating the coefficient of determination (R²) and the lack of fit.

Optimization was performed by maximizing the expansion index, a key variable for an expanded extrudate, while also maximizing phenolic compound contents and minimizing bulk density and the crispness work (WCr). Derringer's desirability function methodology was employed to generate optimal conditions for the extrusion process variables, considering all the properties of the extruded snacks. The optimal conditions were validated by applying them in the process and determining all associated variables. Validation was verified through a one-sample t-test performed for each variable, comparing the experimental values with those calculated by the model.

Beyond the variables included in the optimization study, additional compositional analyses were performed to provide a deeper understanding of the effects of extrusion on the phenolic compound profile of the extrudates. These included quantification of phenolic compounds by HPLC-DAD as well as the structural characteristics of proanthocyanidins, i.e., their mean degree of polymerization and constitutive units. These analyses were carried out on a subset of nine treatments corresponding to the factorial points of the central composite design and one replicate of the central point (run 15), selected to represent the experimental design space. Although not used in the construction of the predictive models, these data offered complementary insights into the compositional changes induced by the processing conditions and contributed to a more comprehensive evaluation of the extrudates.

## 3. Results and discussion

The response surface equations obtained from the experimental data were generated using Design-Expert software and are presented in Table 3. Each equation describes the relationship between the independent variables and the corresponding response, integrating the linear, quadratic, and interaction effects. The table also reports the main statistical parameters of the models. For all responses, the models were statistically significant (p < 0.05), with non-significant lack of fit and consistently high values of R², adjusted R², and predicted R². These parameters respectively indicate the proportion of variability explained by the model, the adjustment for the number of predictors included, and the model's ability to predict new observations (Téllez-Morales et al., 2022), thereby confirming the adequacy, reliability, and predictive performance of the fitted equations.

### 3.1. Effect of extrusion conditions on system parameters: specific mechanical energy (SME)

Specific mechanical energy (SME) which determines the degree of mechanical energy delivered to the material, was significantly affected by the linear and quadratic factor of SBPC as well as by the linear factor of FM and its interaction with SBPC and DT (p < 0,05) (Table 2). Hence, increased strawberry by-products content led to a significant decrease in SME (Fig. 1 a), indicating that high strawberry by-product content facilitated flow through the extruder, thus reducing SME as previously observed by (Mohamad Mazlan et al., 2020) after inclusion of spinach, citrus fiber, and mango peel. Moreover, low values were observed at both high moisture levels and die temperatures (Fig. 1 b). A strong correlation was observed between SME and EI (r = 0.84), L* (r = 0.85), and BD (r = -0.76). Similar correlations have been reported previously (Karkle et al., 2012), indicating that high moisture content facilitates flow during extrusion, a phenomenon commonly referred to the lubricating effect (Mohamad Mazlan et al., 2020). This effect aids flow through the barrel reducing both the pressure and the energy needed for expansion. SME directly influences the extent of macromolecular modifications and the rheological properties of the melt (Guerrero et al., 2014). SME measures the energy transferred into the extrusion system and dissipated as heat through shear friction during effective conveying. This heat generation leads to the gelatinization of starch, a process particularly enhanced under high mechanical energy conditions (Mohamad Mazlan et al., 2020). In contrast, Mohamad Mazlan et al. (2020) found no evidence of a lubricating effect in extrudates containing corn grits and mango peel probably due to a low moisture range (15.5 %–21.5 %). However, they report high SME values with increased temperature.

These findings highlight the complex interplay between formulation and processing variables in determining SME. The observed trends confirm that SBPC and FM play a central role in driving energy input during extrusion.
<table><caption>**Table 2**
Effect of extrusion on extrudate properties depending on processing conditions.</caption><thead><tr><th>Run</th><th colspan="10">System and physical properties</th></tr><tr><th></th><th>SME (KJ/Kg)</th><th>EI</th><th>BD (g/cm³)</th><th>L*</th><th>a*</th><th>b*</th><th>C*</th><th>h (°)</th><th>CI</th></tr></thead><tbody><tr><td>1</td><td>3300.43 ± 451.90<br>ab</td><td>2.28 ± 0.15a</td><td>0.25 ± 0.03abc</td><td>58.99 ± 0.42a</td><td>3.06 ± 0.04a</td><td>5.92 ± 0.06abc<br>0.64 ± 0.02de</td><td>6.66 ± 0.07 ab</td><td>62.64 ± 0.28<br>ab<br>62.50 ± 0.12<br>ab</td><td>8.76 ± 0.17abc<br>9.37 ± 0.06de<br>8.40 ± 0.06cf</td></tr><tr><td>2</td><td>2554.80 ± 114.04cd</td><td>1.93 ± 0.17bcd</td><td>0.33 ± 0.02def</td><td>55.55 ± 0.24b</td><td>3.36 ± 0.01bh</td><td>6.45 ± 0.02de</td><td>7.28 ± 0.01c</td><td>63.58 ± 0.11cde</td><td>8.40 ± 0.06cf</td></tr><tr><td>3</td><td>3469.91 ± 445.14a</td><td>2.10 ± 0.14acd<br>0.14acd</td><td>0.22 ± 0.02a</td><td>59.20 ± 0.17 ag</td><td>3.29 ± 0.03bc</td><td>6.62 ± 0.05e</td><td>7.39 ± 0.06c</td><td>62.22 ± 0.25a</td><td>9.49 ± 0.17de</td></tr><tr><td>4</td><td>2879.03 ± 288.16bd</td><td>1.83 ± 0.14bf<br>0.30 ± 0.03bcd</td><td>0.33 ± 0.02def</td><td>55.52 ± 0.45b</td><td>3.82 ± 0.03d</td><td>7.25 ± 0.13f</td><td>8.19 ± 0.13d</td><td>62.22 ± 0.37a</td><td>9.76 ± 0.21eg</td></tr><tr><td>5</td><td>2252.80 ± 40.33cef<br>202.67cef</td><td>1.62 ± 0.16 fg</td><td>0.41 ± 0.02</td><td>53.85 ± 0.45c</td><td>2.74 ± 0.06e</td><td>5.22 ± 0.15gh<br>0.52 ± 0.18gh</td><td>5.90 ± 0.15ef</td><td>62.26 ± 0.37a</td><td>9.76 ± 0.21eg</td></tr><tr><td>6</td><td>2160.95 ± 202.67cef</td><td>1.28 ± 0.10hi</td><td>0.51 ± 0.04g</td><td>49.62 ± 0.31de</td><td>2.47 ± 0.06f</td><td>5.26 ± 0.18gh</td><td>5.81 ± 0.18e</td><td>64.82 ± 0.45<br>fg</td><td>9.47 ± 0.26de<br>8.58 ± 0.14acf</td></tr><tr><td>7</td><td>1977.76 ± 138.67ef<br>274.22cef</td><td>1.43 ± 0.13gh</td><td>0.80 ± 0.05i</td><td>52.00 ± 0.32f</td><td>2.32 ± 0.07f</td><td>5.21 ± 0.14g</td><td>5.74 ± 0.15e</td><td>65.26 ± 0.34g</td><td>8.26 ± 0.12f</td></tr><tr><td>8</td><td>2120.49 ± 274.22cef</td><td>1.38 ± 0.10h</td><td>0.77 ± 0.05h</td><td>49.29 ± 0.34d</td><td>2.15 ± 0.06g</td><td>5.27 ± 0.14gh</td><td>5.70 ± 0.14e</td><td>67.84 ± 0.28i</td><td>8.26 ± 0.12f</td></tr><tr><td>9</td><td>3112.40 ± 390.53<br>ab</td><td>2.26 ± 0.19ae</td><td>0.24 ± 0.02 ab</td><td>59.65 ± 0.41g</td><td>2.72 ± 0.06e</td><td>5.53 ± 0.11hi</td><td>6.23 ± 0.12 fg</td><td>64.91 ± 0.32g</td><td>8.25 ± 0.15f</td></tr><tr><td>10</td><td>2335.18 ± 102.24cf</td><td>1.83 ± 0.11bf</td><td>0.31 ± 0.02cdef<br>0.02cdef</td><td>53.15 ± 0.50h</td><td>3.30 ± 0.09bc</td><td>6.61 ± 0.12e</td><td>7.39 ± 0.15c</td><td>64.55 ± 0.21fgh</td><td>9.39 ± 0.04ce</td></tr><tr><td>11</td><td>2557.06 ± 348.51cd</td><td>1.85 ± 0.09bf</td><td>0.36 ± 0.05bcd<br>0.05bcd</td><td>55.72 ± 0.23b</td><td>3.25 ± 0.07c</td><td>5.74 ± 0.13aci<br>0.13aci</td><td>6.60 ± 0.14 ab</td><td>60.51 ± 0.16j</td><td>10.15 ± 0.08g</td></tr><tr><td>12</td><td>2552.53 ± 136.48cd</td><td>1.66 ± 0.13f</td><td>0.58 ± 0.04g</td><td>54.54 ± 0.36i</td><td>2.83 ± 0.03e</td><td>5.82 ± 0.07aci<br>0.07aci</td><td>6.47 ± 0.08 ag</td><td>64.04 ± 0.17cch<br>0.17cch</td><td>8.93 ± 0.06abh<br>8.41 ± 0.19f</td></tr><tr><td>13</td><td>3062.30 ± 236.41<br>ab</td><td>2.17 ± 0.14ade</td><td>0.29 ± 0.02abcd<br>0.02abcd</td><td>58.86 ± 0.51a</td><td>3.47 ± 0.07h</td><td>7.00 ± 0.08f</td><td>7.81 ± 0.11h</td><td>63.65 ± 0.32cde</td><td>8.62 ± 0.21acf</td></tr><tr><td>14</td><td>1812.88 ± 200.09e</td><td>1.09 ± 0.10i</td><td>0.82 ± 0.08hi</td><td>50.16 ± 0.50e</td><td>2.07 ± 0.04g</td><td>4.78 ± 0.05j</td><td>5.20 ± 0.05i</td><td>66.62 ± 0.36k</td><td>8.85 ± 0.15</td></tr><tr><td>15</td><td>2411.34 ± 368.34cdf</td><td>1.92 ± 0.07bc</td><td>0.36 ± 0.03ef</td><td>55.53 ± 0.33b</td><td>2.94 ± 0.03a</td><td>5.99 ± 0.04abc<br>0.04abc</td><td>6.67 ± 0.05 ab</td><td>63.81 ± 0.25cdh<br>0.25cdh</td><td>8.85 ± 0.15<br>ab</td></tr><tr><td>16</td><td>2493.66 ± 316.49cdf</td><td>1.95 ± 0.18bcd</td><td>0.31 ± 0.03cdef<br>0.03cdef</td><td>55.87 ± 0.20b</td><td>3.07 ± 0.03a</td><td>6.16 ± 0.11bd<br>0.11bd</td><td>6.88 ± 0.11b</td><td>63.53 ± 0.21cde<br>0.21cde</td><td>8.91 ± 0.10abh<br>8.86 ± 0.05</td></tr><tr><td>17</td><td>2546.25 ± 258.78cd</td><td>2.04 ± 0.09bcd<br>0.09bcd</td><td>0.34 ± 0.02def</td><td>57.00 ± 0.20b</td><td>3.11 ± 0.04a</td><td>6.17 ± 0.04bd<br>0.04bd</td><td>6.91 ± 0.05b</td><td>63.21 ± 0.21bde<br>0.21bde</td><td>8.91 ± 0.05bch<br>ab</td></tr><tr><td>18</td><td>2387.92 ± 175.21cdf</td><td>1.91 ± 0.12bc</td><td>0.31 ± 0.02cdef<br>0.02cdef</td><td>57.16 ± 0.07b</td><td>3.15 ± 0.01a</td><td>6.04 ± 0.01abc<br>0.01abc</td><td>6.82 ± 0.01b</td><td>62.46 ± 0.09<br>ab</td><td>9.13 ± 0.05bch<br>ab</td></tr><tr><td>19</td><td>2467.54 ± 246.37cdf</td><td>1.95 ± 0.07bcd</td><td>0.36 ± 0.03ef</td><td>57.08 ± 0.13b</td><td>3.09 ± 0.01a</td><td>6.07 ± 0.07<br>ab<br>0.07abc</td><td>6.82 ± 0.07b</td><td>62.96 ± 0.30abe<br>0.30abe</td><td>8.93 ± 0.13abh<br>8.93 ± 0.13abh</td></tr><tr><td>20</td><td>2437.37 ± 125.56cdf</td><td>1.86 ± 0.16bcf</td><td>0.38 ± 0.02f</td><td>56.05 ± 0.40b</td><td>3.07 ± 0.09a</td><td>5.90 ± 0.13abc<br>0.13abc</td><td>6.65 ± 0.15 ab</td><td>62.48 ± 0.17b</td><td>9.29 ± 0.02ch</td></tr></tbody></table>

<table><thead><tr><th>Run</th><th colspan="6">Textural properties and phenolic compound contents</th></tr><tr><th></th><th>H (N)</th><th>FCr (N)</th><th>NSr (mm<sup>-1</sup>)</th><th>WCr (N.mm)</th><th>FPC (mg/g)</th><th>BPC (mg/g)</th></tr></thead><tbody><tr><td>1</td><td>38.46 ± 4.57ade</td><td>17.53 ± 1.28abc</td><td>17.09 ± 1.72abcdef</td><td>1.03 ± 0.03 ab</td><td>0.61 ± 0.01abc</td><td>2.30 ± 0.11 ab</td></tr><tr><td>2</td><td>41.00 ± 5.10ade</td><td>18.47 ± 1.42abcd</td><td>16.32 ± 1.90abcdefg</td><td>1.13 ± 0.05a</td><td>1.06 ± 0.09de</td><td>2.76 ± 0.12acde</td></tr><tr><td>3</td><td>25.94 ± 3.54a</td><td>13.36 ± 2.03b</td><td>18.28 ± 1.61abcdef</td><td>0.73 ± 0.05c</td><td>0.74 ± 0.03 fg</td><td>3.06 ± 0.32cde</td></tr><tr><td>4</td><td>33.16 ± 4.25ad</td><td>15.23 ± 1.49bc</td><td>23.25 ± 2.66f</td><td>0.66 ± 0.01c</td><td>1.16 ± 0.07d</td><td>5.02 ± 0.59h</td></tr><tr><td>5</td><td>62.32 ± 11.92b</td><td>28.32 ± 2.99ad</td><td>15.99 ± 2.22abcdefg</td><td>1.77 ± 0.06d</td><td>0.57 ± 0.04 ab</td><td>3.24 ± 0.17ce</td></tr><tr><td>6</td><td>56.64 ± 10.84bef</td><td>30.74 ± 3.78d</td><td>14.01 ± 1.69cdeg</td><td>2.19 ± 0.01e</td><td>0.95 ± 0.03eh</td><td>2.30 ± 0.19 ab</td></tr><tr><td>7</td><td>117.65 ± 17.26c</td><td>45.43 ± 7.85e</td><td>10.59 ± 1.24 dg</td><td>4.29 ± 0.24f</td><td>0.50 ± 0.04bi</td><td>2.17 ± 0.04 ab</td></tr><tr><td>8</td><td>133.68 ± 22.76c</td><td>56.62 ± 5.97ef</td><td>9.89 ± 1.50g</td><td>5.73 ± 0.28g</td><td>0.82 ± 0.03fj</td><td>2.41 ± 0.12abd</td></tr><tr><td>9</td><td>38.18 ± 5.50ade</td><td>19.32 ± 2.55abcd</td><td>12.35 ± 1.53cdg</td><td>1.56 ± 0.01h</td><td>0.42 ± 0.0045i</td><td>3.45 ± 0.14ef</td></tr><tr><td>10</td><td>46.66 ± 3.58bdef</td><td>24.11 ± 2.31abc</td><td>15.07 ± 2.74acdeg</td><td>1.60 ± 0.14h</td><td>0.97 ± 0.07eh</td><td>4.21 ± 0.23g</td></tr><tr><td>11</td><td>41.58 ± 4.69ade</td><td>21.00 ± 1.82abc</td><td>20.98 ± 1.70abf</td><td>1.00 ± 0.01 ab</td><td>0.54 ± 0.03abi</td><td>1.76 ± 0.13b</td></tr><tr><td>12</td><td>60.15 ± 4.57bf</td><td>27.29 ± 4.22ac</td><td>15.99 ± 2.32abcdefg</td><td>1.71 ± 0.02dh</td><td>0.56 ± 0.03 ab</td><td>2.70 ± 0.11 acd</td></tr><tr><td>13</td><td>27.27 ± 5.55ad</td><td>13.46 ± 1.98b</td><td>18.82 ± 2.44abcdef</td><td>0.72 ± 0.01c</td><td>0.87 ± 0.03hj</td><td>4.14 ± 0.07 fg</td></tr><tr><td>14</td><td>133.72 ± 23.11c</td><td>59.45 ± 10.45f</td><td>10.89 ± 1.65 dg</td><td>5.46 ± 0.14i</td><td>0.56 ± 0.04 ab</td><td>2.78 ± 0.29acde</td></tr><tr><td>15</td><td>42.32 ± 5.59adef</td><td>19.35 ± 3.00abcd</td><td>20.25 ± 3.00aef</td><td>0.96 ± 0.01b</td><td>0.63 ± 0.02acg</td><td>2.65 ± 0.26ac</td></tr><tr><td>16</td><td>40.87 ± 4.74ade</td><td>19.11 ± 2.44abcd</td><td>19.50 ± 2.87abef</td><td>0.98 ± 0.02b</td><td>0.61 ± 0.02abc</td><td>2.44 ± 0.19abd</td></tr><tr><td>17</td><td>37.71 ± 5.60ade</td><td>18.72 ± 2.85abcd</td><td>20.69 ± 2.18abef</td><td>0.90 ± 0.04b</td><td>0.71 ± 0.004cfg</td><td>2.65 ± 0.14 acd</td></tr><tr><td>18</td><td>37.60 ± 6.16ade</td><td>18.97 ± 1.70abcd</td><td>20.11 ± 2.43abef</td><td>0.94 ± 0.03b</td><td>0.61 ± 0.02abc</td><td>2.78 ± 0.34acde</td></tr><tr><td>19</td><td>38.99 ± 6.40ade</td><td>19.16 ± 3.42abcd</td><td>22.04 ± 2.97bf</td><td>0.87 ± 0.04b</td><td>0.56 ± 0.05 ab</td><td>2.65 ± 0.15 acd</td></tr><tr><td>20</td><td>39.67 ± 4.44ade</td><td>19.09 ± 2.68abcd</td><td>20.62 ± 3.17abef</td><td>0.93 ± 0.01b</td><td>0.60 ± 0.03abc</td><td>2.47 ± 0.13a</td></tr></tbody></table>

SME: Specific Mechanical Energy; EI: Expansion Index; BD: Bulk density; L*, a*, b*, C*, h: Color parameters; CI: Color Index; H: Hardness; FCr: Crushing Force; NSr: Number of spatial ruptures; WCr: Crispness Work; FPC: Free Phenolic Compounds; BPC: Bound Phenolic Compounds. Different letters in the same column indicate significant differences (p < 0.05) by Tukey test.

## 3.2. Effect of extrusion conditions on physical properties: expansion index and bulk density

EI ranged from 1.6 to 2.3 (Table 2), depending on the extrusion

conditions, in accordance with Wójtowicz et al. (2019), who reported similar results for strawberry inclusions above 15 %. BD ranged from 0.2 to 0.6 g/cm³ (Table 2). Similar values were registered by Wójtowicz et al. (2019) at 20 % of strawberry inclusion.
<table><caption>**Table 3**
Best-fit response models in terms of coded variables terms.</caption><thead><tr><th>Parameter</th><th>Response surface model</th><th>p of F (model)</th><th>Lack of fit</th><th>R<sup>2</sup></th><th>Adjusted R<sup>2</sup></th><th>Predicted R<sup>2</sup></th></tr></thead><tbody><tr><td>SME</td><td>SME = 2457.01-189.85SBPC<sup>a</sup> + 12.49DT - 424.21FM<sup>a</sup> + 48.67SBPC*DT + 173.42SBPC*FM<sup>a</sup> - 101.15DT*FM<sup>a</sup> + 96.42 SBPC<sup>2</sup> + 36.67 DT<sup>2</sup> - 4.77 FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.14</td><td>0.98</td><td>0.97</td><td>0.89</td></tr><tr><td>EI</td><td>EI = 1.94-0.13SBPC<sup>a</sup> - 0.05DT - 0.31FM<sup>a</sup> + 0.04SBPC*DT + 0.03SBPC*FM + 0.02DT*FM + 0.02SBPC<sup>2</sup> - 0.08DT<sup>2</sup> - 0.12FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.40</td><td>0.98</td><td>0.96</td><td>0.89</td></tr><tr><td>BD</td><td>BD = 0.34 + 0.03SBPC<sup>a</sup> + 0.07DT + 0.17FM<sup>a</sup> - 0.02SBPC*DT - 0.01SBPC*FM + 0.09DT*FM<sup>a</sup> - 0.02SBPC<sup>2</sup> + 0.05DT<sup>2</sup> + 0.08FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.49</td><td>0.99</td><td>0.98</td><td>0.94</td></tr><tr><td>H</td><td>Ln (H) = 3.67 + 0.05SBPC<sup>a</sup> + 0.11DT<sup>a</sup> + 0.47FM<sup>a</sup> + 0.05SBPC*DT<sup>a</sup> - 0.03SBPC*FM + 0.26DT*FM<sup>a</sup> + 0.04SBPC<sup>2</sup> + 0.10DT<sup>2</sup> + 0.16FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.41</td><td>0.99</td><td>0.99</td><td>0.97</td></tr><tr><td>FCr</td><td>Ln (FCr) = 2.95<sup>a</sup> + 0.06SBPC<sup>a</sup> + 0.08DT<sup>a</sup> + 0.44FM<sup>a</sup> + 0.03SBPC*DT<sup>a</sup> + 0.01SBPC*FM<sup>a</sup> + 0.19DT*FM<sup>a</sup> + 0.04SBPC<sup>2</sup> + 0.08DT<sup>2</sup> + 0.14FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.54</td><td>1.00</td><td>1.00</td><td>1.00</td></tr><tr><td>NSr</td><td>NSr = 20.53 + 0.45SBPC - 0.72DT - 2.77FM<sup>a</sup> + 0.88SBPC*DT - 0.86 SBPC*FM - 2.21DT*FM<sup>a</sup> - 2.35SBPC<sup>2</sup> - 0.66DT<sup>2</sup> - 1.95FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.12</td><td>0.95</td><td>0.91</td><td>0.69</td></tr><tr><td>WCr</td><td>Ln (WCr) = -0.07 + 0.04SBPC<sup>a</sup> + 0.14DT<sup>a</sup> + 0.63FM<sup>a</sup> - 0.02SBPC*DT + 0.06SBPC*FM + 0.34DT*FM<sup>a</sup> + 0.19SBPC<sup>2</sup> + 0.12DT<sup>2</sup> + 0.27FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.11</td><td>1.00</td><td>0.99</td><td>0.97</td></tr><tr><td>L*</td><td>L* = 56.48-1.83SBPC<sup>a</sup> - 0.29DT - 2.87FM<sup>a</sup> + 0.16SBPC*DT + 0.02SBPC*FM - 0.29DT*FM - 0.23SBPC<sup>2</sup> - 0.68DT<sup>2</sup> - 0.90FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.26</td><td>0.96</td><td>0.93</td><td>0.79</td></tr><tr><td>a*</td><td>a* = 3.07 + 0.10SBPC<sup>a</sup> - 0.05DT - 0.45FM<sup>a</sup> + 0.04SBPC*DT - 0.16SBPC*FM<sup>a</sup> - 0.18DT*FM<sup>a</sup> - 0.03SBPC<sup>2</sup> - 0.02DT<sup>2</sup> - 0.11FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.06</td><td>0.96</td><td>0.93</td><td>0.76</td></tr><tr><td>b*</td><td>b* = 5.95 + 0.22SBPC<sup>a</sup> + 0.12DT<sup>a</sup> - 0.66FM<sup>a</sup> + 0.01SBPC*DT - 0.13SBPC*FM - 0.19DT*FM<sup>a</sup></td><td>&lt;0.0001</td><td>0.07</td><td>0.95</td><td>0.92</td><td>0.84</td></tr><tr><td>C*</td><td>C* = 6.79 + 0.24SBPC<sup>a</sup> + 0.09DT - 0.79FM<sup>a</sup> + 0.03SBPC*DT - 0.19SBPC*FM<sup>a</sup> - 0.24DT*FM<sup>a</sup> + 0.001SBPC<sup>2</sup> - 0.1DT<sup>2</sup> - 0.11FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.06</td><td>0.97</td><td>0.94</td><td>0.81</td></tr><tr><td>h</td><td>h = 63.08 + 0.22SBPC<sup>a</sup> + 0.92DT<sup>a</sup> + 1.04FM<sup>a</sup> - 0.15SBPC*DT + 0.83 SBPC*FM<sup>a</sup> + 0.67DT*FM<sup>a</sup> + 0.54 SBPC<sup>2</sup> - 0.33 DT<sup>2</sup> + 0.68 FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.43</td><td>0.94</td><td>0.88</td><td>0.70</td></tr><tr><td>CI</td><td>CI = 8.99 + 0.22SBPC<sup>a</sup> - 0.34DT<sup>a</sup> + 0.03FM + 0.06SBPC*DT - 0.29SBPC*FM<sup>a</sup> - 0.27DT*FM<sup>a</sup> - 0.05SBPC<sup>2</sup> + 0.20 DT<sup>2</sup> - 0.16 FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.41</td><td>0.93</td><td>0.87</td><td>0.66</td></tr><tr><td>FPC</td><td>FPC<sup>-0.6</sup> = 1.28-0.19SBPC<sup>a</sup> - 0.004DT + 0.09FM<sup>a</sup></td><td>&lt;0.0001</td><td>0.07</td><td>0.75</td><td>0.70</td><td>0.58</td></tr><tr><td>BPC</td><td>BPC = 2.62 + 0.22SBPC<sup>a</sup> + 0.27DT<sup>a</sup> - 0.39FM<sup>a</sup> + 0.34SBPC*DT<sup>a</sup> - 0.39SBPC*FM<sup>a</sup> - 0.50DT*FM + 0.37SBPC<sup>2</sup> - 0.20DT<sup>2</sup> + 0.24FM<sup>2</sup></td><td>&lt;0.0001</td><td>0.09</td><td>0.97</td><td>0.94</td><td>0.79</td></tr></tbody></table>

SBPC: Strawberry by-products content; DT: Die temperature; FM: Feed Moisture; EI: Expansion index; BD: Bulk density; H: Hardness; FCr: Crushing force; NSr: Spatial frequency of ruptures; WCr: Crispness work; L*: Luminosity; a*: redness; b*: yellowness; C*: Chroma; h: Hue; CI: Color index; SME: Specific mechanical energy; FPC: Free phenolic compounds; BPC: Bound phenolic compounds. a Statistically significant at p < 0.05.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Response surface curve showing the effect of extrusion process variables on specific mechanical energy (SME): (a) FM vs SBPC and (b) FM vs DT. SBPC: Strawberry by-product content; DT: Die temperature; FM: Feed moisture.</figcaption></figure>

EI was significantly negatively influenced by all factors (Table 3), particularly by feed moisture (FM) and the inclusion level of strawberry by-products (SBPC). Additionally, the quadratic effects of FM and temperature (DT) impacted its behavior (p < 0.05) (Table 3). BD was also affected by FM and DT, their interactions (p < 0.05), and observable effects from all quadratic terms (Table 3).

Feed moisture had a negative effect on EI and a positive effect on BD, hence as FM increased, EI decreased and BD increased, indicating that great expansion was achieved with low feed moisture levels (Fig. 2a and b). The inverse behavior observed between BD and EI was expected as it was shown that a reduction in expansion is inversely proportional to the measurement of bulk density (Wójtowicz et al., 2019). Similar effects of feed moisture were previously reported on snacks enriched with fruit materials (Falfán Cortés et al., 2014; Rathod & Annapure, 2017). This

behavior could be due to the water plasticizer role in the starch granule amorphous regions, where it directly participated in gelatinization and contributed to the rheological properties of the melt and the formation of gas bubbles (Oliveira et al., 2017). As a consequence, dough viscosity and resistance to flow were reduced which led to gas retention reduction and less expansion (Natabirwa et al., 2018; Sahu et al., 2022).

The die temperature had a positive effect on expansion up to 180 °C, beyond which expansion decreased (Fig. 2a) in accordance with Rathod and Annapure (2016, 2017). Expansion depends on starch gelatinization, and die temperature is a critical factor influencing starch modification during extrusion (Rathod & Annapure, 2016, 2017). However, excessive starch modification led to expansion decreased, which may explain the decline in expansion beyond 180 °C.

Increasing strawberry by-products contents reduced expansion index
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Response surface curve showing the effect of extrusion process variables on (a) expansion index (EI) and (b) bulk density (BD). DT: Die temperature; FM: Feed moisture.</figcaption></figure>

as previously reported for extrudates with addition of fruits such as chokeberry, elderberry and strawberry (Wójtowicz et al., 2019) or products such as apple pomace (Ačkar et al., 2018), passion fruit shell (Alves et al., 2018) and mango peel (Mohamad Mazlan et al., 2020). Hence, the behavior observed could be due to dietary fiber introduced by strawberry by-products as expansion of extruded products is a complex process that occurs under specific conditions of high temperature and low feed moisture and which is driven by biopolymer structural modification due to shear forces and pressure, phase transitions, and formation of air bubbles (Falfán Cortés et al., 2014; Neder-Suárez et al., 2024). Hence fiber from strawberry by-products bound water more effectively than starch, leading to less water loss at the die temperature and reduced expansion capacity. Additionally, the fiber structures may interfere with the formation and stability of the gas cells during

expansion, limiting bubble growth (Ačkar et al., 2018; Bisharat et al., 2013). Moreover, fruit material inclusion could act as a lubricant, reducing friction inside the extruder and lowering shear forces which could decreased extrusion pressure and limited expansion (Ménabréaz et al., 2021).

Expansion index (EI) and bulk density (BD) are key parameters in extruded snacks, with a high EI being a main physical quality indicator for these products (Neder-Suárez et al., 2024).

### 3.3. Effect of extrusion conditions on textural properties

The values of the textural properties ranged from 22 to 134 N for H, 13–59.5 N for FCr, 9.9 to 23.3 mm⁻¹ for NSr and 0.6–5.5 N mm for WCr (Table 2). All individual factors had a significant effect on textural

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Response surface curve showing the effect of extrusion process variables on textural properties: (a) hardness (H), (b) crushing force (FCr), (c) fracture work (WCr) and (d) spatial frequency of ruptures (NSr). DT: Die temperature; FM: Feed moisture.</figcaption></figure>
properties ($p < 0.05$) (Table 3).

For hardness (H), DT and FM were the factors with the greatest influence as well as their interactions and all quadratic individual factors ($p < 0.05$) (Table 3; Fig. 3 a). The H values were close to those reported by Wójtowicz et al. (2019) with values between 44 and 105 N depending on strawberry inclusion level, with high values at high inclusion levels. Similarly, Gumul et al. (2023) reported high hardness values with increased cherry and chokeberry pomace contents. Moreover, the highest H values were obtained with combinations of high FM and DT levels (Fig. 3 a), which aligned with Sahu (2020) results for maize-millet extrudates fortified with soy. In the same way, Falfán Cortés et al. (2014) observed a similar effect of moisture increased without significant effect of temperature. Medina-Rendon et al. (2023) also observed a similar effect of temperature in extrudates enriched with mango by-products, but they reported an inverse effect for moisture which could be due to the moisture range evaluated (17–21 %) lower than in the present study. The H behavior may be due to the expansion degree of the extrudates, as a strong negative correlation was found between both variables ($r = -0,821$). Increased expansion resulted in low fracture resistance, which was expected because of a low compaction of extrudates (Uribe-Wandurraga et al., 2020) and to long but thin cell walls, making them easier to crush under compression (Kesre & Masatcioglu, 2022). Increasing feed moisture at each die temperature led to an increase of H levels. This may be attributed to the plasticizing effect of water on the starch granule amorphous region which could reduce viscosity and the capacity for mechanical energy dissipation in the extruder. Consequently, the final product became denser and harder with a cell growth restriction (Kesre & Masatcioglu, 2022; Natabirwa et al., 2018; Sahu et al., 2022).

FCr and WCr results showed high positive correlations with H ones ($r = 0.99$ in both cases). FCr was significatively affected by all factors (lineal and quadratic) and their interactions ($p < 0.05$), while WCr was mainly influenced by FM and DT and to a lesser extent by SBPC, and a negligible influence was observed for SBPC and DT interactions ($p < 0.05$) (Table 3). FCr and WCr highest values were observed with a combination of high levels of DT and FM (Fig. 3b and c). A similar behavior of WCr was also observed by Wójtowicz et al. (2019). As for H, a high and inverse association was observed with the extrudate expansion for both FCr and WCr ($r = -0.86$ and $r = -0.79$, respectively) which was also observed in extruded snacks by Karkle et al. (2012) and Ačkar et al. (2018) which may be related to the changes occurring during the extrusion explained above.

Regarding NSr, a negative association was observed with H ($r = -0.78$) and no significant effect was found for SBPC, DT and $DT^2$ or for interactions between SBPC and DT or FM (Table 2). However, the interaction between DT and FM was highly significant. NSr reached its highest values under a combination of low feed moisture and high die temperature, whereas the lowest values were observed when both variables were at low levels or both at high levels (Fig. 3 d). Wójtowicz et al. (2019) showed little or no effect of dried strawberry inclusion level on the crispness of extruded snacks. In contrast, Karkle et al. (2012) observed a significant effect of apple pomace content in extruded snacks, but as in this study, an increase in NSr with a moisture decrease, indicating a more fragile porous structure and greater crispness (Renoldi et al., 2021). NSr was directly associated with the expansion index. In fact, a low but positive correlation was observed between these parameters ($r = 0.56$). Longer walls, caused by a high expansion in the direction of compression, combined with high variation in individual wall lengths, increased the sensitivity to applied force and resulted in a high number of sequential fractures as the weakest cells buckled first. This allowed for the detection of individual fractures, leading to more distinct crushing peaks over the same probe travel distance when the moisture decreased (Karkle et al., 2012). With respect to the temperature factor, unlike this study, other authors found a significant increase of crispness with a rise in die or barrel temperature, though the temperature range was lower than in this study (110–150 °C) (Kesre & Masatcioglu, 2022; Sahu, 2020; Zambrano-Moreno et al., 2015). Hence,

crushing, hardness, and crispness are key parameters in determining the quality and consumer acceptance of expanded extruded snacks as they significantly influence the sensory experience and overall satisfaction of the product (Yadav et al., 2018).

### 3.4. Effect of extrusion conditions on color

The snack visual appearance plays an important role in determining consumer acceptance. $L^*$ values ranged from 50.2 to 59.2, $a^*$ values from 2.07 to 3.82 and $b^*$ values from 5.2 to 7.2 (Table 2). The color parameters ($L^*, a^*, b^*$) were significantly influenced by both SBPC and FM, while DT only affected the $b^*$ values. Moreover, interactions and quadratic terms affected the color profile in different ways ($p < 0.05$) (Table 3; Fig. 4 a, b, c).

The behavior of lightness $L^*$ was correlated with expansion results and a strong correlation between these two parameters was observed ($r = 0.95$). As previously discussed, low moisture levels resulted in more expanded extrudates, likely due to the formation of less compact cells and a greater number of bubbles during extrusion process. This structural change may affect porosity and consequently the degree of light reflection, i.e., $L^*$ values (Chan et al., 2019).

As expected, $a^*$ and $b^*$ values increased with high levels of by-product content due to the presence of pigments in strawberries as previously observed by Wójtowicz et al. (2017). Additionally, extrudates with high levels of by-products appeared darker. This phenomenon could be due to the progressive darkening induced by increasing amounts of by-products (Aussanasuwannakul et al., 2022), but also to chemical reactions such as pigment degradation, hydrolysis, oxidation, phenolic compound degradation and non-enzymatic browning processes such as sugar caramelization and the Maillard reaction, that occurred during extrusion cooking and that may affect color (Aussanasuwannakul et al., 2022; Ačkar et al., 2018; Falfán Cortés et al., 2014). The same trends were also observed for extruded snacks enriched with strawberry, chokeberry, cherry and blackcurrant by-products (Wójtowicz et al. (2017); Gumul et al. (2023)).

Moreover, high feed moisture led to a decrease in lightness ($L^*$), redness ($a^*$) and yellowness ($b^*$) values. High moisture levels are typically associated with increased gelatinization and reduced energy dissipation within the system due to low viscosity. Consequently, the energy requires to process the melt in the screw channel come mainly from thermal input from the barrel walls rather than from molecular friction which potentially intensifying thermochemical changes (Akdogan, 1999; Chiodetti et al., 2024). These changes may affect pigments such as flavonoids associated with both reddish and yellowish hues (Lu et al., 2021), and especially anthocyanins linked to redness (Neder-Suárez et al., 2021).

$C^*$ and $h$ values were in the range of 5.20–7.81 and 60.51–67.84° respectively (Table 2). Extrudates were shifting away from the red color characteristic of strawberries, showing a greater tendency towards orange or yellow hues ($h$ values), particularly at high moisture and temperature levels, but with low intensity ($C^*$ values) (Fig. 4a and b). In contrast, Neder-Suárez et al. (2021) reported high $h$ and $*C$ values at high temperature (142 °C) but under low moisture conditions (22 %). Respect to CI values, which ranged from 8.25 to 10.15 (Table 2, Fig. 4c), indicating desaturated yellow and orange hues with low intensity (Vignoni et al., 2006). Téllez-Morales et al. (2022) reported similar values and trends for corn flour-based extrudates containing other types of biological material. The color parameters Chroma ($C^*$) and lightness ($L^*$) were significantly influenced by both SBPC and FM, while DT affected the hue angle ($h$) and color index (CI). Additionally, $h$ was also influenced by FM, and CI significantly by SBPC ($p < 0.05$) (Table 3). As previously said, these color parameters modification could be associated with chemical reactions that occurred during extrusion cooking and pigment degradation (Aussanasuwannakul et al., 2022; Ačkar et al., 2018; Falfán Cortés et al., 2014), especially anthocyanins responsible for red hues (Neder-Suárez et al., 2021).
<figure><img src="image_6.png" /><figcaption>Fig. 4. Response surface curve showing the effect of extrusion process variables on color parameters: (a) Chroma (C*), (b) Hue (h) and (c) color index (CI). DT: Die temperature; FM: Feed moisture.</figcaption></figure>

## 3.5. Effect of extrusion on phenolic compounds

### 3.5.1. Free and bound phenolic compounds contents

Free Phenolic Compound (FPC) contents ranged from 1.16 mg/g DM to 0.42 mg/g DM and Bound Phenolic Compound (BPC) contents ranged from 5.02 mg/g DM to 1.76 mg/g DM (Table 2). As expected, high SBPC led to high phenolic compound contents. FPC were significantly affected by SBPC and FM, while DT had no significant effect in the range of the study (Table 3) whereas BPC were significantly influenced by all factors, including interactions and quadratic factors (Table 3).

Natabirwa et al. (2018) and Medina-Rendon et al. (2023) did not find any effect of temperature on the free phenolic compound contents. However, in some cases, depending on the temperature range evaluated, an increase in free phenolic compounds was observed with the rise of temperature (Bisharat et al., 2015; Leyva-Corral et al., 2016; Neder-Suárez et al., 2021), which could be associated to the release of phenolic compounds bound to the cell walls. Moreover, the increase of temperature could also lead to phenolic compound thermal degradation (Lohani & Muthukumarappan, 2017a; 2017b; Ortiz-Cruz et al., 2020). FPC decreased as the feed moisture content rise (Fig. 5 a) as previously observed (Natabirwa et al. (2018) whereas BPC had an inverse trend (Fig. 5b). Bisharat et al. (2015), Leyva-Corral et al. (2016) and Natabirwa et al. (2018) also showed a significant drop in the phenolic contents in extrudates with an increase in feed moisture from 14 to 19 %, 25–28 % and 15–20 % respectively. BPC compounds exhibited a behavior depending on the combination of temperature and moisture. This phenomenon could be due to chemical reactions of phenolic compounds promoted by water excess leading to the formation of process-induced compounds. These compounds may further bound to

the cell walls reducing their extractability and leading to an increase of bound phenolic compound contents. (Brennan et al., 2011; Lohani & Muthukumarappan, 2017; Natabirwa et al., 2018).

### 3.5.2. Quantification of phenolic compounds

Proanthocyanidins (PAC) were the main phenolic compounds, followed by flavonols (mainly quercetin and kaempferol glycosides), monomeric flavan-3-ols, phenolic acids, and finally anthocyanins (Table 4). The phenolic compound profile of strawberry pomace was different as the fruit one where anthocyanins were the second phenolic classes (Rincon et al., 2025). Pomace resulted from juice production which led to anthocyanin solubilization in the juice and degradation during processing as they are highly sensitive molecules (Patras et al., 2010; Z. Wang et al., 2024). PAC were composed of (+)-catechin and (−)-epicatechin both as terminal and extension units, and epiafzelechin which was only found as extension units.

The extrusion process significantly influenced the phenolic profile of the extrudates and the proanthocyanidin structure (Tables 4–6). The highest levels of all compounds were achieved at a die temperature of 180 °C, 20 g/100g moisture content, and 25 g/100g DM by-product.

Processing conditions affected both content and structural characteristics of proanthocyanidins (Tables 4–6). PAC contents increased significantly with both the inclusion level of strawberry by-products, as expected, and moisture levels whereas temperature had no effect. However, a significant interaction between moisture and temperature was observed. High PAC content were determined at high moisture levels, consistent with findings reported by Hirth et al. (2015) for proanthocyanidins contents in extrudates containing chokeberry extract. This could be attributed to the reduced specific mechanical

<figure><img src="image_7.png" /><figcaption>Fig. 5. Response surface curve showing the effect of extrusion process variables on phenolic compound contents: (a) Free (FPC) and (b) bound (BPC) phenolic compounds. SBPC: Strawberry by-product content; DT: Die temperature; FM: Feed moisture.</figcaption></figure>
<table><caption>**Table 4**

Phenolic compound contents (ug/g dry matter.) in extrudates enriched with strawberry by-products depending on processing conditions.</caption><thead><tr><th>Run</th><th>(+)-Cat</th><th>(-)-Epi</th><th>PACS</th><th>p-C-HexA</th><th>Q</th><th>Q-3-GlcA</th><th>K-3-GlcA</th><th>K-3-MalGlc</th><th>Pg-3-Glc</th></tr></thead><tbody><tr><td>1</td><td>2.3 ± 0.1 ab</td><td>1.8 ± 0.1a</td><td>109.0 ± 6.5<sup>b</sup></td><td>0.6 ± 0.0<sup>ac</sup></td><td>0.2 ± 0.0<sup>a</sup></td><td>3.9 ± 0.2<sup>a</sup></td><td>0.8 ± 0.0<sup>a</sup></td><td>ND</td><td>0.4 ± 0.0<sup>a</sup></td></tr><tr><td>2</td><td>8.3 ± 0.5c</td><td>3.6 ± 0.3b</td><td>168.5 ± 8.1<sup>b</sup></td><td>2.2 ± 0.2<sup>b</sup></td><td>0.6 ± 0.0<sup>d</sup></td><td>12.8 ± 0.7<sup>c</sup></td><td>4.4 ± 0.4<sup>b</sup></td><td>0.1 ± 0.0<sup>a</sup></td><td>1.4 ± 0.1<sup>b</sup></td></tr><tr><td>3</td><td>3.2 ± 0.2de</td><td>0.7 ± 0.0c</td><td>126.8 ± 1.4<sup>c</sup></td><td>0.6 ± 0.1<sup>c</sup></td><td>0.2 ± 0.0<sup>a</sup></td><td>5.4 ± 0.4<sup>d</sup></td><td>1.0 ± 0.1<sup>ac</sup></td><td>ND</td><td>ND</td></tr><tr><td>4</td><td>2.9 ± 0.1de</td><td>2.2 ± 0.2a</td><td>162.4 ± 5.7<sup>b</sup></td><td>0.6 ± 0.0<sup>ac</sup></td><td>0.3 ± 0.0<sup>b</sup></td><td>4.8 ± 0.1<sup>d</sup></td><td>1.3 ± 0.1<sup>cd</sup></td><td>ND</td><td>ND</td></tr><tr><td>5</td><td>2.7 ± 0.1bd</td><td>2.8 ± 0.1d</td><td>158.2 ± 6.1<sup>bd</sup></td><td>0.4 ± 0.0<sup>a</sup></td><td>0.2 ± 0.0<sup>a</sup></td><td>2.9 ± 0.1<sup>e</sup></td><td>1.0 ± 0.1<sup>ac</sup></td><td>ND</td><td>ND</td></tr><tr><td>6</td><td>3.5 ± 0.1e</td><td>5.4 ± 0.3e</td><td>251.4 ± 3.9<sup>e</sup></td><td>1.3 ± 0.1<sup>d</sup></td><td>0.5 ± 0.0<sup>c</sup></td><td>7.6 ± 0.4<sup>f</sup></td><td>2.7 ± 0.1<sup>e</sup></td><td>0.1 ± 0.0<sup>b</sup></td><td>0.8 ± 0.0<sup>c</sup></td></tr><tr><td>7</td><td>2.0 ± 0.1a</td><td>2.2 ± 0.1a</td><td>160.4 ± 2.5<sup>bd</sup></td><td>0.7 ± 0.0<sup>c</sup></td><td>0.3 ± 0.0<sup>b</sup></td><td>3.8 ± 0.2<sup>ae</sup></td><td>1.5 ± 0.1<sup>d</sup></td><td>ND</td><td>ND</td></tr><tr><td>8</td><td>2.1 ± 0.2 ab</td><td>4.2 ± 0.3f</td><td>222.7 ± 7.6<sup>f</sup></td><td>1.4 ± 0.1<sup>d</sup></td><td>0.5 ± 0.0<sup>c</sup></td><td>7.9 ± 0.4<sup>f</sup></td><td>3.1 ± 0.2<sup>f</sup></td><td>ND</td><td>ND</td></tr><tr><td>Central point</td><td>1.7 ± 0.2a</td><td>1.0 ± 0.1c</td><td>146.2 ± 4.4<sup>d</sup></td><td>1.0 ± 0.1<sup>e</sup></td><td>0.2 ± 0.0<sup>a</sup></td><td>3.3 ± 0.2<sup>ae</sup></td><td>1.0 ± 0.1<sup>ac</sup></td><td>ND</td><td>ND</td></tr></tbody></table>

(+)-Cat: (+)-Catechin; (-)-Epi: (-)-Epitechin. PACs: Proanthocyanidins; p-C-HexA: p-coumaroylhexoside acid; Q: Quercetin; Q-3-GlcA: Quercetin-3-O-glucuronide; K-3-GlcA: Kaempferol-3-O-glucuronide; K-3-MalGlc: Kaempferol-3-O-malonylglucoside; Pg-3-Glc: Pelargonidin-3-O-glucoside. ND: Not detected. Different letters in the same column indicate significant differences (p < 0.05) by Tukey test.

<table><caption>**Table 5**

Mean degree of polymerization (mDP) and proportion of constitutive units (%) of proanthocyanidins in extrudates containing strawberry by-products depending on processing conditions.</caption><thead><tr><th>Run</th><th>mDP</th><th>% (+)-Cat T</th><th>% (-)-Epi T</th><th>% (+)-Cat E</th><th>% (-)-Epi E</th><th>% Epiafz E</th></tr></thead><tbody><tr><td>1</td><td>7.6 ± 0.5<sup>ab</sup></td><td>10.3 ± 0.6<sup>a</sup></td><td>2.9 ± 0.2<sup>a</sup></td><td>31.7 ± 0.3<sup>a</sup></td><td>49.2 ± 0.5<sup>a</sup></td><td>6.0 ± 0.4<sup>ab</sup></td></tr><tr><td>2</td><td>6.2 ± 0.2<sup>c</sup></td><td>14.2 ± 0.5<sup>b</sup></td><td>2.0 ± 0.1<sup>b</sup></td><td>40.1 ± 1.0<sup>b</sup></td><td>38.5 ± 0.8<sup>bc</sup></td><td>5.2 ± 0.3<sup>c</sup></td></tr><tr><td>3</td><td>8.3 ± 0.3<sup>a</sup></td><td>6.8 ± 0.4<sup>c</sup></td><td>5.3 ± 0.1<sup>c</sup></td><td>30.6 ± 1.8<sup>a</sup></td><td>52.5 ± 2.3<sup>a</sup></td><td>4.9 ± 0.3<sup>cd</sup></td></tr><tr><td>4</td><td>5.6 ± 0.2<sup>cd</sup></td><td>13.4 ± 0.2<sup>bd</sup></td><td>4.5 ± 0.4<sup>d</sup></td><td>38.0 ± 0.8<sup>b</sup></td><td>41.3 ± 0.4<sup>b</sup></td><td>2.8 ± 0.1<sup>e</sup></td></tr><tr><td>5</td><td>6.3 ± 0.4<sup>ee</sup></td><td>12.2 ± 0.9<sup>d</sup></td><td>3.8 ± 0.3<sup>e</sup></td><td>37.5 ± 0.6<sup>b</sup></td><td>41.1 ± 0.6<sup>b</sup></td><td>5.5 ± 0.2<sup>bc</sup></td></tr><tr><td>6</td><td>5.2 ± 0.1<sup>d</sup></td><td>18.0 ± 0.2<sup>e</sup></td><td>1.3 ± 0.1<sup>f</sup></td><td>39.8 ± 1.8<sup>b</sup></td><td>36.0 ± 2.1<sup>c</sup></td><td>4.9 ± 0.2<sup>cd</sup></td></tr><tr><td>7</td><td>5.9 ± 0.2<sup>cd</sup></td><td>13.0 ± 0.4<sup>bd</sup></td><td>4.1 ± 0.2<sup>de</sup></td><td>37.7 ± 1.9<sup>b</sup></td><td>40.0 ± 1.8<sup>b</sup></td><td>5.3 ± 0.3<sup>bc</sup></td></tr><tr><td>8</td><td>5.3 ± 0.2<sup>d</sup></td><td>16.3 ± 0.5<sup>f</sup></td><td>2.6 ± 0.2<sup>ab</sup></td><td>38.8 ± 0.7<sup>b</sup></td><td>38.0 ± 0.9<sup>bc</sup></td><td>4.3 ± 0.2<sup>d</sup></td></tr><tr><td>Central point</td><td>7.0 ± 0.2<sup>be</sup></td><td>12.2 ± 0.3<sup>d</sup></td><td>2.1 ± 0.2<sup>b</sup></td><td>38.6 ± 0.2<sup>b</sup></td><td>41.0 ± 0.8<sup>b</sup></td><td>6.3 ± 0.3<sup>b</sup></td></tr></tbody></table>

mDP: proanthocyanidin mean degree of polymerization; % (+)-Cat T: Percentage of (+)-catechin as terminal unit; % (-)-Epi T: Percentage of (-)-epicatechin as terminal unit; % (+)-Cat E: Percentage of catechin as extension unit; % (-)-Epi E: Percentage of (-)-epicatechin as extension unit; % Epiafz E: Percentage of epiafzelechin as extension unit. Different letters in the same column indicate significant differences (p < 0.05) by Tukey test.

<table><caption>**Table 6**

Analysis of variance (ANOVA) on the individual phenolic compounds and proanthocyanidin characteristics (mDP and constitutive units) depending on processing conditions.</caption><thead><tr><th rowspan="2">Responses</th><th colspan="6">p-value by source</th></tr><tr><th>SBPC</th><th>DT</th><th>FM</th><th>SBPC x FM</th><th>SBPC x DT</th><th>FM x DT</th></tr></thead><tbody><tr><td>p-coumaroylhexoside acid</td><td>0.0007*</td><td>0.3903</td><td>0.2576</td><td>0.0014*</td><td>0.0025*</td><td>&lt;0.0001*</td></tr><tr><td>Quercetin</td><td>0.0007*</td><td>0.0211*</td><td>0.2003</td><td>0.0037*</td><td>0.0002*</td><td>&lt;0.0001*</td></tr><tr><td>Quercetin-3-O-glucuronide</td><td>0.0058*</td><td>0.2348</td><td>0.0549</td><td>0.0035*</td><td>0.0097*</td><td>&lt;0.0001*</td></tr><tr><td>Kaempferol-3-O-glucuronide</td><td>0.0004*</td><td>0.3456</td><td>0.8119</td><td>0.0095*</td><td>0.0031*</td><td>&lt;0.0001*</td></tr><tr><td>Kaempferol-3-O-malonylglucoside</td><td>0.0218*</td><td>0.0218*</td><td>0.0218*</td><td>&lt;0.0001*</td><td>&lt;0.0001*</td><td>&lt;0.0001*</td></tr><tr><td>Pelargonidin-3-O-glucoside</td><td>0.4942</td><td>&lt;0.0001*</td><td>&lt;0.0001*</td><td>&lt;0.0001*</td><td>&lt;0.0001*</td><td>&lt;0.0001*</td></tr><tr><td>(+)-Catechin</td><td>0.0004*</td><td>0.0005*</td><td>0.0007*</td><td>0.0003*</td><td>0.006*</td><td>0.1617</td></tr><tr><td>(-)-Epitechin</td><td>&lt;0.0001*</td><td>0.001*</td><td>&lt;0.0001*</td><td>0.4976</td><td>0.2839</td><td>0.4869</td></tr><tr><td>PACS</td><td>&lt;0.0001*</td><td>0.3909</td><td>&lt;0.0001*</td><td>0.0037*</td><td>0.0017*</td><td>0.033*</td></tr><tr><td>mDP</td><td>&lt;0.0001*</td><td>0.8096</td><td>&lt;0.0001*</td><td>0.317</td><td>0.0039*</td><td>0.5729</td></tr><tr><td>% (+)-Cat T</td><td>&lt;0.0001*</td><td>0.0024*</td><td>&lt;0.0001*</td><td>0.8555</td><td>0.3974</td><td>0.036*</td></tr><tr><td>% (-)-Epi T</td><td>&lt;0.0001*</td><td>&lt;0.0001*</td><td>0.0016*</td><td>0.2151</td><td>0.0136*</td><td>0.0009*</td></tr><tr><td>% (+)-Cat E</td><td>&lt;0.0001*</td><td>0.0717</td><td>&lt;0.0001*</td><td>0.2956</td><td>&lt;0.0001*</td><td>0.2747</td></tr><tr><td>% (-)-Epi E</td><td>&lt;0.0001*</td><td>0.006*</td><td>&lt;0.0001*</td><td>0.2771</td><td>&lt;0.0001*</td><td>0.0337*</td></tr><tr><td>% Epiafz E</td><td>0.0001*</td><td>0.0002*</td><td>0.2386</td><td>0.1014</td><td>0.1636</td><td>0.0093*</td></tr></tbody></table>

SBPC: Strawberry by-products content; FM: Feed moisture; DT: Die temperature; PACs: Proanthocyanidins; mDP: Proanthocyanidin mean degree of polymerization, % (+)-Cat T: Percentage of (+)-catechin as terminal unit; % (-)-Epi T: Percentage of (-)-epicatechin as terminal unit; % (+)-Cat E: Percentage of catechin as extension unit; % (-)-Epi E: Percentage of (-)-epicatechin as extension unit; % Epiafz E: Percentage of epiafzelechin as extension unit. *Significant at 95 %.

energy (SME) associated with high water content during processing. Reduced SME may minimize shearing and thermal stress, thereby preventing compound degradation (Lohani & Muthukumarappan, 2017). Although high moisture levels favored PAC retention, they also led to the decrease of their mean degree of polymerization (mDP), i.e., number of constitutive units. The mDP was influenced by SBPC and FM, with a strong negative correlation observed between these variables (r = -0.806), indicating that increasing SBPC and FM levels were associated with low mDP values. In contrast, DT did not significantly affect mDP in this study, unlike Khanal et al. (2009), who reported that high extrusion

temperatures led to a decrease of procyanidins' mDP in blueberry pomace, resulting in increased monomer, dimer, and trimer contents. In the present work, the lowest mDP values were observed at the highest levels of FM and SBPC, with no consistent variation due to temperature. High FM led to high water content which may enhance the solvation effect and increase water polarity, facilitating ionization processes and the formation of new compounds (Liu, Le Bourvellec et al., 2021; Loncarić et al., 2018) or disruption of PAC interflavanic bond (Liang et al., 2024) leading to low mDP. Such reactions may also involve the release and/or rearrangement of monomers during thermal processing,
as proposed by previous studies (De Taeye et al., 2014; La Mantia et al., 2023). Both (+)-catechin (as a terminal unit) and (−)-epicatechin (as an extension unit) were influenced by all the factors studied, whereas epiafzelechin (as a terminal unit) was affected by SBPC and DT, but not by FM. However, significant interactions between FM and DT influenced epiafzelechin content. mDP was positively associated with the percentage of (−)-epicatechin as extension unit (r = 0.89) and negatively associated with (+)-catechin as extension unit (r = −0.79). This could be associated with a higher (−)-epicatechin thermal stability than (+)-catechin (Ananingsih et al., 2013; Lončarić et al., 2018), which may contribute to its high prevalence in polymerized structures under extrusion conditions.

The extrusion process also affected flavan-3-ol monomers. (+)-Catechin was significantly influenced by all factors and their interactions, except for the interaction between DT and FM. Similarly, (−)-epicatechin was affected by all factors. Temperature led to a decrease in their content. In contrast, as the moisture level increased, (+)-catechin content decreased, whereas (−)-epicatechin content increased. Several studies previously reported changes in their ratio during food processing (Fernández-Romero et al., 2020; Hurst et al., 2011; Kofink et al., 2007; Lončarić et al., 2018; Valverde et al., 2020). These changes could be due to thermal degradation but also to proanthocyanidins degradation in hot acidic conditions (Le Bourvellec et al., 2013), as well as changes in their extractability (Cao et al., 2021), and epimerization reactions that occur at high temperatures converting (+)-catechin into (−)-epicatechin (Ananingsih et al., 2013).

Among the flavonol group, quercetin was affected by SBPC, DT, and interactions, particularly between DT and FM, reaching a maximum value of 0.6 µg/g DM. For quercetin-3-O-glucuronide, the maximum content was 12.8 µg/g DM. Similar factors influenced this compound, except for the linear term of DT, which did not show a significant effect. The temperature effect depends on molecular structure and conjugated forms, such as quercetin-3-O-glucuronide, may exhibit greater thermal stability due the conjugated group (Gopalakrishna et al., 2023; Lin et al., 2022; Wang et al., 2016) than aglycone form. A similar structural differentiation was observed between kaempferol-3-O-glucuronide and kaempferol-3-O-malonylglucoside. The former was influenced by SBPC and all interactions between factors, while the latter was affected by all terms (linear and interactions). Kaempferol-3-O-malonylglucoside was detected only in the treatments with high strawberry by-product contents and low temperature. The instability of malonyl esters of kaempferol has been previously reported and was attributed to demalonation reactions due to the low stability of the malonyl group. Hence, during processing of vegetables, flavonoid malonylglucosides may lose carbon dioxide, potentially leading to the formation of corresponding flavonoid acetylglycosides (Dupont et al., 2000).

Anthocyanins were strongly affected by the extrusion process. After extrusion pelargonidin-3-O-glucoside was only detected in three treatments, all of them at low temperature. SBPC levels did not significantly influence anthocyanin content, probably due to the high effect of extrusion process conditions. Neder-Suárez et al. (2021) reported a decrease in anthocyanin contents, including cyanidin-3-O-glucoside and pelargonidin-3-O-glucoside, in extruded snacks due to temperature and screw speed. Similarly, Hirth et al. (2015) show a reduction in cyanidin glycosides and total anthocyanin levels after extrusion of a mixture between starch and chokeberry extract. Anthocyanins are polyhydroxy and polymethoxy derivatives of 2-phenylbenzofuran cations with inherently unstable structures, making them particularly susceptible to temperature (Patras et al., 2010; Wang et al., 2024), but also to non-thermal factors such as water, light, oxygen and pH (Li et al., 2021). Their degradation occurs through oxidation and cleavage of covalent bonds, leading to process-induced products which depend on the intensity and nature of the heating (Cunha et al., 2023; Enaru et al., 2021).

### 3.6. Multi-response optimization and validation

The process optimization was performed using an experimental design approach based on the Response Surface Methodology (RSM). This method enabled the modeling and analysis of the influence of processing parameters on the evaluated variable, yielding adequate $R^2$ values and model fit levels (Table 7), which indicate validity and high predictive capacity.

For multi-response optimization, the desirability function was employed, which combines multiple responses into a single weighted global index. In this study, the maximization of FPC, BPC, and EI was prioritized, with varying degrees of levels assigned. Simultaneously, the minimization of WCr and BD was targeted. Similar to other studies (Lohani & Muthukumarappan, 2017; Medina-Rendon et al., 2023; Natabirwa et al., 2018), some parameters were identified as critical attributes to obtain extrudates with suitable physicochemical characteristics.

The optimal extrusion conditions identified through the response surface methodology were DT of 192 °C, FM of 18.6 g/100 g DM, and SBPC of 16.5 g/100 g DM (Table 7). These conditions were selected to achieve a compromise between physical quality attributes and total phenolic content, rather than maximizing individual responses. Experimental validation confirmed the model's predictive accuracy ($p < 0.05$) across the studied response variables, supporting its suitability for describing process behavior within the experimental range evaluated.

## 4. Conclusions

Extrusion of tapioca starch enriched with strawberry by-products demonstrated that feed moisture (FM) and die temperature (DT) were the main factors influencing expansion index (EI) and bulk density (BD). Strawberry by-product content (SBPC) reduced expansion, likely due to its high fiber content. Hardness (H), fracture work (WCr), and crushing force (FCr), increased with increasing FM and DT. Crispness (NSr) increased under low moisture and high temperature conditions, highlighting the relevance of these parameters in designing porous and brittle structures. FM negatively affected both free (FPC) and bound (BPC) phenolic compounds. SBPC increased proanthocyanidin content, but high FM led to a degradation of their structure leading to proanthocyanidin with low mDP. Optimal conditions (DT: 192 °C, FM: 18.6 %, SBPC: 16.5 %) balance physicochemical quality (high EI, low BD) and phenolic compound retention (FPC: 0.67 mg/g; BPC: 3.52 mg/g). The identified parameters align with existing processes, ensuring scalability and reproducibility for valorizing agro-industrial by-products and offers a sustainable solution to transform by-products into functional snacks, reducing waste and adding value.

However, our dataset had some limitations. Only one strawberry by-product coming from juice production was tested whereas part of the production is processed into various products such as jam, puree and juice. Hence response of by-product to extrusion process will depend on its structure and composition mitigated by the processing pathway they came from. So, it is necessary to test other by-product types (jam, puree, etc ...) to broaden industrial applications. No analysis of polysaccharide content and composition before and after extrusion process was done. However, it will help to understand structural and nutritional changes. Moreover, only native phenolic compounds were quantified whereas during by-product production and extrusion process phenolic compounds may evolved leading to the formation process-induced molecules. They could influence snack's color, snack color's stability during storage, and health benefits.

Future studies should include:

- validation of consumer acceptability and product stability (see supplementary material S1),

- analyze of polysaccharide structural and nutritional changes
<table><caption>**Table 7**
Optimum extrusion process conditions and validation of responses.</caption><thead><tr><th></th><th>Importance</th><th>Target</th><th>Calculated value</th><th>Experimental value</th><th>Desirability</th><th>p-value (t-test)</th></tr></thead><tbody><tr><td colspan="7"><strong>Experimental factors</strong></td></tr><tr><td>DT (°C)</td><td>3</td><td>Optimum</td><td>192</td><td></td><td rowspan="3">0.732</td><td></td></tr><tr><td>FM (g/100g)</td><td>3</td><td>Optimum</td><td>18.6</td><td></td><td></td></tr><tr><td>SBPC (g/100 g dry matter)</td><td>3</td><td>Optimum</td><td>16.5</td><td></td><td></td></tr><tr><td colspan="7"><strong>Response variables</strong></td></tr><tr><td>SME (KJ/Kg)</td><td>3</td><td>Range</td><td>3584.84</td><td>3544.01 ± 429.37</td><td></td><td>0.88</td></tr><tr><td>EI</td><td>2</td><td>Maximize</td><td>2.22</td><td>2.15 ± 0.05</td><td></td><td>0.11</td></tr><tr><td>BD (g/cm3)</td><td>2</td><td>Minimize</td><td>25.05</td><td>25.03 ± 0.02</td><td></td><td>0.95</td></tr><tr><td>H (N)</td><td>3</td><td>Range</td><td>12.77</td><td>12.71 ± 2.82</td><td></td><td>0.99</td></tr><tr><td>FCr (N)</td><td>3</td><td>Range</td><td>17.54</td><td>19.86 ± 1.26</td><td></td><td>0.94</td></tr><tr><td>NSr (mm-1)</td><td>3</td><td>Range</td><td>0.73</td><td>0.7 ± 1.14</td><td></td><td>0.07</td></tr><tr><td>WCr (N.mm)</td><td>2</td><td>Minimize</td><td>0.22</td><td>0.25 ± 0.09</td><td></td><td>0.56</td></tr><tr><td>L*</td><td>3</td><td>Range</td><td>59.96</td><td>60.61 ± 0.23</td><td></td><td>0.98</td></tr><tr><td>a*</td><td>3</td><td>Range</td><td>3.28</td><td>3.23 ± 0.04</td><td></td><td>0.19</td></tr><tr><td>b*</td><td>3</td><td>Range</td><td>6.83</td><td>6.87 ± 0.04</td><td></td><td>0.20</td></tr><tr><td>C*</td><td>3</td><td>Range</td><td>7.51</td><td>7.51 ± 0.04</td><td></td><td>0.97</td></tr><tr><td>h (°)</td><td>3</td><td>Range</td><td>64.35</td><td>65.68 ± 0.18</td><td></td><td>0.09</td></tr><tr><td>CI</td><td>3</td><td>Range</td><td>7.98</td><td>7.76 ± 0.1</td><td></td><td>0.13</td></tr><tr><td>FPC (mg/g)</td><td>3</td><td>Maximize</td><td>0.67</td><td>0.67 ± 0.06</td><td></td><td>0.71</td></tr><tr><td>BPC (mg/g)</td><td>2</td><td>Maximize</td><td>3.67</td><td>3.52 ± 0.16</td><td></td><td>0.24</td></tr></tbody></table>

SBPC: Strawberry by-products content; DT: Die temperature; FM: Feed Moisture; EI: Expansion index; BD: Bulk density; H: Hardness; FCr: Crushing force; NSr: Spatial frequency of ruptures; WCr: Crispness work; L*: Luminosity; a*: redness; b*: yellowness; C*: Chroma; h: Hue; CI: Color index; SME: Specific mechanical energy; FPC: Free phenolic compounds; BPC: Bound phenolic compounds. Significance level of t-test for one sample (p > 0.05).

- investigate chemical modifications of phenolic compounds during extrusion, i.e., structure of process-induced compound, to optimize stability and functional properties.

- study bioaccessibility and/or bioavailability of phenolic compounds, beyond variations in their content in the snack depending on processing conditions, to assess their real health benefits

- for robust industrial integration, it is recommended to conduct pilot-scale trials to validate technical and economic feasibility and process sustainability.

### CRediT authorship contribution statement

**Jeimy Rocío Bonilla Méndez:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Carine Le Bourvellec:** Writing – review & editing, Supervision, Methodology, Funding acquisition, Formal analysis, Data curation.
**Jeffrey León Pulido:** Writing – review & editing, Supervision.

### Declaration of competing interest

The authors declare no conflict of interest.

### Acknowledgement

The authors acknowledge the International Agroindustrial Biotechnology Center of the University of Cauca for its valuable technical support and the provision of infrastructure and equipment throughout the development of this research. We also express our gratitude to INRAE for access to laboratory facilities and analytical equipment, as well as to its technical staff for their valuable assistance during the experimental and analytical phases of the study. This work was supported in part by the L'Oréal-UNESCO For Women in Science program (Colombia), whose financial contribution facilitated key research activities related to the present study.

The authors warmly thank Marielle Bogé, Caroline Garcia and Claire Demenitroux for their precious technical help.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.lwt.2025.118752.

### Data availability

Data will be made available on request.

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