# Wheat flour dry heat treatment (DHT): effect of time-temperature parameters on wheat flour functionalities in the case of sponge cake applications

Alejandra Velasquez, Eve-Anne Norwood, Delphine Queveau, Anthony Oge, Alain Le-Bail

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## To cite this version:

Alejandra Velasquez, Eve-Anne Norwood, Delphine Queveau, Anthony Oge, Alain Le-Bail. Wheat flour dry heat treatment (DHT): effect of time-temperature parameters on wheat flour functionalities in the case of sponge cake applications. Journal of Cereal Science, 2025, 125, pp.104257. 10.1016/j.jcs.2025.104257. hal-05253956v2

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Submitted on 13 Nov 2025

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# Wheat flour dry heat treatment (DHT): effect of time-temperature parameters on wheat flour functionalities in the case of sponge cake applications

Alejandra Velasquez, Eve-Anne Norwood *, Delphine Queveau, Anthony Oge, Alain Le-Bail **

Oniris, Nantes Université, CNRS, GEPEA, UMR 6144, F-44000, Nantes, France

## ARTICLE INFO

### Keywords:

Flour heat treatment

Starch functionalities

Pasting properties

Cake volume

## ABSTRACT

The application of a Dry Heat Treatment (DHT) on wheat flour is of interest regarding the clean label high ratio cakes production as it helps to maintain the cake structure without altering the final volume. This study aims at further exploring the DHT impact on the flour's functionalities such as the starch gelatinization enthalpy and the pasting properties using the Chopin technologies-MIXOLAB 2 equipment. This study also relates to the batter and cake properties including the batter viscosity and the cake volume and shape. The application of a DHT process at different time and temperature combinations – ranging from 108 to 151 °C and from 10 to 40 min – led to modifications on the protein quality and starch characteristics of the wheat flour. The batters made with DHT flours had higher viscosities than the control batter. Furthermore, the sponge cakes made with standard flour (control) and flours treated at 108 °C-25 min had similar specific volumes. However, the cake structure collapsed when the cake was made with the standard flour, as it did not when the cake was made of the heat treated flour. The enhancement of starch's gelatinization is suggested to be the main effect to take place during the DHT process leading to a higher viscosity, which may eventually alter the cake specific volume. The heat treated flour has shown enhanced functionalities that are suitable for a clean label high ratio sponge cake. Their application in different food matrices is still yet to be investigated.

## 1. Introduction

Wheat is one of the most grown crops worldwide, highly studied and well recognized for its functionalities. This is mainly attributed to its two main components: gluten and starch, needed for many well-known food products such as pasta, bread and cakes. Among the cake ingredient classification, two types of cakes can be distinguished: high and low ratio cakes (Godefroidt et al., 2019). The former one is characterised by a $\frac{\text{sugar}}{\text{flour}} > 1$ and the latter by a $\frac{\text{sugar}}{\text{flour}} \le 1$. The high ratio cake has a porous structure resulting from a highly aerated emulsion-based batter (high level of fat) or foam-based batter (low level of fat, no shortening, margarine or oil added). Typically, they are composed of wheat flour, eggs, sugar, leavening agent, and, in some cases, oil and/or surfactants (Godefroidt et al., 2019). The challenge regarding the production of a high ratio cake lies in obtaining a non-collapsing structure. The high amount of sucrose of these types of cakes competes for water regarding the flour, hindering the starch functionalities and disrupting the cake

structure setting (Catterall, 2000). Moreover, the challenges in producing a sponge cake, based on a foam batter recipe, with a homogenous structure and a desirable volume relies on the absence of fat in the matrix. In fact, an aqueous liquid foam is formed during the mixing step leading to an unstable system presenting coalescence, disproportionation and rise (Dickinson, 2010). To avoid instability of such a system, surface-active components are added. The latter are labelled as food additives and numerous of them have been pointed out to have a serious impact on human health (Sellem et al., 2023). The literature showed that functionalised starch granules could potentially be used as emulsifiers (Saari et al., 2016). This mechanism can be referred as the Pickering stabilisation, which is defined by the adsorption of solid particles at the droplets interface of a dispersed system (Dickinson, 2010). To improve the cake system, clean label solutions are currently being explored through the modification of wheat flour's functionalities by applying heat treatments (Hong et al., 2021; He et al., 2025) such as a dry heat treatment process (DHT). This process delivers a "clean label" ingredient considered as an alternative to food additives or to the

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: eve.anne.norwood@oniris-nantes.fr (E.-A. Norwood), alain.lebail@oniris-nantes.fr (A. Le-Bail).</footnote>
**Glossary**

<table><tr><td>a<sub>w</sub></td><td>Water activity</td></tr><tr><td>DHT</td><td>Dry Heat Treatment</td></tr><tr><td>μDSC</td><td>Micro differential scanning calorimetry</td></tr><tr><td>T<sub>0</sub></td><td>Onset Temperature</td></tr><tr><td>T<sub>p</sub></td><td>Peak temperature</td></tr><tr><td>T<sub>f</sub></td><td>Offset Temperature</td></tr><tr><td>wb</td><td>Wet basis</td></tr><tr><td>ΔE</td><td>Change in total colour</td></tr><tr><td>ρ</td><td>Density</td></tr><tr><td>d</td><td>diameter</td></tr><tr><td>h</td><td>height</td></tr></table>

chlorinated flour used in the EU until the year 2000. From this year, the chlorinated flour was banned due to health concerns (Catterall, 2000). The DHT can be performed in rotating drums, continuous screw conveyors, mechanically agitated beds via a heated surface, stirred expanded beds, microwave heating, and others (Nicholas et al., 1978). Depending on the authors, the DHT process consists in exposing the flour at a target temperature in the range of 100–150 °C for a specific period of time. This step will deliver the desired product characteristics (Chesterton et al., 2015; Neill et al., 2012). The most common operational parameters are 120–140 °C for 0–30 min (Keppler, 2017) and for lower temperature range treatments, around 100 °C, generally requires longer holding times. It is important to note that parameters settings must avoid dextrinization and off-taste (Doe and Russo, 1970). Doe and Russo (1970) reported that the holding time at 125 °C or less is not critical while at higher temperatures from 135 °C the exposure time should be as fast as possible to avoid the previous mentioned consequences. Neill et al. (2012) reported that the optimum time-temperature for a base flour heat treatment for the application of high ratio Madeira cake, was 120–130 °C for 30 min, including drying step, with an initial moisture content of about 12.5 % on a wet basis, in order to have finer texture and lower bulk density characteristics. Meza et al. (2011) finally reported that a heat treatment with temperature parameters set of 125–140 °C for 20 min could lead to desirable characteristics for a high ratio cake such as an improved foam stability. The DHT application will give a functionalised flour with enhanced properties. For example, Ozawa et al. (2009) observed, through the analysis of the Amylograph profile of dry-heated wheat flour at 120 °C for 120 min, an earlier onset time and higher peak viscosity than the control profile. This was thought to be a consequence of the gluten fraction modification. More recently, Hong et al. (2023) supported these results by showing that protein denaturation significantly affects the pasting viscosity of wheat flour. The flour may even present new properties such as an increase in the starch granule surface hydrophobicity (Nakamura et al., 2008). With the characteristics mentioned above, an increase in viscosity and a Pickering effect at the air bubble interface could be observed in the cake batter system. These would help to achieve a non-collapsing clean labelled cake through a better structure setting and an improvement of the batter stability. The mechanisms behind the functionalities improvement are still not well understood from a molecular point of view, but it is widely accepted that the modification of the starch properties leads to the desired cake structure (Keppler, 2017). The underlying mechanisms are thought to be linked to the rearrangement, interactions, and/or denaturation of proteins located at the surface of the starch granule (Keppler, 2017). It is well known that proteins are heat sensitive molecules. Thus, it is expected that proteins found in the flour (i.e. gluten, proteins located at the starch surface, etc.) would undergo conformational changes during the DHT (i.e. denaturation). However, the baking performances can be negatively affected if the protein denaturation rate is not well mastered during the DHT. This study aims at investigating the effect of a

conventional DHT on wheat flour functionalities as a function of time and temperature parameters through the baking performance of a high ratio sponge cake. The substitution rate chosen for this study was 100 % of the standard flour.

**2. Materials and methods**

**2.1. Materials**

A commercial wheat flour type 45 has been used for this study. The flour moisture content and a<sub>w</sub> were of 13.79 ± 0.12 % wet basis (wb) and 0.677 ± 0.081, respectively. The moisture content was determined by weighing 3 g of samples on aluminium cups before and after 14 h at 105 °C in a drying cabinet (Memmert-Germany), using an analytical balance (Scaltec, instruments Heiligenstadt, Germany). The a<sub>w</sub> was determined using a PreAquaLab equipment (Decagon devices Inc; USA). The flour was stored in the original paper bag (5 kg presentation) at –20 °C. Prior to use, the flour bag was kept for 2 days at ambient temperature inside a plastic bag to defreeze.

**2.2. Heat treatment processes**

The heat treatment was performed as a direct process. The heat treatment was carried out in a stirred-heated stainless steel cylinder, which was designed at Oniris (Fig. 1). It consisted in three outer layers 1) a silicon heating mat layer (thickness 1.5 mm) and two layers of cork insulation (thickness 3.8 mm/layer). The temperature of the cylinder was set and tracked by the temperature setting system. The system's internal temperature was monitored with two thermocouples located at the centre and at the bottom extreme of the steering blade. Both were used to record the sample's temperature. The temperature was tracked with an OMEGA temperature logger. The come-up time, defined as the time when the flour reached a constant temperature, was determined for the processes to ensure that the sample was exposed to a constant treatment temperature during the experimentation. An air flow was added to the system in order to prevent moisture accumulation inside the device. For this process, the come up time was of 1 h for all temperature values. In addition, the steering speed was set to be of 12 rev/min. After the heat treatment was performed, the treated flour was cooled down at room temperature in the reception tray of the system. Then it was manually sieved with a kitchen sieve – fine mesh of 1 mm² – to remove any possible agglomerated sample. At last, the treated sample was stored in sealed plastic containers at room temperature.

**2.3. Experimental design**

The time-temperature combinations were obtained with the Statigraph18 software applying the Box-Wilson Central Composite Design (CCD). This type of design starts with fractional numbers with centre points and adds axial points, known as star points that will estimate the curvature. This distance is called α and depends on the quantity of factors in the factorial part of the design. In this case, two factors – time and temperature – were used giving a factorial portion of 2², hence α = 1.414. The experimental analysis was run with 5 degrees of freedom. Table 1 displays the 11 parameter combinations obtained from the experimental design. The heat treatment parameter combination was applied after reaching the come-up time of 1 h.

**2.4. Flour analysis**

The untreated and heat-treated flours were analysed in triplicate.

**2.4.1. Starch gelatinization enthalpy**

The starch's gelatinization enthalpy was determined using Differential Scanning Calorimetry (μDSC K ∈ p technologies Setaram instrumentation, India). Samples of 600 mg were analysed in two different
<figure><img src="image_5.png" /><figcaption>Fig. 1. Equipment used for the heat treatment process.</figcaption></figure>

<caption>**Table 1**</caption>

Parameters combinations for flour heat treatment, with experiments 9, 10 and 11 as central points.

<table><thead><tr><td>Experiment</td><td>Temperature (°C)</td><td>Time (min)</td></tr></thead><tbody><tr><td>1</td><td>115.00</td><td>15.00</td></tr><tr><td>2</td><td>115.00</td><td>35.00</td></tr><tr><td>3</td><td>145.00</td><td>15.00</td></tr><tr><td>4</td><td>145.00</td><td>35.00</td></tr><tr><td>5</td><td>108.79</td><td>25.00</td></tr><tr><td>6</td><td>151.21</td><td>25.00</td></tr><tr><td>7</td><td>130.00</td><td>10.86</td></tr><tr><td>8</td><td>130.00</td><td>39.14</td></tr><tr><td>9</td><td>130.00</td><td>25.00</td></tr><tr><td>10</td><td>130.00</td><td>25.00</td></tr><tr><td>11</td><td>130.00</td><td>25.00</td></tr></tbody></table>

media: water and sucrose solution. Water was taken as a reference in the reference cell. The sample made in the water media consisted in a starch/water ratio of 30/70 %. The sample made in the sucrose solution media aimed at mimicking the high ratio cake system, therefore the sucrose concentration was added to a flour/syrup ratio of 35.04 %. The sucrose solution at a concentration of 42.05 % was prepared 24 h before the analysis to ensure the sucrose full dissolution. Samples were heated from 20 °C to 120 °C at 1.2 °C.min⁻¹ and cooled down at the same rate. From the curve given by the program, the onset temperature (T₀), peak temperature (Tₚ), offset temperature (Tₜ) and starch gelatinization (J. g⁻¹ of dried starch) were determined.

### 2.4.2. Pasting properties

Chopin technologies-MIXOLAB 2 equipment was used to analyse the dough behaviour when mixed at different temperatures following the protocol set by the program Chopin+. An adapted hydration was applied in order to reach 1.1 ± 0.05 N m torque.

Six parameters were extracted from each sample curve. These parameters are based on the typical curve given by the MIXOLAB which displays information related to the protein quality and the starch characteristics. The six characteristics are described as follow: 1) the dough stability (min) which is the time elapsed until the loss of consistency is lower than 11 % of the maximum consistency reached during the mixing, 2) the dough development time (DDT) which is the time needed to reach the maximum torque at 30 °C, 3) protein weakening C2 (N.m), 4) starch gelatinization C3 (N.m), which is the maximum torque produced after the heating stage, 5) stability of the formed starch gel C4 (N.m),

which is the minimum torque reached at the cooling stage, and, 6) starch retrogradation C5 (N.m), which is the torque after cooling at 50 °C.

### 2.5. Baking trials

A high ratio sponge cake recipe was used for the baking trials. Each ingredient percentage values were based on total flour weight: eggs 215 %, caster sugar 120 % and flour 100 %. When using the heat treated flour, water was added to have the same amount of solid and liquid percentage on each recipe (variation due to the initial moisture content of the flours).

A multistage mixing process was used to incorporate air into the batter. As a first stage, eggs at room temperature (20 °C) were whipped in the mixer (KitchenAid, Michigan, USA) at maximum speed (10 grade) for 2 min. Then, sugar was added and the mixing continued for 2.50 min. As a second stage, at low speed (grade 1), water and the previously sieved flour were added. To have a homogeneous system, the mixing was abruptly elevated to speed 10 and quickly turned off. 30 g of the cake batter were placed in 5 × 5 moulds and baked at 170 °C for 30 min in a Ramalhos deck oven (Agueda, Portugal). At the starting point, steam was introduced in the oven and then extracted in order to improve the final product's volume. Cakes were quickly removed from the moulds and cooled down for 1 h at ambient temperature on top of an aluminium grill. Baking trials were done with 100 % replacement of the 11 different heat-treated flours.

### 2.6. Batter and cake characterisation

#### 2.6.1. Batter viscosity

The viscosity was measured with a rheometer (Haake Mars 40, Germany), with a parallel-plate (serrated, 35 mm) adapted at a constant shear rate of 10.00 s⁻¹. The plates were set at constant gap, height of 1 mm. For each sample, viscosity measurements were run at an increasing temperature from 20 to 95 °C, with a heating rate set at 3 grades per minute. Samples were prepared just before the experiments as explained in 2.5. without the cooking step, and followed by a degassing step in a vacuum sealer (Multiva, France). For each treated sample, the C point, representing the onset temperature of rapid viscosity increase, was determined as shown by Shelke et al. (1990).

#### 2.6.2. Cake specific volume

The volume was determined after 1 h of cooling using the rapeseed
displacement method (Seguchi and Matsuki, 1977). For each batch, six
cakes were measured.

3. Results

3.1. Starch gelatinization enthalpy

The starch gelatinization was studied as it contributes to the cake
structure, and especially to the sponge cake volume (Nakamura et al.,
2010). The enthalpy value of starch increases in the presence of sucrose.
The results showed that the untreated flour ΔH (J.g⁻¹ starch) in water
and sucrose were 6.93 ± 0.13 and 9.74 ± 0.08 respectively. Moreover,
the starch gelatinization onset and offset temperatures in the water
medium were of 47.82 ± 1.58 and 76.09 ± 0.73 °C, respectively, which
were lower than the ones in sucrose solution, 63.01 ± 0.01 and 91.11 ±
0.04 °C, respectively.

Fig. 2 shows the starch gelatinization curves comparing the un-
treated flour and the different treatments applied in both water and
sucrose media (Fig. 2A and B). It also gathers the surface response
graphs of the water medium regarding the ΔH and Tp (Fig. 2C and D).
Regarding the water medium, the heat-treated flours had only a signif-
icant positive effect on the enthalpy value considering the linear tem-
perature factor with an R² = 0.782. In fact, the enthalpy recorded for the
untreated flour and flours heat-treated at 108 °C and 151 °C for 25 min
were of 6.93 ± 0.13 J.g⁻¹starch, 6.91 ± 0.28 J.g⁻¹starch and 7.25 ± 0.22 J.
g⁻¹starch, respectively. In addition, the two factors – time and temperature
– had a negative effect on the starch gelatinization Tp. A slight

displacement of the T<sub>p</sub> towards lower temperatures was observed from
60.54 ± 0.10 °C and 60.18 ± 0.06 °C to 59.53 ± 0.01 °C for the un-
treated flour and after the flours have been heat-treated for 25 min at
108 °C and 151 °C, respectively. The results also showed a linear and
quadratic significant negative effect from the temperature factor with a
R<sup>2</sup> = 92.934. In the sucrose medium, the two studied factors had not any
significant effect on the heat-treated flours' enthalpy. The enthalpy of
the untreated flour was of 9.74 ± 0.08 J.g<sub>starch</sub><sup>-1</sup>. Only the temperature
had a significant negative effect on the T<sub>p</sub>. The T<sub>p</sub> values that were
recorded for the solutions made with untreated flour and flours heat-
treated for 25 min at 108 °C and 151 °C were of 75.58 ± 0.03 °C,
75.81 ± 0.10 °C and 75.08 ± 0.17 °C, respectively.

3.2. Pasting properties

Fig. 3A shows the MIXOLAB curves of the untreated flour, and the low and high extremes of the heat-treated flour. The results showed that the curve section displaying the starch changes increased as a function of the heat treatment severity. From the spider web graph (Fig. 3B) which shows the curves parameters converted into flour indexes, an increase in the absorption capacity of the heat-treated flour was observed compared to the untreated flour.

The treated flour samples did not show any significant effect on the dough stability time. The time and temperature factors had a significant influence on the other five parameters evaluated from the curve and presented a R² of 0.936. The temperature had the most significant effect. Fig. 3C shows the summarised results obtained from the surface

<figure><img src="image_6.png" /><figcaption>Fig. 2. Starch gelatinization curve, comparison of untreated T45 flour and DHT samples in A) water and in B) sucrose media. Red line to emphasise on the Tp slight difference, higher temperature treatment, lower value for the start of gelatinization processes; C) estimated surface response of variable of Tp in water medium and D) estimated surface response of variable ΔH in water medium. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<table><caption>**Fig. 3.** Mixolab analysis mixing flour and water under adapted hydration - A) curves of the untreated T45 flour (blue line) and flour samples heat-treated at 108 °C for 25 min (orange line) and 151 °C for 25 min (green line) along with the block temperature (black line) and the dough temperature (red line); B) indexes of the untreated T45 flour (blue line) and flour samples heat-treated at 108 °C for 25 min (orange line) and 151 °C for 25 min (green line); C) significant factors affecting the pasting properties parameters obtained from the MIXOLAB analysis. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</caption><thead><tr><td>Parameters</td><td>Significant factors</td><td>R<sup>2</sup></td></tr></thead><tbody><tr><td>Dough development time (min)</td><td>-T, -T<sup>2</sup></td><td>0.936</td></tr><tr><td>C2 (N.m)</td><td>+T, -T<sup>2</sup></td><td>0.972</td></tr><tr><td>C3 (N.m)</td><td>+T, +T<sup>2</sup>, +t, +Tt</td><td>0.994</td></tr><tr><td>C4 (N.m)</td><td>---</td><td>0.68</td></tr><tr><td>C5 (N.m)</td><td>+T, -T<sup>2</sup></td><td>0.93</td></tr></tbody></table>
response graphs (data not shown). The temperature significantly and negatively affected the dough development time (DDT). It is worth noting that the surface response graph seemed to show an increase from 100 to 120 °C with a R² of 0.936. When this specific temperature stage was passed, a reverse effect was observed leading to a decrease in the DDT values. The protein weakening behaviour (C2 variable) was positively and linearly affected by the temperature until 130 °C. Over 130 °C, the linear tendency changed to quadratic behaviour, and C2 response seemed to become constant. The C2 values of the doughs made with the untreated flour and flours heat-treated 25 min at 108 °C, 130 °C and 151 °C were of 0.58 ± 0.01 N m, 0.58 ± 0,00 N m, 0.68 ± 0,01 N m and 0.72 ± 0,01 N m, respectively. The starch gelatinization (C3 variable) was significantly and positively influenced by the temperature factor. As the temperature increased, the time became also positively significant. The C3 values for the doughs made with the untreated flour and flours heat-treated for 15 min and 35 min at 115 °C were of 1.70 ± 0.06 N m, 1.68 ± 0.01 and 1.69 ± 0.01 N m, respectively, as for the ones made with flours heat-treated for 15 min and 35 min at 145, which were of 1.96 ± 0.03 and 2.09 ± 0.00 N m, respectively. The results showed that the time and temperature had no significant effect on the C4 variable (stability of the starch gel).

The starch retrogradation parameter (C5 variable) was only significantly affected by the temperature. It had a positive linear significance and a negative quadratic significance on the C5 variable. The C5 values of the dough made with the untreated flour and flours heat-treated 25 min at 108 °C and 151 °C were of 2.54 ± 0.03 N m, 2.62 ± 0.01 N m and 2.83 ± 0.02 N.m.

## 3.3. Batter and cake characterisation

The batter properties will determine the final cake characteristics, such as the amount of air retain in the batter that will influence the final cake volume.

### 3.3.1. Batter viscosity

At ambient temperature, the viscosities of the batters made with untreated sample followed by the ones made with dry heat-treated flour samples at 108 °C, 130 °C and 151 °C for 25 min were 1.40 ± 0.08 Pa s,

1.76 ± 0.09 Pa s, 1.31 ± 0.12 Pa s and 2.71 ± 0.12 Pa s, respectively (Fig. 4). On the one hand, the viscosity of the batter made with the untreated flour was significantly lower than the ones made with dry heat-treated flours at 108 °C for 25 min and 151 °C for 25 min within the temperature interval investigated. The latter sample had the highest viscosity values. On the other hand, the batter made with the sample dry heat treated at 130 °C for 25 min did not show a significantly different viscosity compared to the batter made with the untreated sample until 68 °C. At this temperature, the increase in viscosity occurred significantly faster for the batter made with the heat-treated flour. The C-points were calculated from the same curves. They were of 79.3 ± 0.5 °C, 78.5 ± 0.5 °C, 78.2 ± 0.8 °C and 76.7 ± 0.8 °C for the batter made with the untreated flour followed by the ones made with dry heat treated flour at 108 °C, 130 °C and 151 °C for 25 min, respectively.

### 3.3.2. Baking trials

The two main characteristics that have been studied during the baking process are the expansion of the air cells coupled with the setting of the cake structure. The baking trials have been performed through a 100 % flour replacement. The specific volume of the sponge cake obtained from the untreated flour was of 2.69 ± 0.20 cm³ g⁻¹ and the highest among all the studied samples, however not significantly. Regarding the specific volume of the sponge cakes made with the heat-treated flours, the highest specific volume was reached by the sample heat-treated for 25 min at 108 °C, with a value of 2.58 ± 0.00 cm³ g⁻¹. However, the sponge cake made with the untreated flour presented the greatest structure collapse (Fig. 5A and B). The sponge cakes made from the heat-treated flours had a different outcome. Even though, the sponge cake made with heat-treated flour had a similar specific volume than the control cake, it showed a more uniform inner and outer structure. In fact, the crumb structure was made of small bubble with more homogeneous size than the control sponge cake. Moreover, no structure collapse was observed. The surface response of the specific volume for cakes made with heat-treated flours and their pareto graph are shown in Fig. 5C. The results showed that the specific volumes of the sponge cakes made with heat-treated flours decreased according to the severity of the heat treatment, however with no significant effect of the time or temperature parameters.

<figure><img src="image_8.png" /><figcaption>Fig. 4. Viscosity as a function of temperature at a constant shear rate (10.001/s) of the cake batter made with untreated flour (blue line), flour sample heat treated at 108 °C for 25 min (orange line), 130 °C for 25 min (black line) and 151 °C for 25 min (green line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 5. Specific volumes of sponge cakes made with A) untreated flour, B) heat treated flour at 108 °C for 25 min; and C) surface response of the specific volume for sponge cakes and the associated pareto graph.</figcaption></figure>

## 4. Discussion

The untreated wheat flour had initial moisture content and water activity values in line with soft wheat flours that are tempered at 15 % w.b. during the extraction process. The MIXOLAB was used in order to characterise the flour functional changes thanks to DDT, C2, C3, C4 and C5 variables. The results showed differences in protein quality and starch characteristics. The MIXOLAB values increased for each parameter as a function of heat treatment conditions. This behaviour is in accordance with the results obtained by Tawii et al. (2017). They focused on heat-treated wheat flour functional performance in high- and low-ratio cake formulations. In the latter study, the increase in the parameters values was attributed to the denaturation of gluten network and the partial gelatinization of starch granules. In our study, the C2 parameter, related to the protein quality, increased. It was supposed that the heat treatment induced protein denaturation and cross-linking. In fact, the temperatures that were performed in the dry heat treatment were above the denaturation temperature of wheat proteins (Leon et al., 2003). The increase in C3 and C5 parameters values, linked to the starch characterisation, upon heat treatment could be translated into an increase in starch gelatinization, which is further confirmed by an increase in the absorption capacity index of the heat-treated flours. Starch gelatinization is believed to increase the batter viscosity (Cook, 2002). Nevertheless, the absorption capacity index along with the high gluten index, two of the characteristics that are translated from the parameters given by the program, could reflect a change in viscosity and be influenced by the role of arabinoxylan molecules. These latter affect the viscosity and the gelation of wheat batters, concomitantly with the increase of gas retention during baking (Kiszonas et al., 2013). Therefore, the change in viscosity could not only be attributed to the starches' improved swelling and dispersion capacity but also to arabinoxylan molecules changes. For this reason, to isolate the influence of starch "gelatinization" effect over the heat treatments, the full starch gelatinization enthalpy was evaluated, and defined as the heat content in a

system per unit mass first in the water medium and then in the sucrose medium. The starch gelatinization curve of the untreated flour in the water medium showed two peaks, the second being smaller than the first one. The first peak was located at 60 °C and the second at 70 °C. In the sucrose medium, the second peak did not appear. This can be explained by the starch gelatinization peak displacement. In such conditions, the term gelatinization is not appropriate. Indeed, "gelatinization" implies that the starch granule is in excess of water and refers to the loss of crystallinity of the starch granule followed by disruption. In the present conditions, one should rather consider a loss of crystallinity resulting from the melting of starch amylopectin and amylose followed by the dispersion of these biopolymers. In the following, the term "gelatinization" will be used as a generic one covering the loss of crystallinity and dispersion even though the conditions are not "in excess of water" but rather under "limited" access to water. In the water medium, the estimated surface response for heat-treated flours' $\Delta H$ showed first a decreased followed by an increase in relation to the temperature rise. The lowest enthalpy value occurred with a heat treatment at 130 °C for 25 min. This time and temperature combination is in line with the typically ideal parameter combination suggested by the literature, also known as the optimal flour heat treatment (Chesterton et al., 2015; Neill et al., 2012; Meza et al., 2011; Doe and Russo, 1970). The decrease in the flour gelatinization enthalpy was attributed to the proteins located at the starch granule surface by Keppler (2017); however due to the small amount of proteins located at the starch granule surface, the variation may also be attributed to a depolymerisation of amylopectin. Such assumption was proposed by Levine and Slade (1988) in the case of high temperature and low water content treatment for starch. The former changes would first facilitate the granule water absorption, hence decreasing the flours $\Delta H$. As the severity of the treatment increases, interactions between the components located at the surface of the starch granules would occur, creating complexes and eventually leading to a "cement" formation at the particle surface (González et al., 2020). Consequently, the water absorption of the starch granule from the heat-treated flour could be
hindered. This was highlighted by the DSC analysis, especially with the increase in the amount of energy needed for the most severe heat-treatment to reach the same temperature as the reference cell. However, the overall water absorption was still higher than the one from the untreated flour. The minimums revealed by the estimated surface graphics are likely to be the optimum points for the starch gelatinization according to this specific process configuration. Further trials must be done to confirm this hypothesis. In order to understand the starch behaviour in a high-ratio cake, the enthalpy of the starch gelatinization for each heat-treated flours has been studied in a medium containing sugar. The results showed a displacement of the enthalpy peaks towards higher temperatures. This was explained by a reduced accessibility to water in the system. In a cake system, sucrose is thought to shift starch's gelatinization towards higher temperatures as it competes for free water. Therefore, the time for the batter to change from a semi-liquid system to a solid crumb system increases. This delay allows the batter to expand during the baking process before the structure sets. The ΔH value of the heat-treated samples in the sucrose medium was not significantly affected by the heat treatment parameters. Moreover, the results showed that for a heat treatment <130 °C, the T<sub>p</sub> values were slightly higher than the untreated flours' and that for a heat treatment >130 °C, the T<sub>p</sub> values were slightly decreasing as a function of temperature. This can be translated as a lower gelatinization temperature than the untreated sample, leading eventually to a different behaviour during the baking process. The results from the baking trials did not show any significant effect of the severity of the DHT on the specific volume of sponge cakes, even though a decrease of the sponge cake specific volume was observed. Nevertheless, the cakes containing the heat-treated flour had a structure that was less collapsed and a crumb with more homogeneous bubble size than the cake made from the untreated flour. This was possibly due to the early start of gelatinization concomitantly with a rapid viscosity increase. The gelatinization process refers to the stage at which the starch crystal structure starts to melt followed by dispersion of starch's biopolymers, resulting in an earlier setting of the gas holding capacity of the batter within the batter-to-crumb transition. Thus, this mechanism allowed an earlier expansion and the gel to set. The literature states that the change in the viscosity of the batter – due to the starch and arabinoxylan molecules swelling behaviour – was leading to a smaller volume expansion during baking process (Van Steertegem et al., 2013). The fact that the specific volume did not improve in our study is supported by the work of Moiraghi et al. (2012) and Nakamura et al. (2010), who suggested that an optimum viscosity value should be reached in order to produce a positive effect on a sponge cake volume. They also reported that less viscous sponge cake batters produced larger sponge cakes. Our results showed that the viscosity of the batters made with heat-treated flour at 130 °C was not different from the batter made with untreated flour. It was also observed that the batters made with heat-treated flours at 108 °C and 151 °C had higher viscosities at ambient temperature than the one made with untreated flour. Both of the latter also showed increasing water absorbance with the increase of the DHT severity. However, the DSC results in the water medium showed that the starch functionality changed in excess of water as the ΔH first increased, and then started to decrease after a heat treatment at 130 °C. On the one hand, it is presumed that the viscosity increases from the untreated to the heat-treated flour at 108 °C through the increase of the swelling capacity of starch granules and the absorbing capacity of arabinoxylan molecules. On the other hand, it is believed that the flour heat-treated at 151 °C showed poor water absorption capacity from the starch possibly due to a cement formation at the granule surface. The increase in water absorption capacity, and thus in viscosity, could be due to the presence of arabinoxylan molecules and the denatured proteins (Hong et al., 2023). Despite their low content, 2–3 % in regular flour, arabinoxylan molecules contribute to one fourth of the doughs absorption (Kulp, 1968). Keppler (2017) reported that the solvent accessibility increased of approximatively 40 % after the flour was heat-treated 20 min at

150 °C. The changes in viscosity during baking at a temperature below 70 °C are thought to enhance the batter air retention by decreasing the destabilizing phenomena such as the air bubbles coalescence. The difference in the viscosity behaviour of all the samples was also observed through the C-point results. The C-point represents the onset temperature of a rapid viscosity increase. At this point, the starch granules start to gelatinize leading to an increase in viscosity. This stage precedes the inter-granular contacts that forms a rigid gel-like structure as well as the protein coagulation from the egg in a cake batter. In our study, the C-point of the untreated flour was found at 79.3 ± 0.5 °C, whereas it was of 76.7 ± 0.8 °C for the sample made of the flour heat-treated at 151 °C for 25 min. This can be translated as an earlier rapid viscosity increase for the batters made with heat-treated flours than for the one made with untreated flour. According to Kulp et al. (1972), the chlorinated soft wheat flours accelerated the setting of the cake batter. They also reported that the soft wheat flour chlorination induced a decrease in the C-point. The difference between the C-points they determined and ours may rely on the type of flour treatment as well as the cake recipe. In addition, Frazier et al. (1974) showed that the gel strength of flour and water slurries increased at temperatures above 60 °C for both untreated and chlorinated flours. The previous study supports the hypothesis that DHT flour could enhance the batter physical strength that is needed to produce a high ratio cake, which will not collapse. Another important phenomenon to take into account is that hydrophobic properties of the flours could be enhanced at more severe heat treatment (>130 °C). One could think of the cement properties located at the surface of the starch granule as well as the denatured gluten proteins. The induced hydrophobicity would further stabilize the batter through the absorption of the starch granules at the air/batter interface possibly due to the Pickering effect. Nakamura et al. (2008) indicated that the Japanese sponge cake had a more stable foam system due to the increase in hydrophobicity of wheat flours heat-treated at 120 °C for 10–120 min interval. This latter mechanism occurring at severe heat treatment needs to be further studied.

## 5. Conclusion and prospects

The DHT parameters used in this study enabled protein and starch modifications, leading mainly to functional changes of the starch. The latter are believed to affect the cake batter physical properties such as viscosity, hence influencing the volume development of the cake during baking. This study suggested that the starch gelatinization reached a maximum capacity and then decreased as the severity of the heat treatment increased, eventually affecting the batter viscosity. Nevertheless, it also showed that the viscosity did not significantly affect the cake specific volume although it prevented the sponge cake's structure to collapse. It is thought that the use of DHT flours led to a higher viscosity of the batter before reaching 70 °C and an earlier viscosity increase during baking improving the batter air retention with smaller bubble size. At more severe flour heat treatments, the air retention may have been further improved thanks to the newly hydrophobic surface of the starch granule despite the fact they had lower swelling capacities. The functionalised starch granules are thought to have acted as solid particle located at the air/batter interface stabilizing the bubbles. However, the faster increase in viscosity eventually hindered the expansion of the cake volume. Such results have shown that different functionalities could be reached, at different rate, depending on the severity of the heat treatment. One could think that two types of ingredients – or more – are produced from the application of a mild (ingredient 1) to a severe (ingredient 2) heat treatment process. Thus, a combination of these, in the right proportion, could improve the cake volume, as they would display different but complementary functionalities. It is important to note that the untreated flour cannot be simply replaced by heat-treated flour due to possible changes in the sensorial properties as well as the economic aspects. Therefore, exploring different baking parameters and recipe (i.e. different level of substitution
of regular flour by DHT flour) to obtain improved results may be
necessary to improve the cakes quality. Understanding the interaction
between the ingredient functional properties, the food system charac-
teristics and the process parameters will enable the user to combine
them for a specific application and/or products final characteristics.

CRediT authorship contribution statement

**Alejandra Velasquez:** Writing – original draft, Methodology, Formal analysis. **Eve-Anne Norwood:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Conceptualization. **Delphine Queveau:** Methodology. **Anthony Oge:** Methodology. **Alain Le-Bail:** Writing – review & editing, Validation, Supervision, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no conflict of interest. This study
was supported by the Région du Pays de la Loire/RFI "Food for
Tomorrow" for funding the "CLEAN" project and the Nantes-Atlantic
National College of Food Process Engineer ONIRIS VetAgroBio.

Acknowledgments

The authors acknowledge the Région du Pays de la Loire/RFI "Food for Tomorrow" for funding the "CLEAN" project together with ONIRIS-GEPEA.

Data availability

The authors do not have permission to share data.

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