# Thermal and Expansion Analysis of the Lebanese Flatbread Baking Process Using a High-Temperature Tunnel Oven

Yves Mansour, Pierre Rahmé, Nemr El Hajj, Olivier Rouaud

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Yves Mansour, Pierre Rahmé, Nemr El Hajj, Olivier Rouaud. Thermal and Expansion Analysis of the Lebanese Flatbread Baking Process Using a High-Temperature Tunnel Oven. Applied Sciences, 2025, 15 (15), pp.8611. 10.3390/app15158611. hal-05225942

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Article

# Thermal and Expansion Analysis of the Lebanese Flatbread Baking Process Using a High-Temperature Tunnel Oven

Yves Mansour <fnref n="1" />,* ID, Pierre Rahmé <fnref n="2" /> ID, Nemr El Hajj <fnref n="1" /> ID and Olivier Rouaud <fnref n="3" /> ID

## Abstract

This study investigates the thermal dynamics and material behavior involved in the baking process for Lebanese flatbread, focusing on the heat transfer mechanisms, water loss, and dough expansion under high-temperature conditions. Despite previous studies on flatbread baking using impingement or conventional ovens, this work presents the first experimental investigation of the traditional Lebanese flatbread baking process under realistic industrial conditions, specifically using a high-temperature tunnel oven with direct flame heating, extremely short baking times (~10–12 s), and peak temperatures reaching ~650 °C, which are essential to achieving the characteristic pocket formation and texture of Lebanese bread. This experimental study characterizes the baking kinetics of traditional Lebanese flatbread, recording mass loss pre- and post-baking, thermal profiles, and dough expansion through real-time temperature measurements and video recordings, providing insights into the dough’s thermal response and expansion behavior under high-temperature conditions. A custom-designed instrumented oven with a steel conveyor and a direct flame burner was employed. The dough, prepared following a traditional recipe, was analyzed during the baking process using K-type thermocouples and visual monitoring. Results revealed that Lebanese bread undergoes significant water loss due to high baking temperatures (~650 °C), leading to rapid crust formation and pocket development. Empirical equations modeling the relationship between baking time, temperature, and expansion were developed with high predictive accuracy. Additionally, an energy analysis revealed that the total energy required to bake Lebanese bread is approximately 667 kJ/kg, with an overall thermal efficiency of only 21%, dropping to 16% when preheating is included. According to previous CFD (Computational Fluid Dynamics) simulations, most heat loss in similar tunnel ovens occurs via the chimney (50%) and oven walls (29%). These findings contribute to understanding the broader thermophysical principles that can be applied to the development of more efficient baking processes for various types of bread. The empirical models developed in this study can be applied to automating and refining the industrial production of Lebanese flatbread, ensuring consistent product quality across different baking environments. Future studies will extend this work to alternative oven designs and dough formulations.

**Keywords:** Lebanese flatbread; baking process; water loss; thermal profiling; expansion; oven design; industrial baking; empirical modeling

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Academic Editor: Małgorzata Ziarno

Received: 10 July 2025

Revised: 29 July 2025

Accepted: 30 July 2025

Published: 4 August 2025

**Citation:** Mansour, Y.; Rahmé, P.; El Hajj, N.; Rouaud, O. Thermal and Expansion Analysis of the Lebanese Flatbread Baking Process Using a High-Temperature Tunnel Oven. *Appl. Sci.* **2025**, *15*, 8611. https://doi.org/10.3390/app15158611

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Department of Mechanical Engineering, Faculty of Engineering, Holy Spirit University of Kaslik, Jounieh P.O. Box 446, Lebanon; nemrelhajj@usek.edu.lb</footnote>

<footnote n="2">² Department of Industrial and Mechanical Engineering, School of Engineering, Lebanese American University, Byblos P.O. Box 36, Lebanon; pierre.rahme@lau.edu.lb</footnote>

<footnote n="3">³ Department of Food Processing Engineering, Oniris, Nantes University, CNRS, GEPEA, UMR 6144, 44000 Nantes, France; olivier.rouaud@oniris-nantes.fr</footnote>

<footnote>* Correspondence: yves.mansour@usek.edu.lb</footnote>
# 1. Introduction

Flatbreads are among the oldest forms of bread, widely consumed across the Mediterranean, the Middle East, North Africa, and Central and South Asia [1,2]. Typically round, thin, and composed of simple ingredients, such as flour, water, and salt—with or without leavening—flatbreads can be broadly categorized into single-layered (e.g., chapati, naan, focaccia) and double-layered types that form a hollow pocket during baking [1,3].

Lebanese flatbread, often referred to as Arabic bread or pita, is a well-known example of a double-layered flatbread with historical roots tracing back to ancient Middle Eastern civilizations [1]. Its preparation methods have evolved over time, but its core baking characteristics remain consistent: extremely high temperatures (typically 450 °C to 650 °C) applied over a very short baking time (10–12 s) [4,5]. During this rapid process, thin flattened dough disks are placed directly into a hot baking chamber, where the intense heat from above and below causes sudden steam generation and expansion, resulting in the bread's characteristic balloon-like inflation and pocket formation [6]. Once removed from the oven, the inflated bread collapses into two distinct layers, making it ideal for filling.

While Lebanese flatbread is the focus of this study, similar puffing and baking behavior is observed in other double-layered flatbreads, such as Shami bread and certain types of pocket pita commonly consumed throughout the Mediterranean and Levant regions [7]. Due to these shared structural and thermal characteristics, the insights from this work are also broadly applicable to other flatbreads with comparable baking profiles.

While several studies have addressed aspects of bread baking, they often focus on longer baking times and convective heat transfer processes. For example, impingement ovens and home-scale electric ovens have been used in experimental studies [1,3], but these settings do not reflect the extreme and rapid thermal environment of Lebanese tunnel ovens. Recent studies such as that of Bedre-Dine et al. [8] investigated gas permeability and pocket inflation using custom chambers under moderate baking conditions. Khatibi et al. [9] employed numerical simulations to model baking with variable input power in small-scale ovens, and Zhang et al. [10] coupled experimental and simulation methods to study water phase changes in bread. However, these approaches were conducted under controlled, non-industrial settings with lower baking temperatures and longer durations, often exceeding several minutes.

These prior works, although technically rigorous, do not replicate the authentic high-heat and ultra-short-time baking process critical to Lebanese flatbread production. The oven type used, usually electric or impingement, relies predominantly on convective heat transfer, which does not mimic the radiative and conductive heat fluxes present in tunnel ovens [4]. Furthermore, no study to date has simultaneously captured internal temperature profiles and volume expansion in real-time under true tunnel oven conditions. This gap is critical because accurate empirical modeling of the baking process requires data obtained under authentic processing conditions.

Dough composition plays a known role in baking outcomes. Lebanese bread dough typically contains flour (commonly Type 85), water, salt, sugar, and yeast [11,12]. The water-to-flour ratio and fermentation parameters influence the dough's rheological properties [13,14] and its behavior during thermal exposure. Fermentation plays a crucial role in the development of dough structure and flavor [15,16]. Yeast fermentation produces carbon dioxide, which is trapped within the gluten network, causing the dough to rise [17]). The fermentation time and conditions (temperature and humidity) affect the dough's gas retention capacity and, consequently, the bread's texture and volume [18,19]. The Lebanese bread-baking process is highly critical and can only be achieved under the specific set of conditions used in this study. These conditions are essential for maintaining the authenticity of the bread's texture and pocket formation. While this study investigates the
baking process under one set of conditions and a specific dough formulation, it provides valuable insight into the fundamental aspects of the process. This rapid baking causes the dough to puff up, forming a pocket, and results in a distinctive soft texture with a slightly crispy crust [20]. The temperature profiles during baking are critical for understanding the heat transfer mechanisms and their impact on bread quality. Studies have shown that the temperature on the dough's surface rises rapidly, while the core temperature increases more slowly [21]. This temperature gradient is essential for the formation of the crust and the pocket [22,23]. Mass loss during bread baking is primarily due to water evaporation [24,25]. The rate of water loss affects the bread's texture and shelf life. Lebanese bread, due to its thin structure and high baking temperatures, experiences significant water loss compared to other bread types [26]. The final water content is a critical quality parameter, influencing the bread's softness and storability [27].

The expansion of the dough during baking is influenced by the production of carbon dioxide and the rapid vaporization of water [28,29]. The high baking temperatures cause the water in the dough to quickly turn into steam, leading to pocket formation. The extent and uniformity of this expansion are key quality attributes of Lebanese bread [30]. While this mechanism is generally recognized in bread baking [31,32], recent studies have shown that, in thin, double-layered Lebanese flatbreads (~1.5 mm), water vapor plays an even more dominant role due to the dough's low thickness and high permeability. The characteristic expansion and pocket formation in Lebanese flatbread is primarily driven by the buildup of internal steam pressure during baking. As water within the dough rapidly evaporates due to high heat, the resulting vapor increases pressure between the dough layers, forcing them apart and inflating them, resulting in a characteristic balloon shape [8,10]. A recent study has also explored non-conventional baking methods, such as partial vacuum environments, to reduce baking temperature while still achieving proper pocket formation, confirming the critical role of water vapor phase change under varying pressure conditions [33].

Recent reviews emphasize the importance of integrating empirical and physics-based models to capture volume change and expansion in food processes like baking [34] and increasingly focus on optimizing both nutritional quality and traditional sensory attributes by understanding the physicochemical and rheological transformations during the baking process [5,35].

This study uniquely investigates the high-temperature, rapid baking process of Lebanese flatbread, employing an instrumented oven to monitor both temperature and expansion in real time. The use of custom-built experimental equipment and empirical modeling of temperature—expansion relationships in this context is a novel approach, contributing new insights into heat transfer and mechanical behavior while baking.

This study aims to investigate mass loss, expansion, and temperature evolution during the baking process of Lebanese bread. It aims to understand the relationship between temperature changes and bread expansion and provides empirical models to describe the temperature and expansion profiles during baking. The contribution is not only technical but also practical: the findings provide empirical foundations for modeling heat and mass transfer, understanding pocket formation mechanisms, and optimizing process conditions for quality control.

This work addresses a significant gap in the literature by providing high-fidelity, real-world data on temperature, expansion, and water behavior in Lebanese flatbread baking. It uses authentic ovens, realistic bake times, and advanced instrumentation, offering insights applicable not only to traditional bread making but also to broader food processing and modeling domains. This sets the stage for future studies on optimization, energy efficiency, and product quality in high-temperature, short-time baking systems.
## 2. Materials and Methods

The following section outlines the experimental materials, equipment, and procedures used to investigate the thermal and physical behavior of Lebanese flatbread during baking.

### 2.1. Instrumented Oven

An instrumented tunnel oven was specially developed for this study (Figure 1). It consists of a steel plate conveyor that moves through a high-temperature baking chamber equipped with an overhead flame burner. The chamber measures 150 cm in length, 69 cm in width, and 80.5 cm in height. The entry and exit openings are 64 cm wide and feature adjustable gates. The side walls of the chamber include multi-layer insulation with refractory bricks, Rockwool, and ceramic fiber, separated by air gaps for thermal resistance. The material properties are summarized in Table 1. Two inspection windows are located on either side, shielded with heat-resistant glass, allowing visual monitoring during baking. The oven is also instrumented with an array of thermocouples distributed at various locations to accurately measure temperature across the oven and baking plates [2].

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Instrumented oven for experimental tests: (a) oven design with detailed views [4], and (b) manufactured oven.</figcaption></figure>

<table><caption>**Table 1.** Summary of oven material properties.</caption><thead><tr><th>Product</th><th>Material Model and Reference</th><th>Thickness (mm)</th><th>Density ρ (kg/m³)</th><th>Thermal Conductivity (W/m·K)</th><th>Specific Heat Capacity (J/kg·K)</th><th>Emissivity ε</th></tr></thead><tbody><tr><td>Rock wool</td><td>RK120, Rosewool, Zhengzhou, China</td><td>50</td><td>180</td><td>0.035</td><td>900</td><td>0.90</td></tr><tr><td>Ceramic Fiber</td><td>HP, Rosewool, Zhengzhou, China</td><td>50</td><td>100</td><td>0.08</td><td>1000</td><td>0.40</td></tr><tr><td>Refractory bricks</td><td>Viking 330, Högönas, Sweden</td><td>120</td><td>2150</td><td>1.5</td><td>1000</td><td>0.85</td></tr><tr><td>Steel baking plates</td><td>C25 steel, Zcsteel, Jinan, China</td><td>5</td><td>7800</td><td>50</td><td>475</td><td>0.80</td></tr><tr><td>Steel walls (external)</td><td>C25 steel, Zcsteel, Jinan, China</td><td>3</td><td>7800</td><td>50</td><td>475</td><td>0.80</td></tr><tr><td>Stainless steel walls</td><td>Stainless Steel SAE304, Outokumpu, Helsinki, Finland</td><td>1.5</td><td>7800</td><td>15</td><td>500</td><td>0.30</td></tr></tbody></table>

The oven's heating system features a modulating overhead burner (FARHAT model F40V3-130, Beirut, Lebanon) positioned 55 cm above the oven inlet. This diesel-fueled burner is equipped with a 0.6 mm nozzle and a 90 mm air outlet, delivering a thermal power output of 130 kW. The oven is rated for a production capacity of 150 kg of dough per hour [2].
## 2.2. Dough Preparation and Baking Process

A typical Lebanese bread formulation was used in this study, with the dough ingredients detailed in Table 2. Type 85 wheat flour was selected as the primary flour. Type 85 flour used is a medium-extraction wheat flour with an ash content ranging from 0.75% to 0.90%, which indicates a higher mineral and fiber content compared to refined flours. With a typical protein content of 11–13%, it offers good baking performance and dough strength. Type 85 flour is traditionally and widely used in Lebanese flatbread production due to its optimal balance between functionality and nutritional value.

<table><caption>**Table 2.** Lebanese bread dough recipe.</caption><thead><tr><th>Flour-Type 85 (kg)</th><th>Water (kg)</th><th>Sugar (g)</th><th>Salt (g)</th><th>Dry Yeast (g)</th></tr></thead><tbody><tr><td>5</td><td>2.5</td><td>250</td><td>50</td><td>50</td></tr></tbody></table>

The ingredients were mixed using a Fork Mixer (Pietro Berto, Vittoria, Vicenza, Italy), first at 30 rpm for 6 min and then at 90 rpm for an additional 6 min to ensure proper gluten development. Following mixing, the dough was allowed to rest for 30 min at room temperature to initiate yeast fermentation.

The fermented dough was then processed using a screw dough divider (Farhat, Model F1V4-1R, Beirut, Lebanon), which extruded and divided the dough into 100 g portions, shaped into uniform balls. These dough balls were placed in plastic trays and subjected to the first proofing stage at 32 °C for 8 min.

After proofing, the dough balls were sheeted into disks with a diameter of 22 cm and a thickness of 1.5 mm using a pneumatic Flattener (Farhat F3V4-1R, Beirut, Lebanon), a machine commonly used in industrial Lebanese flatbread production. The sheeted disks were then transferred onto stainless steel plates and loosely covered with damp cloths to prevent drying. They were left to undergo a second proofing stage at room temperature (approximately 32 °C) for 9 min.

This procedure closely replicates the industrial manufacturing process of traditional Lebanese flatbread, ensuring that experimental conditions realistically represent commercial baking practices.

After extrusion, proofing, and sheeting, the dough entered the baking stage, which was carried out in the instrumented tunnel oven specifically developed and described in Section 2.1. This oven is equipped with an overhead burner and reflects an industrial configuration typical for Lebanese flatbread production. During this study, direct temperature measurements were conducted inside the oven at various positions along the baking path. The results showed a temperature range from approximately 450 °C at the inlet to 650 °C at the outlet, confirming the strong spatial temperature gradient within the oven. These measurements are in agreement with the detailed experimental and numerical thermal characterization of the same oven model reported by Mansour et al. [4].

The tunnel oven is designed such that the high-temperature flame is located above the moving conveyor surface, with the flame positioned several centimeters above the dough surface. This setup ensures effective heat transfer through radiation, convection, and conduction. While radiation and convection dominate the heat transfer from the oven to the dough's surface, conductive heat transfer occurs within the dough itself. The rate of conduction is influenced by the dough's composition, including its moisture content, density, and structure, which determines how heat is distributed and penetrates the inner layers of the dough.

As the dough passes through the oven, it is subjected to an intense and rapidly changing thermal environment. The total baking time is approximately 10–12 s, during
which the dough undergoes rapid expansion and pocket formation, a quality characteristic of traditional Lebanese flatbread.

This configuration and thermal profile accurately simulate authentic industrial baking conditions and are critical to achieving the desired texture, structure, and sensory quality of the final product.

## 2.3. Mass Loss Measurements

During baking, the dough piece loses mass by evaporating part of its water content. Due to extreme temperatures inside the oven, measuring water loss evolution inside bread during baking is not experimentally possible. Mass loss is measured at the end of the baking process to quantify evaporation. Measurements are automatically recorded in the data acquisition system. The baked dough is automatically dropped on the scaling mechanism mounted directly at the oven exit, instantly allowing accurate mass measurement after baking. Two baking tests were performed over two days with similar initial conditions. Each test was conducted for twenty identical dough pieces. Mass loss measurements are performed with a single-point load Cell Model (OBUH5, Bongshin, Sangwook Yoo, Republic of Korea). The load cell is mounted on a stainless frame and positioned at the oven exit (Figure 2). It is connected to the data acquisition system that detects a measurement resolution of 0.5 g.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Mass loss measurement system: (a) Detailed view showing the load cell used for weighing the baked bread. (b) Assembled system with the load cell positioned at the oven exit for precise mass measurement.</figcaption></figure>

## 2.4. Temperature Measurements Procedure

Temperature measurements were taken using a K-type thermocouple (SAB-T207, SAB Bröckskes, Viersen, Germany), designed for high-temperature industrial environments (up to 800 °C). Its slim 0.25 mm diameter rod and flexible sheath enable easy installation in the thin-sheeted dough layer. The sensor rod is 100 mm long, with a high-temperature cable and armor to withstand extreme temperatures.

The bread's lower surface is in contact with the baking conveyor, while the upper surface is exposed to hot air. Due to the dough's large expansion and high baking temperatures, measuring temperature inside and on the dough's surfaces is challenging. The dough thickness is about 1.5 mm, and it expands to approximately fifty times its initial volume during baking.

One thermocouple was placed beneath the dough at the lower surface, while the other was firmly fixed to the upper surface to ensure stable contact during baking. A third
thermocouple was used to measure the internal dough temperature. Due to the very thin diameter of the thermocouple wire, direct insertion into the dough was not feasible without deformation. Therefore, a 1 mm-diameter rigid stainless-steel tube was embedded into the dough prior to baking to act as a guide channel, into which the thermocouple was inserted. The sensor was secured by fitting it into precision-drilled holes (0.25 mm) along the length of the tube to ensure stability and accurate internal measurement during baking. At the end of the baking process, the internal sensor remains within the air pocket formed inside the bread.

Before each experiment, the thermocouples were calibrated using a reference probe (accuracy ± 0.5 °C) in a 25 °C water bath, ensuring accurate measurements during the baking cycle. The thermocouples were verified to be accurate within ±2 °C under high-temperature conditions. After baking, sensor depth and position were confirmed visually within the bread pocket.

For each baking trial, measurements were recorded every 0.1 s over a 12-s baking period, resulting in 120 data points per trial. A total of ten independent repetitions were conducted under identical conditions, and the results were averaged to ensure accuracy and repeatability. The average measurement uncertainty was estimated at ±4 °C, which accounts for thermocouple precision, variability in placement on the deforming dough surface, and steep local thermal gradients during baking. Surface temperature monitoring was particularly challenging due to the intense heat (~650 °C), the thin dough layer (1.5 mm), and the dynamic inflation of the dough during the short baking period. Statistical analysis and curve fitting methods applied to the data are detailed in the Section 3. Figure 3 shows the thermocouple positions inside the dough before and after baking.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Temperature measurement on the lower surface, upper surface, and inside the dough during baking: (a) Thermocouple placement inside the dough before baking. (b) Thermocouple positioned inside the dough after baking to measure core temperature. (c) K-type thermocouple (SAB-T207, SAB Bröckskes, Viersen, Germany).</figcaption></figure>

## 2.5. Measurement of Dough Expansion During Baking

The expansion of Lebanese bread is primarily driven by the vaporization of water inside the dough, which causes overpressure and results in the formation of pockets. Additionally, the CO₂ produced during fermentation also contributes to the dough's expansion, but its effect is secondary to the vaporization of water during the high-temperature baking [31,32]. Expansion of the dough during baking was studied by capturing its expansion over time using a Canon D600-SLR camera (Canon, Tokyo, Japan) equipped with a 1 GB memory card and a frame rate of 30 fps. This allowed for precise monitoring of the
dough's volume changes. The bread's width (X-axis) and height (Y-axis) were measured during baking using a graduated millimetric XY chart placed approximately 40 cm from the oven glass window, directly facing the bread's cross-section, as shown in Figure 4. The measurement setup was calibrated by positioning a 10 cm × 5 cm reference object at the exact location of the dough to ensure spatial accuracy.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Millimetric XY chart is used to record dough expansion during baking.</figcaption></figure>

**2.6. Statistical Analysis**

All experimental measurements were performed in triplicate (n = 3) to ensure re-
producibility. Results are reported as mean values ± standard deviation. No inferential
statistical tests (e.g., ANOVA or post hoc comparisons) were applied. Data processing and
graphical representation were conducted using Microsoft Excel 365.

**2.7. Derivation of Equations**

The equations presented in this manuscript were derived using polynomial regression applied to experimental data. The regression analysis was conducted using MATLAB R2025a and Microsoft Excel, with both tools being used to cross-verify the results. In MATLAB, the Polyfit function was utilized to fit a polynomial model to the data using the least squares method. The experimental data, including temperature readings and baking times, were input into the regression model, and a polynomial of appropriate degree was selected based on data behavior and the goodness of fit.

In Excel, the regression analysis was performed using the Analysis ToolPak, which also applies the least squares method to determine the best-fitting polynomial model. Both methods were used to ensure the consistency and accuracy of the results. The goodness of fit was evaluated using the R-squared value, and both MATLAB and Excel produced similar results, confirming the validity of the derived equations.

**3. Results and Discussion**

**3.1. Mass Loss and Water Content**

The water content in bread is crucial for determining its quality and texture [28]. Generally, bread water content (ratio of bread water mass over total mass) is around 30% to 40% [29,36]. However, flatbreads generally have a lower water content than French-style breads due to their larger surface area-to-volume ratio [30].
In this study, water content in Lebanese flatbread was primarily influenced by baking temperature and water evaporation. Table 3 shows the water content of dough and bread after baking for two trials. For each test, 100 g of dough was used, and 20 pieces were baked under identical conditions over two days. The results indicate that the water loss due to evaporation corresponds to 14.8% of the initial dough mass, with a maximum loss of 16 g and a minimum of 12 g.

<table><caption>**Table 3.** Mass loss measurements.</caption><thead><tr><th></th><th>Number<br>Pieces/Test</th><th>Dough<br>Water<br>Content %</th><th>Dough<br>Sample<br>Mass (g)</th><th>Bread<br>Average<br>Mass (g)</th><th>Average<br>Water Mass<br>Loss (g)</th><th>Bread Water<br>Content (g)</th><th>Bread Water<br>Content<br>%</th><th>Average Mass<br>Loss (%)</th></tr></thead><tbody><tr><td>Test 1<br>(Day 1)</td><td>20</td><td>31.8%</td><td>100</td><td>85.1 ± 1.49%</td><td>14.9 ± 8.89%</td><td>16.9</td><td>20%</td><td>14.9%</td></tr><tr><td>Test 2<br>(Day 2)</td><td>20</td><td>31.8%</td><td>100</td><td>85.3 ± 1.39%</td><td>14.7 ± 8.30%</td><td>17.1</td><td>20%</td><td>14.7%</td></tr></tbody></table>

The water content values presented in Table 3 are calculated on a wet basis, where they are expressed as a percentage of the total mass of the dough or bread, including both water and dry ingredients.

Moisture content was calculated based on the difference between the amount of water added during dough preparation and the water lost during baking. While this method introduces potential measurement errors, the results reported are the average of triplicate measurements.

These results show that the water content in baked Lebanese bread is remarkably lower than in Western bread (30% to 50%) [37].

The mass loss in Lebanese bread baking is greater than in other flatbreads, such as chapati, which shows an average weight loss of 9% [38]. This difference is likely due to the significantly higher baking temperature used for Lebanese bread (~650 °C) [2], compared to the chapati baking temperature (~250 °C) [38], despite similar baking durations. Due to its thin structure and exposure to such high heat, Lebanese bread experiences substantial water loss, typically resulting in a final moisture content of around 20%. In contrast, breads like naan and Western-style loaves retain more moisture, with final water contents ranging from 30% to 50% [32,39]. The short baking time at extreme temperatures also distinguishes Lebanese bread from breads such as French loaves, which are baked for approximately 22 min at 250 °C [32]. These baking conditions also contribute to the rapid crust formation and distinctive pocket formation. The high temperature in Lebanese bread causes the crust to form quickly, which acts as a barrier to further water loss [40] and preserves the softness of the interior [41]. In contrast, other flatbreads such as naan, bake at lower temperatures, which results in a more uniform texture and slower evaporation of water during baking.

## 3.2. Temperature Measurements

For the lower, core, and upper dough surfaces, the mean temperature evolution throughout baking is shown in Figure 5. At the beginning of the baking process, the upper surface is directly subjected to radiative and convective heat flux, while the lower surface is in contact with the baking plate, subjected to conductive flux. Thus, the temperature increases inside the dough more slowly than on the surfaces. The upper and lower surface temperatures continuously increase during baking, reaching 170 °C on the lower surface and 245 °C on the upper surface. Bread surfaces cross 100 °C within 7 s for the lower surface and 5.8 s for the upper, responsible for crust formation [42,43]. The temperature on the upper surface is higher than on the lower surface. The lower surface is in contact with the baking band at 400 °C, subjected only to conductive heat fluxes of 7.8 kW/m². In
contrast, the upper surface is directly exposed to the burner flame and the baking chamber walls, thus exposed to convective and radiative heat fluxes. In the baking chamber walls, the temperature is around 600 °C, with heat flux evolving from 8.3 to 28.6 kW/m² along the baking chamber [4].

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Dough lower, upper, and core temperature measurements.</figcaption></figure>

The bread core temperature increases slowly during the first four seconds of baking, followed by a more rapid, ramp-like rise over the next six seconds, until a plateau is reached at around 99 °C, which corresponds to the boiling point of water. As the dough heats up, surface water begins to evaporate early in the process. As internal temperatures continue to rise, ebullition occurs within the dough core, where water transitions to steam, absorbing latent heat and temporarily slowing further temperature increases. As more moisture evaporates and the dough becomes drier, the rate of temperature increase resumes. A temperature gradient gradually develops from the crust region toward the core, and the internal temperature approaches the boiling point independently of the oven's external temperature [31,44]. Such thermal behavior, especially in high-temperature bread ovens, can be effectively modeled and visualized using CFD simulations [45,46]. This temperature trend has been seen in most baking processes and for different types of bread [29,47,48]. Although the core temperature typically stabilizes at around 99 °C, this value alone does not confirm that starch gelatinization or full internal structural development is complete, particularly in short baking cycles [49].

In this study, baking was considered complete when the bread reached its final puffed shape, the core temperature plateaued near 99 °C, and visual indicators confirmed full expansion and crust formation typical of commercially baked Lebanese bread.

The temperature curves illustrated in Figure 5 exhibit consistent trends without significant variation in slope across the three measured locations (lower surface, core, and upper surface). This consistency under constant oven operating conditions justifies the development of empirical time-temperature relationships, as expressed in Equations (1)–(3), respectively, determined from the experimental plot:
$$T_{LS} = 0.465 t^2 + 6.66 t + 34.19 \quad (1)$$

$$T_C = 0.0031 t^5 - 0.1070 t^4 + 1.1638 t^3 - 3.95878 t^2 + 5.2797 t + 31.589 \quad (2)$$

$$T_{US} = 1.218 t^2 + 3.14 t + 36.43 \quad (3)$$

where $T_{LS}$ is the lower surface temperature in °C, $T_C$ the core temperature in °C, $T_{US}$ the upper surface temperature in °C, and $t$ the baking time in seconds.

The empirical temperature fittings were developed to smooth measurement variability and provide a continuous, analytical representation of the thermal evolution at different depths of the dough. This allows for the estimation of instantaneous thermal gradients within the flatbread thickness, which are otherwise difficult to extract directly from discrete experimental data. These fitted curves form the basis for comparing the rate and extent of heat penetration across the dough layers. Specifically, the regression analysis for the lower surface temperature ($T_{LS}$) yielded an $R^2$ value of 0.9936 and a Residual Standard Error of 3.9, indicating a strong correlation between the modeled and observed temperatures. For the upper surface temperature ($T_{US}$), the $R^2$ value was 0.9975 with a Residual Standard Error of 3.5, suggesting a very accurate prediction of the temperature changes on the upper surface exposed to direct heat. Finally, the core temperature analysis ($T_C$) demonstrated the highest accuracy, with an $R^2$ value of 0.9994 and a Residual Standard Error of 0.7, reflecting a near-perfect alignment between the predicted and observed core temperatures. However, it is important to distinguish between good fitting and prediction ability. While high $R^2$ values and low Residual Standard Errors indicate the model fits the observed data well, prediction ability refers to the model's ability to predict unseen data. In this study, the high $R^2$ values suggest that the model fits the training data well, but further testing on new data is required to assess its predictive power.

These findings contribute to understanding the thermal behavior during baking and provide insight into how different regions of the bread heat up, an aspect closely linked in the literature to pocket formation and crust development, which are key quality indicators in flatbread.

The results show a higher temperature on the upper surface than on the lower surface. Temperature plays a key role in browning, which occurs through the Maillard reaction and caramelization, heat-induced chemical reactions between sugars and proteins in the dough, contributing to the bread's flavor and aroma [50]. The Maillard reaction is optimal between 150 °C and 180 °C, while caramelization occurs around 170 °C to 190 °C. Browning can be influenced by baking time, temperature, and dough water, as demonstrated by Giovanelli and Cappa [23], who found a strong correlation between browning indices and Maillard reaction products, particularly under more intense thermal conditions. Thus, the temperature difference between the upper and lower surfaces contributes to the observed difference in browning, with the upper surface reaching 150 °C more quickly and achieving a higher temperature than the lower surface, which is associated with the bread's surface characteristics (Figure 6).

While the Maillard reaction plays a crucial role in the desirable browning, flavor, and aroma formation during baking, it is also associated with the formation of potentially harmful compounds such as acrylamide, particularly under high-temperature conditions. This dual effect must be carefully considered when optimizing thermal profiles for product quality and safety.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Surface color of baked Lebanese bread: (a) Upper surface (orange-brown), (b) Lower surface (light brown).</figcaption></figure>

### 3.3. Expansion Measurements

Expansion measurements show significant swelling in both the X (width) and Y (height) directions during the rapid heating phase, particularly as the core temperature approaches the boiling point of water (~100 °C). This supports the hypothesis that steam expansion and moisture loss are key contributors to pocket formation, even though internal pressure was not directly measured. This expansion is most pronounced in areas where steam accumulates beneath the dough's surface. Typically, the layers of dough have uniform thickness after baking [1,3]. The expansion is documented through video recordings and regular snapshots during baking (Figure 7).

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Dough expansion at different baking times: (a) 4 s, (b) 7 s, (c) 10 s, (d) 12 s (end of baking).</figcaption></figure>

As shown in Figure 7a, no expansion occurs in the first 4 s of baking. This phase is followed by the uniform formation of a few gas bubbles after 7 s, as shown in Figure 7b. At 8 s of the baking process time, a fast volume expansion occurs, and the bread gradually starts to inflate, taking an elliptical shape and reaching its maximum at the end of the baking period (Figure 7c,d). A pocket formation between two thin layers of baked bread is visible, resulting from the thermal expansion of gases during baking (Figure 3b). The expansions in the X (width) and Y (height) directions (Figure 4) measured during baking are presented in Figure 8. It shows the X (width) and Y (height) changes of the bread during the baking period for four representative tests conducted under identical conditions. These four tests were selected from a larger series of repeated trials based on their consistent dough preparation, baking profiles, and expansion behavior, providing a reliable basis for averaging and analysis. A slight continuous decrease in width (X direction) from 22 cm at the start to 17.5 cm at the end of baking is observed. A slight increase in the height (Y direction) at the beginning of the baking, reaching 25 mm after 7 s, is followed by a fast
expansion, where the dough pieces' height increases from 25 mm to 100 mm in 4 s. A slight decrease in height is noticed at the end of the baking process. This is due to crack formation on the bottom surface of the baked bread and to gas leaking out, slightly reducing the pocket volume.

<figure><img src="image_11.png" /><figcaption>Figure 8. Expansion in the X and Y directions during baking for four tests.</figcaption></figure>

On the other hand, small deviations in Y (height) expansion between tests are monitored. These variations may be attributed to minor differences in dough handling or proofing conditions, which could influence the initial dough structure and indirectly affect the location and behavior of pocket formation.

However, it should be emphasized that, consistent with prior studies on flatbreads, water vapor expansion remains the dominant mechanism for pocket formation at high baking temperatures, while the contribution of CO₂ generated during fermentation is considered negligible under these condition [51]. The test is repeated several times with identical initial conditions. The average uncertainty across these measurements is found to be approximately 1.8 mm and 2.2 mm on X (width) and Y (height) expansion, respectively. The maximum uncertainty is found around 5 mm and 6.5 mm on X (width) and Y expansion, respectively. These values are considered small for such applications.

Figure 9 shows the average expansion in both directions along with the corresponding trendlines derived from polynomial regression analysis. Equations (4) and (5) also illustrate the average expansion in the X (width) and Y (height) directions concerning baking time, based on these empirical data.

$$DEF_{X(t)} =0.0230 t^3-0.3893 t^2-1.9397 t + 222.1223 \quad (4)$$

$$DEF_{Y(t)} =0.0038 t^6-0.0607 t^5+0.8899 t^4-5.7830 t^3+17.0551 t^2-17.9077 t + 3.1502 \quad (5)$$

where DEF<sub>X(t)</sub> and DEF<sub>Y(t)</sub> represent the expansion in the X (width) and Y (height) directions, respectively, as functions of baking time, with t denoting the baking time in seconds. The regression analysis yielded the following values: for DEF<sub>X(t)</sub>, R² value of 0.9987 and a Residual Standard Error of 0.57. For DEF<sub>Y(t)</sub>, R² value of 0.9932 and Residual Standard Error = 4.0. This empirical modeling approach, using polynomial regression to describe expansion over time, aligns with methods highlighted by Purlis [34,52] as
effective for food process optimization when full physics-based modeling is impractical or
data-limited.

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Average expansion of the dough in the X and Y directions during baking.</figcaption></figure>

For advanced analysis, the mean expansions in X (width) and Y (height) directions and the inside bread temperature are illustrated in Figure 10. To measure the bread core temperature during baking, the thermocouple is inserted inside the dough. This insertion may cause some gases to leak out, which possibly affects the bread expansion. Experimental tests performed on bread without and with sensor insertion using the same initial conditions showed that expansion in the X (width) and Y (height) axes follows the same pattern without remarkable deviation, concluding that sensor insertion had no significant effect on expansion.

<figure><img src="image_13.png" /><figcaption>**Figure 10.** Relationship between expansion in the Y direction and core temperature during baking.</figcaption></figure>

In Figure 10, the Y (height) expansion and temperature show a similar trend with a
small phase delay between the two curves. Inflation begins, and fast temperature increases
are time-shifted by 1 s. The fast temperature increase begins in the 4th second, starting at a temperature of 60 °C and reaching a peak of 99 °C after 10 s. One second later, the Y (height) inflation starts, rapidly increasing towards a maximum of 100 mm after 11 s of baking and stabilizing. Similarly, the temperature and Y (height) expansion stabilize with the same time shift of one second. On the other hand, the X (width) expansion shows a slight contraction as the Y (height) expansion rapidly increases, indicating a redistribution of dough volume during baking. This behavior aligns temporally with the sharp rise in bread core temperature, suggesting a correlation between thermal expansion and structural transformation. While not strictly linear, these expansion trends reflect the physical mechanism of internal steam expansion driving vertical puffing and lateral contraction, characteristic of pocket formation in Lebanese flatbread.

To characterize the thermal expansion behavior of the bread during baking, Figure 11 illustrates the expansions plot as a function of the temperature inside the bread core. This behavior is empirically described for the X (width) and Y (height) expansion in Equations (6) and (7), respectively:

$$DEF_{X(T_c)} = -0.008 T_c^3 + 0.1145 T_c^2 - 6.601 T_c + 339.7454 \quad (6)$$

$$DEF_{Y(T_c)} = 1.6778 \times 10^{-5} T_c^4 - 3.8 \times 10^{-3} T_c^3 + 0.3163 T_c^2 - 10.52 T_c + 123.31 \quad (7)$$

where $T_c$ is the core temperature in °C. The regression analysis shows the following values: for $DEF_{X(T_c)}$, $R^2$ value of 0.9933, and a Residual Standard Error of 1.3 for $DEF_{Y(T_c)}$.

<figure><img src="image_14.png" /><figcaption>Figure 11. Expansion and volume as function of the bread core temperature.</figcaption></figure>

$R^2$ value of 0.99682 and Residual Standard Error = 2.4. Assuming that the bread expansion is symmetrical and has an ellipsoid shape, the swelled volume can be calculated using Equation (8):

$$V = \frac{4}{3} \times \pi \times \left( \frac{DEF_x}{2} \right)^2 \times \left( \frac{DEF_y}{2} \right) \times 10^{-3} \quad (8)$$

where $DEF_x$ is the expansion in the X (width) direction in mm, $DEF_y$ the expansion in the Y (height) direction in mm, and V the swelled volume in ml.
Figure 11 depicts the detailed progression of expansion in both the X and Y directions as a function of the core temperature during the baking process. Additionally, it illustrates the corresponding changes in the volume of the bread pocket, providing a comprehensive overview of how these variables evolve simultaneously.

The volume expansion behavior observed in this study is primarily governed by two key phenomena: carbon dioxide release during fermentation and rapid water vaporization during baking [44]. However, the primary factor for pocket formation is the rapid expansion of water vapor at the high temperatures of the tunnel oven. The water in the dough rapidly vaporizes, creating internal pressure that separates the upper and lower layers and forms the characteristic pocket. Steam expansion is significantly more dominant than CO₂ production, particularly given the short baking time of Lebanese flatbread. This finding aligns with earlier observations in similar flatbreads baked at lower temperatures in an electrical deck oven [8,53] and these results confirm that this mechanism holds true even under higher energy transfer conditions typical of tunnel ovens operating at up to 650 °C. The thin structure of the dough, combined with rapid moisture loss, facilitates this rapid expansion [53]. Additionally, the temperature gradient between the upper and lower surfaces, evident from our thermocouple measurements, plays a key role in determining the pocket’s shape and uniformity, as the upper surface heats more rapidly due to direct flame exposure, accelerating crust formation [9].

### 3.4. Energy Analysis

The minimum energy demand for bread baking is the sum of the energy required to cook the dough, including the amount of heat needed to gelatinize the starch within the dough, and the energy required for water evaporation [54–56]. The required energy to bake the dough is given by the following:

$$Q_{bd} = m_{d} C_{pd} (T_{fd} - T_{bd}) \qquad (9)$$

where $m_d$ is the mass of dough, $C_{pd}$ the mean value of the specific heat and $T_{bd}$ and $T_{fd}$ the dough’s initial and final temperatures, respectively. The required energy for water evaporation is given by the following:

$$Q_v = m_W L_W \qquad (10)$$

where $m_w$ is the evaporated water mass during Lebanese bread baking, and $L_w$ is the water latent heat.

The required energy to increase the baked bread temperature is as follows:

$$Q_{bb} = m_b C_{pb} (T_{fb} - T_{bb}). \qquad (11)$$

where $m_b$ is the mass of baked bread, $C_{pb}$ the mean value of the specific heat of bread and $T_{bb}$ and $T_{fd}$ the bread’s initial and final temperatures, respectively.

Determining the actual surface temperature of the bread during baking presents a unique challenge due to several factors. The measured values during baking do not precisely represent the actual surface temperature, primarily due to the influence of gas temperatures and heat fluxes in the oven. Additionally, measurements taken immediately after baking must account for rapid temperature gradients as the bread exits the oven, as cooling begins quickly. To address this, the method used to estimate surface temperatures involves averaging the recorded values, which incorporates both the experimentally measured temperatures from various points within the baking chamber and the temperature readings taken immediately post-baking.
Furthermore, it is essential to acknowledge that the upper and lower surfaces of the bread do not exhibit identical temperatures during baking. This discrepancy arises because of different exposure levels to the heat sources and the variation in heat flux levels on each surface. To account for these differences, we treated the bread as two hypothetical layers representing the upper and lower surfaces and then averaged the temperatures of these layers to estimate the final overall surface temperature. Based on these considerations, the estimated average final temperature of the bread was found to be approximately 170 °C. This figure was used to represent the comprehensive thermal conditions experienced by the bread during the baking process.

Table 4 presents the temperature, specific heat, and mass values of Lebanese bread and dough obtained from previous experimental campaigns conducted in our laboratory under conditions similar to those in this study.

<table><caption>**Table 4.** Temperatures, specific heat, and Lebanese bread and dough mass.</caption><thead><tr><th>m<sub>d</sub><br>kg</th><th>m<sub>b</sub><br>kg</th><th>m<sub>w</sub><br>kg</th><th>T<sub>fd</sub><br>°C</th><th>T<sub>bd</sub><br>°C</th><th>T<sub>fb</sub><br>°C</th><th>T<sub>bb</sub><br>°C</th><th>C<sub>pb</sub><br>kJ/(kg·K)</th><th>C<sub>pd</sub><br>kJ/(kg·K)</th><th>L<sub>w</sub><br>kJ/kg</th></tr></thead><tbody><tr><td>1</td><td>0.851</td><td>0.149</td><td>100</td><td>35</td><td>170</td><td>100</td><td>2.4</td><td>3.2</td><td>2250</td></tr></tbody></table>

The total energy required to bake Lebanese bread is estimated at 667 kJ/kg. The oven-rated production capacity is 150 kg/h of dough. Thus, the energy demand for cooking the dough should be only 28 kW. The efficiency of the oven is given by Equation (12):

$$ \eta = \frac{\textit{Power needed to bake the bread}}{\textit{Oven rated power}} \qquad (12) $$

Although the oven's rated power is 130 kW, our energy analysis revealed that the oven operates with a thermal efficiency of only 21% (28 kW/130 kW), consuming 3.15 MJ to bake 1 kg of Lebanese bread. The efficiency drops to 16% when 3 h of preheating are added to a 10-h daily operation.

According to CFD simulations conducted on a similar industrial tunnel oven [2], approximately 37 kW of the total energy input is lost through the oven walls and exit openings. The remaining 65 kW primarily heats the circulating air, with about 50% of this heating energy subsequently lost through the chimney and 29% through the oven walls [4].

This analysis highlights the inefficiency of the Lebanese bread-baking process, where a significant portion of the energy is lost through openings and flue gases. Furthermore, the energy consumption for baking Lebanese bread is higher than that of other flatbreads, such as Egyptian balady bread (which uses 2.8 MJ/kg) and Magr bread (3 MJ/kg) [57,58]. However, it is lower than the energy consumption required for baking French bread, with an average of 4 MJ/kg [49].

Energy consumption varies considerably depending on various factors, such as baking technique, oven type, efficiency, and recipe specifications.

On average, industrial baking processes consume between 1–2 MJ/kg and 4 MJ/kg, with some methods exceeding 4 MJ/kg, with an average of 3.7 MJ/kg depending on factors such as oven efficiency and baking duration [54,59].

## 4. Conclusions

This study presents the first detailed experimental investigation of Lebanese flatbread baking, capturing real-time temperature and expansion data, along with pre- and post-baking measurements of weight loss under high temperature. This study reinforces the well-established understanding that rapid water evaporation is the primary driver of pocket
formation in flatbread, as previously shown in recent studies. These experimental results confirm that this mechanism remains dominant, and even more pronounced, under high-temperature baking conditions characteristic of traditional Lebanese ovens. Additionally, the observed temperature gradients between the upper and lower surfaces further support their role in differential heating, which influences browning behavior during baking.

The findings confirm that Lebanese bread experiences significant water loss due to extreme baking temperatures, a defining characteristic differentiating it from other bread types. The rapid water evaporation and steam expansion lead to the expansion necessary for pocket formation, with temperature gradients contributing to uneven browning and crust formation. Additionally, the energy analysis revealed that the baking process is thermally inefficient, highlighting the need for improvements in oven insulation and alternative heating methods.

This study highlights the critical nature of the Lebanese bread-baking process, which can only be achieved under specific baking conditions. While this study focused on one formulation and one set of conditions, it lays the groundwork for future research to explore the impact of different variables. This study serves as the first step in characterizing the complex baking process of Lebanese flatbread, an area that has not been extensively studied before. Due to the novelty of this subject, future research will be needed to investigate various factors influencing pocket formation, dough behavior, and thermal dynamics during baking. The empirical models developed in this study will form the foundation for future work aimed at refining baking processes and improving product quality.

Further research should focus on extending the investigation into different oven geometries, power configurations, and heating methods to validate and generalize the results. Exploring alternative heat sources, such as infrared or microwave-assisted baking, could offer pathways to improving energy efficiency while maintaining product quality. In parallel, investigating the effects of varying dough formulations and fermentation conditions will provide deeper insights into their influence on baking behavior and final bread characteristics. Additionally, while browning and flavor development are essential quality markers in flatbread baking, future optimization should also address the control of Maillard-related contaminants such as acrylamide to ensure both sensory appeal and food safety.

The empirical models developed in this study can also be extended to other types of flatbreads and used to design more efficient, sustainable, and standardized baking systems. Ultimately, this research lays a foundation for optimizing both the quality and efficiency of Lebanese bread production in industrial settings. By refining baking models and understanding the underlying mechanisms, better quality control and innovation in bread production can be achieved, ensuring that Lebanese bread maintains its traditional appeal while meeting modern production demands.

**Author Contributions:** Y.M.: Methodology, Data curation, Writing, original draft preparation, Writing, Review and editing, Funding acquisition. P.R.: Conceptualization, Supervision, Writing, review, and editing. N.E.H.: Funding acquisition, Writing, review, and editing. O.R.: Conceptualization, Supervision, Writing, review, and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by FARHAT Bakery Equipment, which provided financial support, instrumentation, and the location for conducting the experimental tests.
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.

**Acknowledgments:** We would like to acknowledge the team at FARHAT Bakery Equipment and the staff at the USEK School of Engineering for their valuable administrative and technical support throughout this study. Their assistance and resources greatly contributed to the successful completion of this research.

**Conflicts of Interest:** The authors declare that this study received funding from FARHAT Bakery Equipment. The company provided the instrumentation and the location for conducting experimental tests. The funder had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. The authors declare no conflict of interest.

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