Developing a rotary bread baking machine with an indirect heating system to increase the product quality using numerical techniques

Introduction: Bread is one of the most important food sources for consumers, especially in developing and underdeveloped countries. Large quantities of bread are wasted every year because different factors affect the number of waste products include flour quality, bread production technology, and the storage condition of the bread. Among the factors affecting the quality of the flatbread, technological factors including the source and method of heating the dough and the type of baking bed are very important. Those factors affect the quality and staleness of the bread and the amount of energy consumed per ton of the produced flatbread. All of these issues depend on how heat and mass are transferred. At present, for the baking of the flatbread, direct heat transfer of the burner (torch) to the dough is utilized in the Iranian bread industry. The use of the direct heat method in baking causes non-uniform heat distribution and non-uniform baking of the flatbread. Also, it causes the deposition of some chemical compounds such as benzopyrenes on the surface of the bread due to incomplete combustion of fuel and gases from the burning process of fossil fuels. At present, the change in the structure of bread production from traditional methods to industrial methods is one of the most important factors in the production of safe and standard bread while improving its quality. The objective of this study was to improve existing systems and developed the new flatbread industrial baking machine to increase product quality and product storability. Materials and methods: The design and construction of the oven were performed based on the existing machine in the bread industry. The final dimension of the bread baking machine was 250 ×250 cm, a height of 160 cm. Rotary bread baking machine with an indirect heating system was designed and heat exchangers of the machine were simulated. The uniformity of airflow velocity inside the exchangers and the distribution of heat on the dough on the baking bed were investigated. After achieving the most effective design, the exchangers were constructed and installed inside the baking machine. A rotary baking bed was constrained with cast iron with a diameter of 220 cm and a thickness of 18 mm that was able to rotate horizontally. A three-phase electric motor with a speed of 1400 rpm and a power of 1.5 kW was used for the rotation system of the bed and carousel assembly. Two heat exchangers with an optimized shape were constructed with SS310 coupled with 2 burners. Bread baking experiments with the developed machine and the existing conventional machines were conducted to evaluate the sensory and staling characteristics of the produced flatbreads from different baking machines. Results and discussion: The simulation results indicated that the air velocity in the center of the middle pipes of the upper exchanger with a standard deviation of 0.05 was in the range of 1.552 to 1.72 ms-1, which means an acceptable uniformity. In the lower heat, there was a relatively uniform air distribution. The experimental validation of the developed simulator indicated that there is good agreement between the measured and predicted air velocity. In the top exchanger, R2 and RMSE were 0.9 and 0.36, respectively and in the top exchanger, they were calculated to be 0.97 and 0.218, respectively. The results indicated that the sensory qualities of the bread produced with the indirect heating system in the present study as well as uniformity of baking and sensory acceptability were superior compared to the bread produced from the conventional industrial machines with direct heating systems (with a total sensory score of 5.09 vs. 4.54). Mold was also observed in a few breads, with direct heating samples, after 168 hours of storage. However, no significant changes were observed in the quality of the bread cooked by direct heating methods after 48 hours of storage, and this type of flatbread lasted more than 168 hours without mold. It should be noted that mold is one of the most important quality deterioration factors and its weight in the quality assessment of the bread is greater than the other quality evaluation factors even though it has not been included directly in the evaluation standards. If the bread becomes moldy, it will be unsafe for consumption and fully wasted. Accordingly, the flatbread produced by the indirect heat was the best choice for long-term storage purposes and the reduction of food waste. Another feature that was evaluated in the bread was the uniformity of baking of all parts of the flatbreads. One of the main problems with the industrial direct heat baking machines is the fact that the outer edges of the flatbread burn due to the inadequacy and somehow incompatibility of the exchangers with the baking space, the dimensions of the flatbread, and incorrect design of the heat exchangers. The flatbread baked with indirect heat and the newly designed exchangers was very uniform with no signs of burning in the outer edges of the bread. Conclusion: In general, the results of the current study indicated that the use of the simulation tools, the design and manufacturing of the new exchangers and uniform indirect heating of the baking bed resulted in the production of higher quality bread which could be stored for a longer period of time while reducing the food waste. Also, bread prepared using the new machine was free of any contaminants such as PAHs.