Simulation and experimental validation of heat and mass transfer during Chinese stir-frying considering shrinkage

Abstract: Food stir-frying is one of the most popular and widely used cooking method to prepare vegetables, particularly in southeast Asian countries. This quick food preparation method is used the heat transfer from a hot pan surface to foods, using a small amount of cooking oil. To promote the development, automation, and standardization of Chinese cuisine, it is necessary to carry out a systematic investigation for the heat transfer and the quality changes of food during the cooking process. Numerical simulation can offer one way to explore the heat and mass transfer process of food particles during the food stir-frying. In this study, a heat and mass transfer model was developed, including multiphase coupling phase transition and shrinkage, in order to simulate the stir-frying process of food hygroscopic porous medium, using the porous media theory, Fourier's law, Newton's cooling law, and Darcy's law. An emphasis was also placed on the mechanism of heat transfer, and the variation in maturity and quality of food particles during the Chinese cuisine process. The model considered the non-equilibrium evaporation and shrinkage formulation, energy, momentum and mass conservations of water, and gas governing equations, thereby finally solved by finite element method. The accuracy of model was verified by the temperature history, moisture content, and volumetric shrinkage rate of stir-fried pork loin. The results indicated that the accuracy and robust properties of this model greatly increased after considering the shrinking process. Some parameters, including the water evaporation rate of particle surface, volumetric shrinkage rate, pressure variations, moisture content and temperature distributions, were simulated to reveal the mechanisms of heat and mass transfer inside the food particle for the stir-frying process. The simulation results showed that the main reason for shrinkage was the water loss that induced by strong convection heat transfer. The surface evaporation rate can increase the rate of moisture loss and volumetric shrinkage. The particle internal pressure also determined the volumetric shrinkage. Since the function of volumetric shrinkage rate was analogous to that of water loss rate, the moisture content and shrinkage were associated with an important indicator to evaluate the cooking quality of food particles. Additionally, the heat transfer efficiency of particles was greatly enhanced by shrinkage, because of the increasing surface area to volume ratio. The shrinkage can be used to improve the overall moisture content of food particles, if evaluated from the perspective of cooking quality. Combined with the maturity value theory, further investigation also made on the effect of controlling methods of "Huohou" on the maturity and quality of food particles. The finer cutting technique of food particles can mainly determine to increase in efficiency of heat and mass transfer and internal heating rate, whereas, to decrease in average moisture content. The stirring operations can be vigorous, as the preheating oil temperature increased. The food particles can reach its maturity values, before the average moisture content rapidly decreased under the appropriate "Huohou" controlling. Thus, it infers that the control of "Huohou" can be used to dramatically reduce the average time of food particles for reaching cooking maturity termination, indicating a significant (P <0.01) effect on the cooking quality. The finding can bring important advantages to obtain food particles with better cooking quality during the food stir-frying process.