Review of moisture and heat transfer mechanism during drying process of noodles

The drying process of Chinese dried noodles was a complex process coupled with moisture and heat transfer, and it was difficult to control. Improper drying conditions would lead to the quality decline, which was mainly affected by the moisture and heat transfer. Chinese dried noodles had compact structure, low porosity and small thermal conductivity, so the drying process was a falling rate period, in which the internal moisture transfer was the major limiting factor of the drying process. Liquid diffusion and thermal conduction were considered as the main moisture transfer and heat transfer mechanism respectively, and vapor transfer was supposed to exist in the drying process. The researchers have not reached consensus on the mechanism of moisture and heat transfer, but mathematical models based on the Fick??s Second Law and effective moisture diffusion coefficient were popularly used in the drying process of food. These models were simple and rapid on solving, and could describe the drying process of foods when several transfer mechanisms were considered in effective moisture diffusion coefficient. During the drying process, heat was transferred along with moisture transfer, so the moisture and heat transfer must be considered simultaneously in order to develop a more accurate drying model. Many models were empirical models or developed with too many assumptions, and model validations were mostly based on the lumped moisture content instead of water distribution of the noodles. Thus, a more accurate model should be developed based on fewer assumptions and more accurate model parameters, such as the effective moisture diffusion coefficient, thermal conductivity, specific heat capacity and shrinkage. Then the model should be validated by the water distribution of the noodles, by which the model could be verified more accurately. The nuclear magnetic resonance (NMR) was a good technology to measure the water distribution and the water status in samples, which could reflect the binding energy of water molecules to the noodles and help determine the moisture transfer mechanism. It also could be used to investigate the glass transition and mechanical properties across its cross-section in pasta, by which some quality problems could be interpreted, such as stress-cracking. Finally, it was easy to control the temperature and humidity in the drying rooms nowadays, but it was hard to confirm why the controlling time and value were the optimum. Thus, it was suggested that mechanism research and mathematical modeling of moisture and heat transfer should be paid more attention in the drying process of Chinese dried noodles. We hope that a more accurate moisture and heat transfer mathematical model was built and embedded into automatic control system. According to the current moisture content and drying time, the mathematical model could work out the proper drying conditions in the rest time. It ensured that all noodles could be dried in personalized conditions, and moisture content and production quality would be stable and uniform. This process could be named as precise drying, and it would bring many benefits, such as less labor, more accurate drying conditions, and more uniform production quality.