When industrial frying machine are processing fried products, the food is placed in a hot oil of a certain temperature first, the oil can provide rapid and uniform conduction heat energy, the surface temperature of the food is rapidly increased, moisture is vaporized, and a dry layer appears on the food surface, form a hard shell. Then, the moisture vaporization layer migrates to the interior of the food. When the temperature of the food surface rises to the temperature of the oil, the temperature inside the food slowly goes to 100°C. At the same time, the caramelization reaction and protein denaturation occur on the surface, and other substances decompose and produce unique deep-fried aroma. The rate of frying infection depends on the temperature difference between the oil temperature and the interior of the food and the thermal conductivity of the food.
 
During the frying process, the dry layer on the surface of the food has the characteristics of porous structure, and the size of its voids is not equal. During the frying process, water and steam are precipitated from these large voids first, and then the space occupied by water vapor is replaced by hot oil. The water migrates through the oil film. The thickness of the oil film interface layer controls the progress of heat transfer, its thickness and oil. The viscosity is related to the flow speed. Similar to the hot air drying, the driving force of dehydration is the difference in the vapor pressure of the moisture inside the food. As the surface layer of the food is hardened during frying, vaporization of the food inside the food is hindered, a certain vapor pressure is formed, the penetration of the water vapor is enhanced, and the food is rapidly cooked. Therefore, the fried food has golden color, crisp and tender taste.

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