Briefly describe the principle of food drying? What are the changes in the food during the drying pr

Food sewage treatment equipment now usually uses food integrated treatment equipment, this equipment can not only be buried on the surface, the surface can also be planted flowers and plants, do not occupy an area, do not need to build a house and heat preservation, the whole equipment has a fully automatic electrical system and equipment failure alarm, relatively safe, usually do not need special management, as long as regular maintenance and repair can be. Weifang Tianyuan Environmental Protection Technology Co., Ltd. is a high-tech enterprise integrating the production and research and development of environmental protection equipment, and is now the cooperation base of Shandong Academy of Water Conservancy Sciences, and was designated as a key recommended enterprise by Shandong Provincial Environmental Protection Bureau in 2011. The company is located in a well-known"The home of dinosaurs"Shandong Zhucheng is an important transportation hub on the Shandong Peninsula.

The company mainly produces air flotation equipment, industrial sewage treatment equipment, domestic sewage treatment equipment, hospital sewage treatment equipment, slaughtering sewage treatment equipment...

In the drying process, if we consider a simple situation, the moisture on the surface of the food is first converted from liquid to gaseous (i.e. water evaporation) after heating, and then water vapor.

Diffusion from the surface of the food to the surrounding medium, so the moisture content of the surface of the food is lower than its interior, and then the moisture difference or moisture gradient is established on the surface of the food and the internal interval, which will promote the continuous transfer of the moisture inside the food to the surface, so that not only the surface moisture is reduced, but also the internal moisture is continuously reduced. However, in complex cases, water evaporation will also occur in certain areas or even in a full range of the food, so the water inside the food may diffuse and transfer outward in a liquid or vapor state. At the same time, when the food is placed in the environment or condition of hot air, as soon as the food is in contact with the hot air, the heat in the hot air will first be transmitted to the surface of the food, and the temperature of the surface will be correspondingly higher than the inside of the food, so there will be a corresponding temperature difference or temperature gradient on the surface and inside the food, and with the extension of time, the temperature inside the food will reach the same temperature as the surface, and the existence of this temperature gradient will also affect the food drying process.

1) Shrinkage and formation of porous structures. When food is dried, the volume decreases due to the removal of water, and the elasticity of muscle tissue cells is partially or completely lost, resulting in shrinkage. The degree of shrinkage is related to factors such as the type of food, drying method and conditions.

After shrinkage, the so-called porous structure may be generated, which is conducive to the rehydration of dry products and the reduction of bulk density of dry products, but the oxidation rate is accelerated, which is not conducive to the preservation of dry products.

2) Case hardening. Case hardening refers to the phenomenon that dry products are dry on the outside but still soft and wet on the inside. It is mainly caused by the excessive dryness of the food surface, and the speed of internal moisture migration to the surface lags behind the vaporization rate of surface moisture, so that the surface layer forms a layer of dry hard film.

After the surface hardening, the air permeability of the food surface will deteriorate, causing the drying speed to drop sharply and prolong the drying process.

3) Dehydration and denaturation of proteins. After rehydration, the appearance, water content and hardness of dried products containing more protein cannot return to the state when they were fresh, which is mainly caused by protein dehydration and denaturation. The degree of change of protein during the drying process mainly depends on the drying temperature, time, water activity, and pH value.

Factors such as fat content and drying method.

4) Lipid oxidation. Although the water activity of dried products is low, and the activities of lipase and lipoxidase are inhibited, due to the lack of water protection, it is easy to automatically oxidize lipids, resulting in the deterioration of dried products. It can be used to reduce the storage temperature and use appropriate relative humidity.

Vacuum-packed, using fat-soluble antioxidants, etc.

5) Discoloration. After drying, food will be affected by the color substances it contains, such as carotene.

Anthocyanins, myorubin, chlorophyll.

and so on, such as yellowing, browning, blackening, etc. One of the most common discolorations is browning. There are two causes of browning, one of which is polyphenols such as tannins.

Tyrosine and other compounds that produce brown compounds under the action of polyphenol oxidase in tissues; The second is non-enzymatic browning.

Drying includes both natural drying and artificial drying. The former is to dry or dry food with the help of sunlight or moving air, such as rice, oilseeds, nuts, fruits, etc., after harvesting; The latter, also known as dehydration, is the application of mechanized production lines to complete the drying process in a relatively short time, such as the dehydration of vegetables and fruits, and the vacuum drying of malt milk essence and fruit juice. As for the baking and frying of food, in addition to the evaporation of water, a series of chemical changes have also occurred, which do not belong to the scope of food drying.

Principle Spoilage of fresh food is caused by many chemical changes caused by enzymes and microorganisms. Enzymes are a component of the food itself, and need proper moisture to play its role, such as reducing the moisture of the food to less than 1, the activity of the enzyme will disappear. Drying does not kill microorganisms, but when food is dried, microorganisms also lose moisture and then become dormant and die (see Food Microbes).

Food is dried and can be stored for a long time because both enzymes and microorganisms are inactive. If the dried food is damp, at the right temperature, the activity of the enzyme will be partially restored, and the residual microorganisms can multiply again, and the food will still rot and deteriorate.

The moisture in fresh food can be divided into 3 categories: capillary water that can be removed by simple methods or thermal action or water adhered to surface tension is called free water or free water; Water that is bound to colloids such as proteins and is generally not easy to evaporate is called physically bound water; Water that is firmly bound to other substances according to the quantity and cannot be separated by a simple method is called chemically bound water. Of these three types of water, only free water can be used by enzymes and microorganisms.

The amount of free water is shown in terms of water activity aw

Dry storage of food. Dry storage of food.

where p is the vapor pressure of the food and p0 is the saturated vapor pressure of pure water at the same temperature. Any food contains water-soluble substances that will reduce the vapor pressure of the food, so P is always lower than P0, i.e. AW is always less than 1. For example, fish, meat, fruits, and vegetables with high water content have an AW of; Rice, soybeans with low moisture, AW is.

The growth of various microorganisms has different requirements for AW: bacteria, yeasts, and molds. AW is below that, the vast majority of microorganisms cannot grow.

Therefore, if the food is dried to less than AW, it can be stored for a long time, except for hyperhalophilic bacteria, hypertonic yeast, and drought-loving molds.

The drying speed of food is determined by the characteristics of the material, such as chemical composition, morphology, surface area, stack thickness, etc., and the type of dryer in the drying conditions, drying temperature, humidity, air flow rate, atmospheric pressure, etc. The time required for different drying processes varies greatly, such as naturally dried raisins and dried cantaloupe, which generally take several weeks, while spray drying to make milk powder and egg powder only takes a few seconds. Regardless of the length of the drying process, the change in moisture can be illustrated by the three typical curves in Figure 1.

These 3 curves are 3 phenomena that occur simultaneously in the same drying process. Xinqiao curve 1 represents the drying curve, which is the absolute water in food.

1) Physical Change:

Shrinkage and cracking: When the food is dried, the volume is reduced due to the removal of water, and the elastic part of the muscle tissue cells or the part of the whole faith is lost;

Surface hardening: a special phenomenon of surface shrinkage and closure of food materials, and food containing high concentration of sugar and soluble substances is prone to surface hardening when drying.

Texture change: the water is removed when drying, due to the concentration of heat and salt mountain envy, it is easy to cause protein denaturation, the denatured protein can not completely absorb water, and the starch and most colloids also change and its hydrophilicity decreases;

Porosity formation: the so-called porous structure may be formed after the dry shrinkage of the slip section;

2) Chemical changes:

Changes in nutrient composition: protein dehydration, denaturation, lipid oxidation, loss of vitamins;

Changes in the color of food: the color of fresh food is generally bright, and drying will change its physical and chemical properties, so that the ability of food to reflect, scatter, absorb and transmit visible light will change, thereby changing the color of food;

Changes in food flavor: The food loses its volatile flavor components.

Increases air velocity.

Increase the temperature. Increase the vacuum.

Reduce the granularity of the material (increase the surface area).

Preferably combined.

For example, in the production of milk powder, it is sprayed into a mist to increase the specific surface, and the temperature of the liquid is increased, so it dries quickly. Some even use vacuum suction and pressure injection, which dries faster.

In the production of raisins, a drying room is used, the temperature is raised, and a fan is used to speed up the air flow.

The drying curve of each material is different.

You'd better read chemistry or food engineering books, principles of chemical engineering, etc.

Seven years after I graduated from food, I have never touched food, and I hope this old instinct can help you.