No physical changeNew substancesGenerated changes.For example, solid ice melts into water when heated, and liquid water evaporates into steam;Water vapor condenses into water, and water freezes into ice.Chemical change is defined as when one molecule contacts another to synthesizemacromolecule；Or molecules are broken and separated to form two or more small molecules;Or the recombination of atoms inside the molecule.Physicochemical changes are reactions in which both physical and chemical changes occur.
A physical change is a change in which no new matter is formed.For example, solid ice melts into water when heated, and liquid water evaporates into steam;Water vapor condenses into water, and water freezes into ice.In the three state change, water only changes its shape and state.No new substance is produced, so it belongs to physical change.Another example is diffusion, aggregation, expansion, compression, volatilization, sublimationfriction generates heat, iron changing magnet, electrified heating luminescence, activated carbon adsorption of chlorine, etc. are all physical changes.Under certain conditions, the transformation of graphite into diamond is not a physical change, but a chemical change, because it becomes another simple substance.Before and after physical changes, the type and composition of substances remain unchangedchemical propertySame as before.The essence of such changes is that the aggregation state of molecules (separation distance, movement speed, etc.) has changed, leading to the change of the shape or state of substances.Physical changes represent the physical properties of the substance.Physical changes are essentially different from chemical changes.
Chemical change is defined as when one molecule contacts another to synthesize macromolecules;Or molecules are broken and separated to form two or more small molecules;Or the recombination of atoms inside the molecule.In order to form changes, chemical reactions are usually related to the formation and fracture of chemical bonds.Essentially, it is a process of atomic recombination, which may involve the exchange of electrons. With the breaking of old bonds and the formation of new bonds, there must be energy changes.
Physical and chemical changes during food drying
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1. Shrinkage and deformation
When the water in the animal and plant tissues composed of cells gradually loses during drying, the cells shrink and the whole body becomes smaller.Sometimes, due to the rapid drying, the pores on the surface shrink, forming a hard film with poor permeability, resulting in a sharp decline in the drying speed. When the internal moisture continues to gasify, internal pressure is formed, resulting in the expansion of bubbles or cracks.
Rapid drying often leads to uneven drying speed of each part of the material, which leads to uneven internal stress contraction of the material, leading to bizarre warping deformation, such as drying of baked bran, frozen tofu, etc. This deformation can be restored to a certain extent when the material is rehydrated.
The deformation of materials during drying is also reflected in the internal organizational structure.When the drying process is slow, the internal and external moisture content gradient is small, and the internal stress of the material is small. When drying and shrinking, the original shape can be relatively maintained, and the organizational structure is relatively dense.If the drying is fast, the surface will be shaped first due to drying. When the interior is further dried and shrunk, greater stress will be formed to form cracks and pores in the structure. For example, this phenomenon occurs when diced potatoes or carrots are dried by fast hot air.
During the freezing vacuum drying process, there is no drying shrinkage or deformation.The wet material has been completely shaped when frozen.Under vacuum conditions, ice sublimates and directly gasifies. After drying, the material remains the size and shape of the original freezing without deformation, but the internal organization is loose.
2. Solute migration
In the water contained in food materials, there are usually solutes dissolved in vacuum, such as sugar, salt, organic acid, soluble nitrogen, etc. When the water is transferred from the inside of the material to the surface in the drying process, the soluble substances will also move to the surface.When the solution reaches the surface, water vaporizes and escapes, and the concentration of solute increases.When the drying speed is fast, the dehydrated solute may accumulate on the surface of the material to crystallize or become dry gel, forming a dry hard film on the surface, or even blocking pores to further reduce the drying speed.If the drying speed is slow, when the concentration of the solute near the surface layer gradually increases, the solute can diffuse to the central layer again with the driving force of the concentration difference, so that the solute in the interior of the material tends to be uniform again.Obviously, the uniform distribution of soluble substances in dried materials is related to drying process conditions and drying speed[1]。
3. Volatile component loss
When water escapes from the dried food materials, it is inevitable to carry some volatile substances in the food, which often represent the flavor of the food and are lost during drying.For example, in the manufacture of instant coffee or instant tea or fruit powder, its characteristic flavor substances are always more or less volatile.When the relative drying surface area (the ratio of drying surface area to material weight) is larger and the drying temperature is higher, the volatilization loss is also larger. It is generally believed that this situation is the most serious in spray drying, while the loss is less in freeze sublimation drying, because the former has a large drying surface area and high temperature, while the latter has a small surface area and low temperature,The water with smaller molecular weight escapes before the organic flavor molecules with larger molecular weight.
At present, there is no economic and reasonable way to reduce the loss of volatile flavor substances in the drying process or to recover them for compensation.
4. Hygroscopicity and resilience
The drying of food is carried out and completed under the condition of the partial pressure difference of water vapor of materials and media. The moisture content of the dried food materials is generally very low, and the vapor pressure is usually lower than the partial pressure of water vapor of normal atmosphere.In other words, the relative equilibrium humidity of dry materials is very low. Under normal atmospheric conditions, it is easy to absorb moisture in the atmosphere and regain moisture. At this time, the water activity in food materials increases again and reduces the quality, which is vulnerable to microbial damage.Therefore, the dried food must be properly packaged to prevent re absorption of moisture before consumption.
grossDehydrated foodRehydration is required before consumption to restore to the state before dehydration as far as possible.However, rehydration is often difficult or not ideal after rehydration.The rehydration time may be very long, or the original shape and sensory quality (color, fragrance, taste) cannot be recovered.It is usually impossible for materials with cellular structure to return to the original state. Except for very special circumstances (such as dry yeast and other single cell materials), drying is an irreversible process.Dehydrated vegetables need to blanch the enzyme before drying, so the cells will lose their vitality, the cell wall will lose its elasticity, resulting in permanent deformation, and the colloidal composition will also have irreversible changes.The dried fresh meat can only absorb a part of the original water content when rehydrating, and its tissue structure is a little crisper than that before dehydration.Thin and fine materials dried in freezing vacuum may be closer to the original shape and quality after rehydration.Liquid substances, such as milk, coffee and fruit juice, can be well rehydrated and recovered after spray drying or freeze drying.However, because the dried milk powder contains fat, and the original emulsification condition has been destroyed after drying, additional emulsifiers and homogenization must be added when rehydrating and reducing.
The dehydrated noodles sold in the market are dehydrated under the condition of frying, so the taste of rehydrated noodles is different from that of the original noodles.For dehydrated noodles and dehydrated rice dried by hot air, the rehydration time is generally longer, and it takes more than 8 minutes to use hot water above 95 ℃. Only frozen and dried products have ideal rehydration performance, but the economic cost is much higher.
5. Nutritional damage
Food ingredients are generally heat sensitive substances, which are inevitably not damaged during the humidification and drying process.Various vitamins are the components with the largest loss ratio in heating and drying.Water-soluble vitaminVitamin C (ascorbic acid) is the most easily damaged under high temperature, and vitamin B (thiamine) is also very sensitive.CaroteneIt will also suffer losses due to oxidation. The carotene loss of vegetables without enzyme passivation is as high as 80% during drying. If the dehydration method is appropriate, it can be reduced to 5%.
carbohydrateIt is easy to decompose and coke when drying at high temperature for a long time.For example, when the drying temperature of Daliyuan jujube is 70 ℃, the sugar loss is 12.3% for 10h and 16.4% for 34h;At 65 ℃, the sugar loss was only 6.5% after 34 h.
proteinSensitive to high temperatureProtein denaturationThe amino acids that make up the protein react with reducing sugar to produce Maillard reaction (carbonyl ammonia reaction) and browning.The rate of browning varies with temperature and time.The browning is obviously serious after drying at high temperature for a long time.When the temperature of the material reaches a critical value, it will become brown quickly.The browning rate is also related to the moisture content of the material.At the beginning of drying, the moisture content of materials is high and browning reaction is slow. However, when drying to a certain intermediate moisture content (usually between 15% and 20%), browning speed reaches the highest value. When the moisture content drops to 1% to 2%, most dehydrated foods can be stable for a long time even at a relatively high storage temperature.Therefore, the drying process should be designed so that the materials can pass the stage of fast browning moisture as soon as possible.
The grease rancidity of dried products is a very serious problem.Generally speaking, high-temperature drying is much more serious than low-temperature drying, and atmospheric drying is much more serious than vacuum drying.Therefore, it is often necessary to addantioxidant。
The rancidity of oil is related to oxygen contact.When the moisture content of food materials is high, the moisture hinders the direct contact between food ingredients and air.When drying, when the moisture content of the material is reduced to a certain extent, that is, the necessary protective water stratification that prevents the direct contact between food ingredients and air is removed, that is, the minimum necessary moisture content of the dried food, the oil in the food is vulnerable to oxidation, the color fading of carotenoids, etcnonenzymatic browning , and even deterioration such as decreased rehydration.According to Saluin's theory, the above necessary protective water layering can be considered as the state in which water is surrounded by a single molecular layer and protects food ingredients, which is not only a physical coating state, but also a state of interaction with the proteincarbohydrateAll active groups ofhydrogen bondCombined.This can prevent the active group from attracting oxygen, and at the same time, it also plays a good role in blocking the interaction between the active groups, effectively maintaining the rehydration of dry food.In addition, because this water molecule is discontinuous at this water content, it will not cause the movement of those metal catalytic substances[2]。
