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First, the composition of food and its metamorphic mechanism In order to maintain the normal life and reproduction of human beings, it is necessary to continuously obtain foods with sufficient energy. Understanding the composition of food and the mechanism of its chemical deterioration not only have guiding significance for the design of food packaging, but also the design basis for oxygen-proof packaging of food.
1. The chemical ingredients of foods are divided into animal foods and plant foods according to their sources. No matter what kind of food its chemical composition is very complicated, in addition to water, volatile components, its solid material composition can be divided into two kinds of organic and inorganic materials. The main chemical components of foods are: proteins, enzymes, sugars, lipids, vitamins, minerals, and water.
2. Chemical deterioration of foods and their mechanisms There are many reasons for food deterioration. In addition to physical changes, they may also cause chemical deterioration for both biological and non-biological reasons. Foods are kept for a long time at about 20°C. Oxidation of substances such as oils, pigments, and vitamins, or browning due to the involvement of enzymes and proteins, results in a decline in food color, aroma, taste and nutritional value, and may even cause foods. Deteriorating or producing toxic substances.
The chemical deterioration of food is mainly manifested in the following two aspects:
(1) Browning of foods In processing or storage, food or food ingredients lose their original color and become brown or dark. This is known as browning. There are two opposite results of browning: For example, bean paste, soy sauce, and black tea may turn brown during the aging process; bread and coffee gradually turn dark during heating, and such browning can make the color and fragrance of the food. Excellent taste. However, food browning to be discussed here generally refers to browning of food spoilage. There are two main types of browning of animal foods: enzymatic and non-enzymatic browning. Animal foods or plant foods themselves contain enzymes that can cause chemical changes to occur without consuming themselves. Browning caused by the enzyme-promoted oxidation of food ingredients is called enzymatic browning. Each enzyme can only catalyze a small range of reactions, such as amylase can only catalyze the role of starch; the speed of biochemical reaction under the action of enzymes varies greatly with the nature of the food; the activity of the enzyme is mainly related to the temperature Relevant, the temperature at which the enzymatic reaction rate is maximized is called the optimum temperature. Enzymes generally begin to fail at 30°C, and are almost completely destroyed at 80°C. When the temperature is higher than the optimum temperature, the enzyme remains catalytic as long as the enzyme is not destroyed. Effect, but the speed of enzymatic reaction is reduced, and the activity of the enzyme is very low at low temperatures.
Therefore, when meat is preserved at -20°C and fish are stored at -25°C to -30°C, they can effectively inhibit the action of enzymes and prevent browning and deterioration of foods. Non-enzymatic browning refers to the browning of foods caused by non-enzymatic oxidation or dehydration reactions and is divided into browning of heating and browning of oxidation. Heat browning is a browning caused by heating foods under substantially oxygen-free conditions, giving foods with satisfactory color, aroma, and taste when heated properly; browning is heating foods under aerobic conditions. The browning reaction that is caused is closely related to heating browning. When browning is heated, an intermediate product is produced, and browning occurs through oxidation reaction. This will make the food dark and produce an offensive odor.
(2) Oxidation of foods The browning and deterioration as described above is oxidation under specific conditions. The oxidation is now the oxidation of food exposed to the air. It is also one of the causes of chemical deterioration of foods. The oxidation of foods is generally reflected in the oxidation of oils, the oxidation of pigments, and the oxidation of vitamin C. Many foods contain oils and fats, which can improve food taste and provide more calories, but oils can cause odors or toxicity after oxidative deterioration, which should obviously be avoided. Oxidation of fats and oils is accompanied by autooxidation, thermal oxidation and enzymatic oxidation. They are all affected by the stability of the oil itself, the partial pressure of oxygen and the contact surface, light irradiation, water and ambient temperature; the pigments contained in the food, Not only gives food a unique color and image, but also is an important indicator of its quality and nutritional value. The causes of discoloration and discoloration of foods are mainly oxidation and browning. Food pigments are basically divided into two types: plant and animal systems. The carmine pigments in tomatoes and watermelons, the carrot pigments in carrots and pumpkins, and the yellow pigments of corn and oranges belong to the plant pigments, and also to muscle pigments such as fresh meat, ham, sausage, and fresh fish fillets. Yellow pigments in crabs belong to animal pigments.
Generally speaking, the pigments in foods are affected by heat, oxygen, and light, and are easily brown and discolored. Especially under light irradiation or under high temperature conditions, the oxidation is more severe; vitamin C is easily oxidized into dehydrogenated vitamin C. Continued decomposition will make vitamin C lose its original role, so that foods lose their proper nutrients.
Second, the aging of macromolecule materials such as oxidized plastics, rubbers, and other macromolecule polymers is the performance of the pyrolysis and cross-linking process under the long-term action of external factors, which leads to deterioration of its physical and mechanical properties, such as stickiness and brittleness. Discoloration and reduction of various mechanical properties. In addition to the aging of polymers, the role of oxygen is the most basic. In addition, oxygen is usually oxidized with polymers under the physical effects of heat, light, mechanical action, ozone, and ion radiation. And its chemical reaction is extremely complicated.
1. Rubber aging rubber is a highly elastic polymer compound. Rubber products are affected by external factors such as oxygen, ozone, heat, light, mold, insects, etc., and their mechanical properties will be degraded because most rubber molecular structures are unsaturated. Easily reacts with reactive oxygen atoms, destroying the original rubber structure from the surface. Oxidation is the basic cause of rubber aging, and heat, light, ionizing radiation, mechanical action and other factors only promote chemical, physical, and biochemical reactions and accelerate rubber aging.
The phenomenon of deterioration of the rubber structure due to the oxidation reaction and the resulting deterioration of the properties are called oxidative aging of the rubber. Oxidative aging of rubber has two main forms: one is in rubbers such as natural rubber and butyl rubber, and the oxidation reaction causes the rubber molecular chain to break (ie, degradation reaction), which makes the surface of the product sticky and the performance is degraded; One is in styrene butadiene rubber, butyl rubber, neoprene rubber, etc., its oxidation reaction will make the rubber molecules generate more cross-linking, making the product hard and brittle, cracking, performance degradation.
In order to prevent the aging of rubber, appropriate antioxidants, such as anti-heat antioxidants and anti-ultraviolet agents, may be added to the rubber ingredients. When using anti-aging agents, pay attention to the limitations, performance, characteristics, and dosage of each anti-aging agent. In addition, during the storage and transportation of rubber, certain protective packaging should be used to prevent oxygen, light (especially violet light and ultraviolet light), prevent heat, moisture, and prevent overheating. Even so, the storage period does not exceed two years. .
2. The mechanism of aging of plastic aging plastics is basically the same as that of rubber, but the oxidation rate of plastics is much slower than that of rubber. Therefore, anti-aging is only considered when long-term storage of extremely thin plastic films.
The commodities that come into contact with everyday life are very different. The types, composition, and structure of the products are different. Then the influence of oxygen on the products is also different. Usually, it is divided into food and non-food products for discussion.