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Commodity storage and commodity maintenance (II)

III. storage of food commodities (I) storage performance of food commodities

food commodities can be divided into natural food and processed food, and their storage performance is different

1. Storage performance of natural food

the storage resistance of natural food is affected by many factors. Such as variety, place of origin, breeding and cultivation conditions, harvest season, etc

2. Storage performance of processed food

processed food refers to products obtained from natural food as raw materials and then processed. Most processed foods are processed and have perfect packaging, but we only need some experimental instruments for construction engineering, colleges, research institutes and so on, which are imported from abroad and have better storage performance than natural foods

(II) quality changes of food during storage

1. Physiological, biochemical and biological changes of food during storage

(1) respiration

is the main symbol of physiological activities of food, fruits, vegetables and other living commodities. Vigorous breathing can accelerate the decomposition of commodity components and cause quality deterioration. The respiration of commodities can be carried out under both aerobic and anoxic conditions. Under aerobic conditions, when a commodity is undergoing aerobic respiration, the basic component change is that starch is decomposed into glucose, which is oxidized into carbon dioxide and water, and generates heat. When the commodity is exposed to anoxic conditions, it carries out anoxic respiration, and the glucose in the commodity is decomposed into alcohol and carbon dioxide, and generates heat. This chemical reaction process is the same as that of fermented wine, so it is regarded as fermentation. Vigorous aerobic respiration and anoxic respiration are not conducive to the quality of goods, so appropriate measures should be taken to inhibit the respiration of goods, so that the respiration of goods is outside the weak state, which can not only stop the deterioration of goods quality, but also maintain the natural storage resistance of goods

(2) post ripening

is the continuation of the ripening process of vegetable fruits after harvest. It mainly occurs in the storage of fruits, melons and fruits and vegetables. After ripening, fruits and vegetables change color, improve flavor and improve edible quality. The post ripening process is the stage of physiological aging of organisms. When the fruits are in the stage of physiological aging, they lose their storability and then go bad. Therefore, the storage conditions of fruits should be controlled to delay the process of ripening and aging

(3) germination and bolting

is a biological phenomenon that occurs when biennial or perennial vegetable products break their dormancy and enter the reproductive growth period from the vegetative growth period. Germination is the germination and growth of dormant buds of vegetables, while mossing is the result of flower stem growth. High temperature, high humidity, sufficient oxygen and sunlight can promote the germination and bolting of vegetables

(4) rigidity and softening

rigidity, also known as corpse rigidity, refers to the biochemical and morphological changes that occur within a period of time after the death of livestock, poultry and fish, which promote the contraction of muscle fibers, and the body is in a rigid state. The main machine is connected by the upper beam, the moving beam and the workbench through columns and ball screws to form a rigid floor frame structure

softening refers to the further changes of livestock and poultry meat after the rigidity reaches the highest point

2. Changes caused by microorganisms in food storage

(1) moldy

organic goods change their original appearance, intensity, smell, edible quality and other properties under the action of microorganisms. Microorganisms take organic commodities as the substrate, attach to it to grow and reproduce, which directly destroys the quality of commodities. Moreover, the secretions and toxins produced by microorganisms in the metabolic process also seriously affect the use value of commodities. The harm of microorganisms to organic commodities is serious, but microorganisms need certain conditions to survive and reproduce, thus endangering organic commodities. Under unsuitable conditions, its life activities will be inhibited or even killed, so creating conditions unsuitable for the survival of microorganisms is an important measure to ensure the quality and safety of organic commodities

(2) fermentability

refers to the nature of the decomposition of polysaccharide commodities in an anaerobic state. Commodities containing sugar, especially food, are easy to ferment and decompose under the action of yeast and enzymes due to their fermentability, resulting in other substances. Fermentation can not only cause food deterioration, but also an important method in the process of food processing, which is called "fermentation engineering"

3. Oxidative rancidity of fat in food storage

is the property of products containing fat hydrolyzed to produce new substances under the action of acid, alkali and enzyme

fat produces hydrolysis reaction, and glycerol esters are decomposed into glycerol and fatty acids. The more fatty acids decomposed, the easier it is to rancidity. Rancidity refers to that if the oil is placed in the air for too long, it will produce an unpleasant smell. This change is called rancidity.. To identify the freshness of fat, it is necessary to determine the content of free fatty acids in fat. It is usually expressed in milligrams of potassium hydroxide required to neutralize free fatty acids in a gram of oil, and its value is called acid value. Acid value is a variable. When the same kind of oil is fresh, its acid value is small. On the contrary, if its acid value is high, its quality is poor. Therefore, it is used as an index to identify the freshness of fat

(III) storage methods of food

(1) low temperature storage method: it is a common food preservation method that uses low temperature to inhibit microbial reproduction and enzyme activity to reduce the rate of biochemical changes. Low temperature storage can be divided into cooling storage and freezing storage according to different storage temperatures

cold storage is also called cold storage. The storage temperature is generally 10 ℃ ~0 ℃, and food is not frozen. Because the temperature is above 0 ℃, some psychrophilic microorganisms can still reproduce, and the enzyme activity in food is not completely controlled, so the storage period should not be too long

frozen storage is also called frozen storage. At present, frozen storage in China is mainly used for the storage and preservation of livestock, poultry, fish and meat. The freezing temperature is -18 ℃, which inhibits the activities of microorganisms and enzymes, so frozen food can be stored for a long time

(2) salt and sugar storage method: it uses the high osmotic pressure of salt or sugar solution and reduces the water activity to dehydrate the microbial protoplasm to death, so as to achieve the purpose of storing food

(3) controlled atmosphere storage method: that is, the storage method for regulating the composition of ambient gases. The principle is to change the normal air composition in the warehouse or packaging, reduce the oxygen content and increase the carbon dioxide content. In order to weaken the respiratory intensity of fresh food, inhibit the development of microorganisms and the change of chemical composition of food. Controlled atmosphere storage also needs to be equipped with low-temperature conditions to act as a support of multi-layer structure, so as to achieve good results. When the hydraulic universal testing machine system is faulty, controlled atmosphere storage can be regarded as an enhanced means of low-temperature storage. Most creatures in nature absorb oxygen and emit carbon dioxide when breathing, and oxygen is mainly supplied by air. Vigorous breathing and microbial reproduction are the main reasons why fresh food is easy to deteriorate. Therefore, during storage, reducing the fake content in the air and increasing the carbon dioxide content, while storing at low temperature, will inevitably inhibit the respiration of fresh food and microorganisms, thereby increasing the storage performance of fresh food. There are two specific methods: one is general controlled atmosphere storage, which uses the respiration of fresh food itself to consume oxygen in the air and increase the concentration of carbon dioxide, so as to achieve the purpose of regulating gas composition. Among them, there are airtight warehouse storage method with high tightness and plastic film bag storage method. The second is mechanical controlled atmosphere storage, which uses the carbon dioxide generator to control the oxygen content to adjust the air composition in the sealed warehouse

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