The focus of food processing techniques is to limit the nutrient content of raw materials and to effectively separate and extract trophic and functional factors in foods.
First, the reasons for low-temperature storage of food
Low-temperature storage of food is the use of low-temperature technology to reduce the temperature of the food, and maintained at low temperatures to prevent spoilage of food spoilage, extend the shelf life of food. Cryogenic storage can not only be used for the storage of fresh materials, can also be used for food processing, semi-finished products storage. Due to the characteristics of various materials are not necessarily the same, its specific requirements are also different.
1, plant food storage principle of low temperature
Breathing is the main cause of plant food spoilage. The process of me***orphism occurs primarily in respiration and in storability (the ability to deplete nutrition by delaying respiration). To solve the above problems, the respiration of plants must be controlled. For long-term storage of plant foods, it is necessary to maintain their living conditions while reducing their respiration. Low temperature is able to weaken the respiration of fruits and vegetables to extend the storage time.
2, animal food storage at low temperature
The main reason for animal food deterioration is the action of microorganisms and enzymes. To solve this major problem, it is necessary to control the activity of microorganisms and the effect of enzymes on foodstuffs. Under the condition of low temperature, the water crystallizes into ice and the vitality of the microorganism is lost and can not multiply. The reaction of the enzyme is severely inhibited. Its effect on the food is small and the food can be stored for a long time. Therefore, the animal food can pass Cryogenic to maintain its freshness.
Second, low-temperature food processing technology introduced
1, cold storage
It is the pre-cooled food, slightly higher than the freezing temperature (0 ℃) under the conditions of storage, the general refrigeration temperature is -1 ℃ ~ 8 ℃, cold storage using this temperature is often referred to as high temperature cold storage. Refrigeration can extend the shelf life of fresh foods and processed products by reducing the rate of biochemical reaction and microbial-induced changes. For most foods, cold storage does not stop the spoilage of food as it is for canning, dehydration or frozen storage, but rather slows the rate of spoilage of the food and therefore is actually a less effective preservation technique.
Fresh fruits and vegetables after cold storage treatment, the loss of vi***in C than normal storage loss. Such as spinach stored at room temperature, the loss of vi***in C up to 50% per day; as in the refrigeration conditions, the loss will not exceed 10%. In addition, the refrigeration technology is also used in the process of cheese ripening, beef tenderization, meat curing and alcohol aging to control the reaction rate of some chemical and biological enzymes during food storage and reduce the metabolic rate of beneficial microorganisms.
However, not all foods can prolong the shelf life under refrigerated conditions. For example, some tropical and subtropical fruits and some vegetables may be chilled when stored at 3 to 10 ° C above their freezing point, while the bread is aged at a low temperature accelerate.
For most foods, adverse effects on the nutritional value of foods over a certain shelf life are much less affected by heat treatment, dehydration, and irradiation as long as they are properly handled. Therefore, cold storage is only applicable to the short-term storage of food, which plays a role in prolonging the supply time of perishable food and its raw materials and alleviating the processing peak of seasonal products.
2, frozen storage
Frozen food is frozen by the method of freezing or freezing the food first, and then kept in the frozen state of the food storage temperature preservation method. Commonly used storage temperature of -12 ~ -23 ℃, and -18 ℃ for the most suitable. Cold storage frozen food storage often called cold storage or cold storage. Frozen storage for long-term storage, a short period of up to a few days, a long year can be.
Food storage is a method of freezing food and storing it in this state. Frozen crystal food can make most or even all of the water ice crystals, thereby reducing the free water, the growth of microorganisms is inhibited, the appropriate low temperature and freezing speed will also promote the death of microorganisms; enzyme activity at low temperatures and the loss of reaction medium Under the same effect is greatly reduced: fatty rancidity, vi***in decomposition and other effects will also slow down when frozen. Frozen storage can delay the corruption of food, but can not completely stop the corruption.
Frozen, frozen during the storage of some foods inevitably vi***in loss, frozen food protein, carbohydrates, fats, trace elements, such as the impact of small, almost negligible. But before freezing or blanching the thawing process, a variety of water-soluble nutrients, such as sugar compounds, water-soluble proteins, amino acids, vi***ins and minerals, have varying degrees of dissolution loss.
Common frozen food, not only need to keep fresh fruits and vegetables, juices, berries, meat, poultry, aquatic products, but also a lot of pre-prepared foods such as bread, snacks, ice cream and a wide variety of pre-cooked and specialty foods, Meals and other dishes. Frozen food due to ease of use, fresh taste, generally as long as thawing and heating can be eaten or reprocessed, it has developed into a broad category of large quantities of food and food ingredients, in foreign countries also become a family, restaurants, canteens meal list Common food. At present, heat-resistant cooking film bags and special thawing furnaces such as microwave ovens make it easier to eat frozen foods.
Among various drying methods, freeze-drying is considered the most "elegant" method since this method can be performed at low temperatures. Vacuum freeze-drying technology is the freezing of aqueous substances into a solid state, and then placed in a vacuum environment, the water sublimated from the solid state to remove the moisture method. Since the food is actually dried at very low temperatures, there is little loss of nutrients and flavorings in the food, preserving the nutritional, taste and aroma of the food to the fullest extent possible.
In the process of freeze-drying, the amino acids and proteins in food can be preserved completely; the preservation rate of vi***in C is 50% -70% or 90% -100%; the preservation rate of carotenoids and vi***in A is above 95% The preservation rate of vi***in B1 is above 75%; the preservation rate of vi***in B2 is over 90%; the preservation rate of other vi***ins except vi***in B6 is less than 80% to 100%.
Lyophilized products rehydration is good, can quickly absorb water recovery, its color, quality and fresh products are basically the same. Complete dehydration, when dry to exclude 90% to 95% of the water, dried at room temperature can be stored for a long time, and because of light weight and easy to transport. Therefore, freeze-drying technology has been favored by many industries.