Food freezing can be divided into slow freezing and quick freezing according to the speed. In terms of time, the central temperature of food dropped from-1℃ to -5℃ within 30 minutes of freezing (that is, the usual maximum ice crystal formation temperature zone, and the fruit generally ranges from 0 to-2.9℃); Or speed, its freezing speed is greater than or equal to 5 ~ 20 cm/hour, that is, quick freezing. According to the cytological characteristics of fruits and the use of frozen fruits, frozen fruits are all used, and the products are also called frozen fruits.
Quick-frozen fruit can keep its color, aroma, taste and texture to the maximum extent, and has a long shelf life, strong ability to adjust the market and production, and the products can be used as processing raw materials for other foods. The disadvantage is that the investment cost is large.
1905 In the eastern part of the United States, fruits were preserved by freezing, but they were all frozen slowly. It was not until 1923 that American Bird's Eye Refrigeration Company developed quick-freezing of food. The food freezing industry in China only developed after 1949, and developed rapidly in 1980s. The types of quick-frozen fruits that have been tried out are litchi, longan, loquat, bayberry, strawberry, guava, plum and peach.
Preservation principle
The preservation function of frozen food lies in low temperature and freezing. Low temperature can inhibit the activity of spoiling microorganisms, inhibit the activity of enzymes that cause quality degradation, and reduce the rate of various chemical reactions. Freezing can reduce water activity, reduce available water, promote partial death of microorganisms and reduce the number of bacteria. In frozen food, microorganisms do not grow and reproduce, nor do they produce toxins, but low temperature and freezing have no bactericidal effect. Once thawed, the remaining microorganisms and enzymes will resume their activities and grow and reproduce, and the toxin-producing bacteria can still produce toxins, causing the products to deteriorate.
When the fruit is frozen, its central temperature changes (figure 1). With the decrease of temperature, latent heat is released and reaches the freezing point quickly. Because the phase transition equilibrium cannot be broken immediately, supercooling occurs, and it does not freeze below the freezing point. Continue to cool down or take other measures to start freezing, release latent heat, and the temperature rises to freezing point. With the continuous cooling, the fruit keeps freezing, and with the increase of protoplasm concentration, soluble solid content and freezing point temperature in the fruit. Until the free water freezes completely, the temperature drops sharply with the temperature of the refrigerant.
Figure 1 During quick-freezing, the temperature of the fruit drops rapidly, the freezing speed is fast, the time to form a temperature zone through the largest ice crystal is short, the supercooling phenomenon is not obvious, and the freezing injury is small; Because of the rapid crystallization speed, large number of ice crystals and small volume, the intercellular space will not be seriously expanded, the protoplasm is close to the cell wall, and the wall and membrane are not damaged, which has little influence on the texture of the fruit. The thawed product can be close to the texture of fresh fruit, and the cell fluid will not flow out in large quantities, and the juice flow is light.
There are still a few chemical reactions during quick freezing and storage of fruits, so it may produce bad flavor; It is also possible that the texture of pulp is softened by pectinase and enzymatic browning occurs by polyphenol oxidase. Chlorophyll turns into pheophytin, which browns the color of green fruit: the contents of vitamin C and other vitamins in fruit will be lost to varying degrees depending on the packaging. After quick freezing, the acidity of fruit will increase due to the solidification of some buffer substances.
Technological process
Including the selection and treatment of raw materials, packaging, freezing, frozen storage, thawing and so on.
Selection and treatment of raw materials
Including the selection of raw material varieties and maturity, fruit quality selection, grading, cleaning and so on. Fruits used for quick freezing must be of suitable variety, fully mature and clean.
Before freezing, reject unqualified fruits, properly grade, clean and remove impurities, and some fruits need special treatment to remove pollutants. The fruit with core or heart should be denuded, and the large fruit should be cut properly. If the enzymatic browning is serious, it needs blanching and other methods to protect the color. Soft pulp is properly hardened.
package
In addition to quick-freezing of single fruit (also known as single quick-freezing), packaging is needed before freezing to prevent water loss during freezing, reduce frosting of freezing equipment and oxidation deterioration of products, and facilitate sales. According to the nature and use of products, syrup or dry sugar and ascorbic acid are often added in packaging to increase flavor, retain good aromatic substances and color, reduce the freezing amount of water at low temperature and reduce oxidative deterioration. Packaging materials are often paper, metal composite cans, polyethylene and polypropylene film bags, metal cans, etc., and raw materials for reprocessing are also packaged in wooden barrels.
freeze
Depending on the device used and the type and variety of fruit. According to the contact between refrigerant and fruit, there are: ① cold air freezing. Cold air cooled by refrigerant is used as the medium of heat transfer, and the product is frozen after a period of time in this cold medium. The traditional method is still cold air freezing in cold storage, which takes a long time and has poor quality. Fruit processing is only used for processing raw materials in large containers or barrels. Blast freezing is an improved method of static air freezing, which is characterized by high-speed flow of cold air and short freezing time. It is suitable for most fruits. Generally, the temperature in the equipment is-29 ~-46℃ and the wind speed is10 ~15m per minute. The device has a small room for intermittent freezing in batches, and a tunnel for continuous freezing with conveyor belt or trolley (Figure 2).
Fig. 2. Vertical upward revolving conveyor belt type device and fluidized bed monomer quick freezing device (fig. 3). Among them, tunnel freezing device is the most widely used in fruit quick freezing, and it can be continuously produced. Fluidized bed device is suitable for small fruits such as strawberries, diced peaches, pineapples and sliced fruits. ② Indirect contact freezing. Refrigerant is not in direct contact with the product, and the common equipment is a flat freezer. The packaged product is placed between two flat plates that can move up and down. Refrigerant flows back in the flat plates, and the product cools and freezes according to the contact with the flat plates, which is suitable for packaged products. ③ Direct contact freezing. Fruits are frozen by direct contact with refrigeration media other than air, and there are two methods: soaking method and spraying method. Common refrigeration media used for impregnation include salt (23% aqueous solution can be cooled to -2 1. 1℃), sucrose (62% aqueous solution can be cooled to -2 1℃), glycerol (67% aqueous solution can be cooled to -46.7℃) and propylene glycol (60%). Cryogenic refrigerants are often used for spraying, such as liquid nitrogen (boiling point-195.8℃) and carbon dioxide (boiling point -78.5℃). Direct contact freezing method is suitable for quick-frozen single fruit or small packaged fruit. ④ Liquefied gas freezing method. The sprayed cryogenic refrigerant does not directly contact the product, but the gasified cryogenic gas contacts the product and freezes. The general device is spraying liquid nitrogen, and the temperature can reach-80 ~-100℃, which is suitable for some precious fruits.
Figure 3 Frozen storage and thawing
Frozen fruits should be stored at low temperature after freezing. The lower the temperature, the better the quality and the longer the shelf life, and the loss of quality is related to the temperature. Any temperature fluctuation in storage, transportation and other treatment can lead to quality degradation. The suitable frozen storage temperature is-18 ~-25℃. The shelf life should be12 ~ 24 months according to the product properties, pretreatment methods, packaging methods and frozen storage temperature (see table).
Frozen fruits should be thawed before eating, in principle, slowly, so that cells can absorb thawed water as much as possible. Frozen fruits are not sterile. After thawing, cells are damaged, and juices flow, which is easy to breed microorganisms and even toxic microorganisms. Therefore, they should be eaten immediately after thawing.
Quick freezing points
Depending on the fruit:
strawberry
Choose varieties with bright red color, small or small fruit shape, small and light seeds and slightly sour taste. There are four kinds of products: sliced sugar, whole fruit or semi-fruit sugar, single fruit quick freezing and processing raw materials in large barrels. After entering the factory, the raw materials are precooled at 0℃ in a shallow tray for 24 hours, sepals are removed and cleaned. Sliced products are cut into 0.63 ~ 0.15cm slices, and semi-fruit products are cut in half. Fructose ratio is 4∶ 1, dry sugar is added, and the mixture is packed in a fiber composite tank with a metal bottom cover. The whole fruit product can be canned with sugar solution or quick-frozen with single fruit, and packed in polyethylene bags. Raw materials for processing are packaged in large tanks or vats, and the fructose ratio is 3 ~ 4:1. Strawberries were frozen at -35℃ by tunnel freezer.
apple
Choose fresh food with good quality, low acidity, round fruit shape, fruit diameter of about 7 cm, light enzymatic browning, tight meat and not easy to fluff. Most of the products are used as raw materials for other food processing, such as baked goods and jelly. The treatment includes grading, cleaning, peeling, slicing, coring, trimming, slicing, packaging in polyethylene bags or cans, and blast freezing. The prevention methods of browning are as follows: ①1~ 3% salt solution for color protection. ② Soak in 0.25% sodium bisulfite solution, and then soak in 0.20% potassium hydrogen phosphate to protect the color. ③ Vacuum. ④ Add syrup and ascorbic acid to the product.
Citrus
Only wide-peel oranges and a small amount of grapes are used for quick-freezing orange slices. There are two kinds of products: single fruit quick-freezing and orange slice quick-freezing. Single-fruit quick-freezing adopts small and medium-sized common satsuma fruits, and after washing, appropriate ascorbic acid solution is added and packaged in polyethylene bags for blast quick-freezing or monomer freezing. Pre-treatment of orange slices is similar to canning, which can be divided into three types: ① monomer quick-freezing, orange slices using monomer quick-freezing and packaging after freezing. (2) Soak in 50-60% syrup until the sugar solution penetrates into the pulp, take it out, add 40% syrup, and pack it in a polyethylene bag for blast freezing. (3) treat orange slices with 3% citric acid solution at 40 ~ 50℃ for 30 ~ 60 minutes, and take out the monomer for quick freezing, so that the outer layer of the product can have a sugar-coated film.
peach
Choosing yellow meat, detached and solute peaches requires bright color, tight meat quality, light enzymatic browning and strong aroma. After cleaning, grading, cutting and coring, peeling with alkali solution, rinsing, soaking in1~ 2% citric acid solution, slicing, adding 40 ~ 50% syrup and 0.1~ 0.2% ascorbic acid, packaging, and quick-freezing by air at -35℃. It can also be dipped or quickly frozen by other methods.
Litchi and longan
There are two kinds: peeled and peeled. Blanching the peeled fruit at 98 ~100℃ for about 20 seconds, cooling, spraying 3% citric acid solution for color protection, blast freezing, and packaging after freezing. The treatment of peeled products is similar to canned food, and small packaging is quick-frozen.
cherry
There are many sour cherries. The fruits are cooled in cold water, graded in size, graded in color, mechanically pitted, and then added with appropriate amount of sugar. They are packed in large acid-resistant paint cans and quickly frozen by air.
Other fruits
Plum and plum are not peeled, and the rest are frozen with single peach. Berries such as blackberries and Vaccinium bracteatum fruits are similar to strawberries. Quick-frozen bayberry single fruit or small package with sugar. Grapes are often used as raw materials for reprocessing. The fruits are heated to 60 ~ 67℃, seeded, bulk and frozen by blast.