1.Selection of raw materials
Basic requirements for quick freezing of fruit and vegetable raw materials:
(1) They must be resistant to freezing. Raw materials that change flavor after freezing are generally not suitable.
(2) Vegetables that need to be cooked before consumption are suitable for quick freezing. Species that need to maintain their raw flavor are not suitable for quick freezing.
2.Precooling of raw materials
After the raw materials are harvested, they need to be cooled before being quickly frozen. This process is called precooling. Precooling can reduce the field heat and various physiological metabolism of fruits and vegetables to prevent corruption and aging. Precooling methods include cold water cooling, cold air cooling and vacuum cooling.
3.Cleaning, sorting and cutting of raw materials
Clean, sort and appropriately cut according to the type, characteristics and processing requirements of the raw materials.
4.Blanching and cooling of raw materials
Blanching can completely or partially destroy the activity of oxidase in the raw materials, playing a certain role in sterilization. For vegetables with more fiber and those suitable for stewing and stir-frying, blanching is generally performed. The blanching time and temperature are determined according to the nature of the raw materials and the degree of cutting, usually 95-100℃, a few seconds to several minutes. For vegetables with less fiber and suitable for fresh consumption, they are generally kept crisp and tender, and are usually not blanched.
5.Sugaring of fruits
Fruits need to maintain their fresh quality, and are usually not blanched in order to destroy the activity of fruit enzymes and prevent oxidation and discoloration. After sorting and cutting, fruits need to be stored in sugar solution or vitamin C solution. Fruit sugar treatment can also reduce the damage of crystallization to the internal tissue of the fruit, prevent the volatilization of aromatic components, and maintain the original quality and flavor of the fruit. The concentration of sugar is generally controlled at 30% to 50%, which varies depending on the type of fruit. The general dosage ratio is 2 parts of fruit plus 1 part of sugar solution. Adding excessive sugar will cause the flesh to shrink. For some varieties of vegetables, 2% salt water can be added for packaging and quick freezing to passivate the activity of oxidase and make the appearance of the vegetables beautiful. In order to enhance the color protection effect, it is often necessary to add 0.1% to 0.5% vitamin C, 0.1% to 0.5% citric acid or a mixture of vitamin C and citric acid to the sugar solution for better results (such as about 0.5% citric acid and 0.02% to 0.05% vitamin C). In addition, after peeling the fruits and vegetables, they can be immersed in 50 mg/kg SO2 solution or 2 to 3% sodium bisulfite solution for 2 to 5 minutes, which can also effectively inhibit browning.
6.Drain (drain water)
After blanching and cooling, the raw materials have a lot of moisture on the surface, which can easily form ice cubes during the freezing process, increasing the volume of the product. Therefore, a certain method must be adopted to drain the moisture. There are two methods for draining:
(1) The raw materials can be placed on a flat carrier to dry.
(2) Drying with a centrifuge or vibrating screen
7.Packaging
(1) Packaging methods for quick-frozen fruits and vegetables
The main packaging methods for quick-frozen fruits and vegetables are ordinary packaging, gas-filled packaging and vacuum packaging. The following mainly introduces the latter two.
① Gas-filled packaging. First, the packaging is evacuated and then filled with gases such as CO2 or N2. These gases can prevent the oxidation of food, especially meat fat, and the reproduction of microorganisms. The amount of gas is generally within 0.5%.
② Vacuum packaging. The gas in the packaging bag is extracted and the bag is sealed immediately. The reduction of gas in the bag is not conducive to the reproduction of microorganisms, which is beneficial to the preservation of product quality and the extension of the storage period of quick-frozen food.
(2) Characteristics of packaging materials
① Temperature resistance. Quick-frozen food packaging materials are generally qualified if they can withstand 100℃ boiling water for 30 minutes, and should also be able to withstand low temperatures. Paper is the most resistant to low temperatures and can still maintain its softness at -40℃, followed by aluminum foil and plastic, which can maintain their softness at -30℃. Plastic will harden when exposed to ultra-low temperatures.
② Breathability. In addition to ordinary packaging, quick-frozen food packaging also has special packaging such as vacuum packaging and vacuum packaging. These packaging must use materials with low breathability to maintain the special aroma of the food.
③ Water resistance. Packaging materials also need to prevent moisture penetration to reduce dry loss. This type of impermeable packaging material is prone to condensation of fog beads on the material due to changes in ambient temperature, which reduces transparency. Therefore, changes in ambient temperature should be taken into account when using it.
④ Light resistance. Packaging materials and printing pigments must be light-resistant, otherwise the material will be exposed to light, which will cause the packaging color to change and the product value to decrease.
(3) Types of packaging materials
The packaging materials of quick-frozen foods can be divided into inner packaging (film), middle packaging and outer packaging materials according to their uses.
Inner packaging materials include polyethylene, polypropylene, polyethylene and glass composite or polyester composite materials, etc., middle packaging materials include waxed paper boxes, plastic pallets, etc., and outer packaging materials include corrugated boxes, water-resistant corrugated boxes, etc.
① Film packaging materials. Generally used for inner packaging, they are required to be resistant to low temperatures and maintain elasticity at -1 to -30°C; they can withstand high temperatures of 100 to 110°C; they are odorless, easy to heat-seal, and have low oxygen permeability; they are oil-resistant and printable.
② Hard packaging materials. Generally used to make pallets or containers, commonly used are polyvinyl chloride, polycarbonate and polystyrene.
③ Paper packaging materials. At present, most quick-frozen food packaging is plastic, and paper packaging is less common. The reason is that paper has the following disadvantages: poor moisture resistance, poor gas barrier properties, and opacity. However, paper packaging also has obvious advantages, such as easy recycling, excellent low temperature resistance, good printing, easy packaging processing, good protection, low price, easy opening, good light shielding, high safety, etc.
In order to improve the freezing speed and efficiency, most fruits and vegetables should be packaged after quick freezing, and only a few leafy vegetables or fruits and vegetables with syrup and salt water should be packaged before quick freezing. The packaging after quick freezing requires rapid and timely, from leaving the quick freezing room to entering the cold storage, strive to control it within 15 to 20 minutes, and the temperature of the packaging room should be controlled at around -5 to 0℃ to prevent the product from softening, freezing quickly and deteriorating in quality.
8.Quick freezing
Select the above-mentioned appropriate freezing methods and equipment for quick freezing of fruits and vegetables, requiring the maximum ice crystal formation zone (-5～-1℃) of fruits and vegetables to be passed at the fastest speed in the shortest time. The freezing temperature is generally controlled at -40～-28℃, and the center temperature of fruits and vegetables is required to reach -18℃ within 30 minutes. Freezing speed is an important factor in determining the intrinsic quality of quick-frozen fruits and vegetables. It determines the formation and size of ice crystals and the amount of juice flow during thawing. In production, methods such as sufficient cooling and draining before freezing, increasing the specific surface area of fruits and vegetables, reducing the temperature of the freezing medium, and increasing the convection speed of cold air are generally adopted to increase the freezing speed. At present, fluidized single quick freezing devices are most widely used in the quick freezing of fruits and vegetables.
9.Frozen storage
Storage of quick-frozen fruits and vegetables is an indispensable step. Generally, the finished products after quick freezing should be immediately packed and stored in warehouses. To ensure that high-quality quick-frozen fruits and vegetables do not deteriorate during storage, the warehouse temperature is required to be controlled at -20℃±2℃, which is the most economical freezing temperature recognized internationally. During frozen storage, large temperature changes should be avoided, otherwise it will cause ice crystal rearrangement, frost, surface drying, browning, taste change, tissue damage and other quality deterioration; the product should also be sealed. If a broken bag is found, the bag should be replaced immediately to prevent dehydration and oxidation of the product. At the same time, the maximum storage time should be determined according to the storage tolerance of different varieties of quick-frozen fruits and vegetables to ensure the high-quality sales of the products.
The storage quality of quick-frozen products mainly depends on two conditions: one is low temperature; the other is to maintain a relatively stable low temperature.
The problems that occur during frozen storage can be summarized into the following three aspects.
(1) The deterioration of quick-frozen fruits and vegetables during frozen storage is mainly caused by physical and biochemical changes, which are manifested as ice crystal growth, color change, taste change, etc. These changes are mainly caused by the freezing conditions and the action of microorganisms and enzymes. In particular, enzymes can still undergo slow changes during long-term frozen storage, causing quality deterioration. For example, many enzymes such as sucrase, esterase, and oxidase can tolerate very low temperatures. In addition, the temperature fluctuation in the frozen storage room can easily cause ice melting and recrystallization, which will cause ice crystals to grow continuously, destroy the product’s organizational structure, affect the quality, and easily cause juice to flow after thawing. Therefore, a stable low temperature must be maintained during the frozen storage period. Quick-frozen fruits and vegetables are usually stored at -18℃. Generally speaking, microorganisms cannot grow and move at such low temperatures. Psychrophilic bacteria stop growing at -10℃, and pathogenic or spoilage bacteria cannot move below -3℃. Therefore, the product deteriorates physically and chemically during the frozen storage period.
(2) Frozen products are prone to ice sublimation during frozen storage, causing the product surface to lose water. Maintaining a layer of ice crystals on the product surface or using water vapor-impermeable packaging materials, as well as increasing relative humidity, can effectively prevent the product from losing water and avoid surface discoloration caused by water loss.
(3) Frozen products also undergo varying degrees of chemical changes during frozen storage, such as degradation of vitamins, decomposition of pigments, oxidation of lipids, and softening of tissues caused by certain chemical changes. These changes are slow at -18℃, and the lower the temperature, the slower the changes. Therefore, quick-frozen fruits and vegetables should be stored below -18℃ as much as possible. If the temperature is too high, there will be obvious browning or quality deterioration. For example, peach slices (4:1 sugar) are very stable when stored at -18℃ after quick freezing, but there will be obvious changes above -18℃. It is beneficial for some European countries to adopt lower storage temperatures.
10.Thawing and use
The thawing and quick freezing of quick-frozen fruits and vegetables are two processes with opposite heat transfer directions, and the speeds of the two are also different. Thawing is slower than freezing for non-fluid foods. Moreover, the temperature change of thawing is conducive to the enhancement of microbial activity and physical and chemical changes, which is exactly the opposite of freezing. Quick freezing and frozen storage of food cannot kill all microorganisms, it just inhibits the activity of surviving microorganisms. After the food is thawed, because its tissue structure has been damaged to a certain extent, the contents will seep out, the temperature will rise, and the microorganisms will be active and the physiological and biochemical changes will be enhanced. Therefore, quick-frozen food should be thawed before consumption, but not too early, and should be eaten immediately after thawing, and should not be left at room temperature for a long time. Otherwise, due to the occurrence of “juice flow” and other phenomena, microorganisms will grow and multiply, causing food spoilage. The faster the frozen fruit is thawed, the less impact on the color and flavor.
The thawing of frozen food is often completed by special equipment. According to the heating method, it can be divided into two types: one is that the external medium such as air, water, etc. transfers heat to the inside through the surface of the food; the other is heat transfer from the inside to the outside, such as high frequency and microwave. According to the different forms of heat exchange, it can be divided into air thawing method, water or salt water thawing method, ice water mixed thawing method, heating metal plate thawing method, low-frequency current thawing method, high-frequency and microwave thawing method and a combination of thawing methods. Among them, there are also three conditions for air thawing: slow thawing in air at 0-4℃; rapid thawing in air at 15-20℃ and rapid thawing in air-steam mixed medium at 25-40℃. Microwave and high-frequency current thawing are ideal thawing methods for most foods. This method heats up quickly and transfers heat from the inside to the outside, so the thawing is quick and uniform. However, the products thawed by this method must have uniform tissue composition to achieve good results. If the internal tissue composition of the food is complex and the ability to absorb radio frequency is inconsistent, it will cause local damage.
Quick-frozen fruits can generally be eaten directly after thawing without heat treatment, such as some frozen berries. Fruits and vegetables used in the production of fruit cakes, jellies, jams or preserves need to undergo certain heat treatment after freezing. After thawing, their pectin content and quality are not greatly lost, and the quality and edible value of the product can still be maintained.
