Due to the need for consumer safety and consumer psychology, modern food processing techniques and techniques require maximum preservation of the color, aroma, taste and nutrients of the food. However, traditional food heat sterilization methods are far from meeting this requirement. In recent years, some new types of cold sterilization technologies have been researched at home and abroad, such as ultra-high pressure sterilization, ultra-high pressure pulsed electric field sterilization, pulsed light sterilization, radiation sterilization, ultraviolet sterilization and other cold sterilization technologies, which have attracted the attention of food science researchers.
Cold sterilization refers to a safe and efficient sterilization method in which the temperature of the food does not rise or rise very low during the sterilization process. Cold sterilization not only helps to maintain the physiological activity of the functional ingredients of the food, but also helps to maintain color, aroma, taste and nutrients.
1. Ultra-high pressure sterilization Ultra-high pressure sterilization is to pack the food materials in a certain way, put them into a liquid medium, and apply them for a period of time under the pressure of 100~1000 MPa to make them meet the requirements of sterilization. The basic principle is the lethal effect of pressure on microorganisms, mainly through the destruction of its cell wall, the coagulation of proteins, the inhibition of enzyme activity and the replication of genetic material such as DNA [1]. In general, the higher the pressure, the better the bactericidal effect. However, prolonging the compression time under the same pressure does not necessarily improve the sterilization effect. Under the pressure of 400 ~ 600 MPa, bacteria, yeasts and molds can be killed, and the bad changes caused by general high temperature sterilization are avoided. The advanced nature of ultra high pressure cold sterilization technology is high pressure and normal temperature sterilization. After the food is processed, it not only has high-efficiency bactericidal properties, but also preserves the nutrients in the food. The food taste is good, the color is natural, the safety is high, and the shelf life is long. This is the advantage that the traditional high-temperature heat sterilization method does not have. At present, ultra-high pressure sterilization has been applied in the production of foreign fruits and vegetables, yogurt, jam, dairy products, aquatic products, egg products, high-viscosity foods [2].
2. Ultra-high voltage pulse electric field sterilization The pulse electric field generated by the high-voltage pulse is used for sterilization. The alternating action of the electric field and the magnetic field generated by the pulse increases the permeability of the cell membrane, weakens the membrane strength, and finally the membrane is broken, the material in the membrane flows out, the extra-membrane material penetrates, and the cell body dies. The electromagnetic field produces ionization, which blocks the normal biochemical reaction and metabolism of the cell membrane, and changes the substance in the bacteria. This technology avoids a series of phenomena such as protein denaturation and vitamin destruction caused by heating [3].
3. Strong magnetic pulse sterilization This technology uses the biological effect of strong pulsed magnetic field to sterilize. On the outside of the infusion tube, a spiral coil is set, and the magnetic pulse generator generates a magnetic field strength of (2~10)T in the coil. When the liquid material passes through the infusion tube, the bacteria therein are killed. The technology has the following characteristics: short sterilization time and high efficiency. Good bactericidal effect and low temperature rise, can not only sterilize, but also maintain the original flavor, taste, color, quality and composition (vitamins, amino acids, etc.) of the food, no pollution, no noise, scope of application widely.
4, pulse glare sterilization Pulse glare sterilization is sterilized by pulsed strong white light flashing method. The inert gas is used to sterilize the ultraviolet to infrared region which is opposite to the solar spectrum but stronger. The use of very short pulses of high intensity white light kills microorganisms on the surface of the food. The high-intensity white light is similar to sunlight, but is reflected only at a fraction of a second, and is stronger than sunlight to kill bacteria quickly. Under the pulse of strong light, the microbial lethal effect is obvious, and it can be completely sterilized. Different light intensity and time should be controlled for different foods and different strains during operation. It can be used to extend the shelf life of foods packaged in transparent materials [4].
5. Microwave sterilization Microwave is an electromagnetic wave with a frequency from 300 MHz to 300 GMHz. The process by which microwaves interact directly with materials to convert ultra-high frequency electromagnetic waves into heat. Microwave sterilization is the result of a combination of microwave thermal effects and biological effects. Microwave on the potential distribution of the bacterial membrane section affects the concentration of electrons and ions around the cell, thereby changing the permeability of the cell membrane. The bacteria are therefore malnourished, unable to normal metabolism, and growth and development are hindered to death. From a biochemical point of view, nucleic acids (RNA) and deoxyribonucleic acid (DNA), which are normally grown and propagated by bacteria, are crimped macromolecules in which several hydrogen bonds are tightly connected. Microwaves cause hydrogen bond relaxation, cleavage and recombination, thereby inducing genetics or Chromosomal aberrations, even breaks. Microwave sterilization is the use of electromagnetic field effects and biological effects to kill microorganisms. The use of microwave devices has obvious advantages in sterilization temperature, sterilization time, product quality maintenance, product shelf life and energy saving. The microwave chamber system developed by Neiss, Germany, has a heating temperature of 72-85 °C for 1~8 min. The sterilization effect is very good. It is especially suitable for packaged bread, jam, sausage, pot cake, snack and storage. Pest control, eggs, etc. Microwave-treated foods have a shelf life of more than 6 months [5].
6. Radiation sterilization Radiation emitted by radioactive isotope usually has three kinds of rays: α, β, and γ. It is used for sterilization of food inside only gamma rays. Γ-ray is a kind of electromagnetic wave with extremely short wavelength, which has strong penetrating power to objects. The cytoplasm of microorganisms is not affected by a structure under certain intensity γ-rays, thus causing variation or death. Nucleic acid metabolism in microbial metabolism can be inhibited by radiation, proteins are denatured by irradiation, and their reproductive functions are most damaged. Radiation does not cause a temperature rise. Generally, bacteria with high heat resistance have a greater resistance to radiation.
7, ultraviolet sterilization Sunlight can kill bacteria, mainly the role of ultraviolet light, the sterilization principle is that the microbial molecules are in an unstable state after being excited, thereby destroying the unique chemical bonds between molecules to cause bacterial death. Microorganisms have different sensitivities to different wavelengths of ultraviolet light, and ultraviolet rays have different lethal doses to different microorganisms. Gram-negative Bacillus sp. is most sensitive to ultraviolet light. The amount of ultraviolet radiation that kills Gram-positive cocci is increased by 5 to 10 times. However, the UV penetration is weak, so it is more suitable for the sterilization of air, water, thin-layer fluid products and packaging containers [6,7].
8. Ozone sterilization Ozone oxidizing power is extremely strong, second only to fluorine, which can rapidly decompose harmful substances. The bactericidal ability is 600 to 3,000 times that of chlorine, and it is rapidly reduced to oxygen after decomposition. Ozone technology using its performance has been widely used in developed countries such as Europe, America, Japan, etc. It is the preferred technology for sterilization, sewage treatment, water purification, food storage, medical disinfection and so on. Medical researchers at the University of Washington in the United States have found that ozone can inhibit the growth of cancer cells; Japan Ishikawajima Hemp Seed Industry Co., Ltd. has proved that ozone water is expected to be the best fruit tree fungicide, and its bactericidal effect is significantly better than sodium hypochlorite; Ozone can effectively kill Neisseria gonorrhoeae and decomposes heavy metals in water.
The test proves that ozone water is a broad-spectrum fungicide, which can effectively kill common pathogens such as Escherichia coli, wax bacillus, dysentery bacillus, typhoid bacillus, and meningococcus, as well as influenza and hepatitis viruses in a very short time. Microorganisms [8]. It can kill and oxidize fish, meat, fruits and vegetables, various microorganisms on the surface of food, and microorganisms that continue to carry out life activities after they have separated from the mother, accelerate the mature ethylene gas, and prolong the shelf life.
As a new type of sterilization technology, cold sterilization technology has been widely used in the food industry. It not only overcomes the shortcomings of traditional thermal sterilization, but also maximizes the original quality of food to meet the needs of consumers, so that the application and research of cold sterilization technology is closely watched in this research field, in the food processing process. The use of cold sterilization technology has become an inevitable trend.