Detailed explanation of the hottest food preservat

Details of food preservation technology (IV)

② heating and sterilization methods

food corruption is often caused by microorganisms and enzymes. Food can be stored for a long time by heating sterilization and inactivation of enzymes, but it must not be contaminated repeatedly. Therefore, it is necessary to sterilize after canning, bottling and sealing, or fill cans under sterile conditions after sterilization. There are many ways to heat and sterilize food. It mainly includes atmospheric sterilization (pasteurization), pressurized sterilization, ultra-high temperature instantaneous sterilization, microwave sterilization, far infrared heating sterilization and ohmic sterilization

atmospheric sterilization: atmospheric sterilization refers to the sterilization operation below 100 ℃. Pasteurization can only kill the vegetative bodies (including pathogenic bacteria) of microorganisms, but it cannot be completely sterilized. At present, water bath, steam or hot water spray type continuous sterilization is more used for atmospheric sterilization. The specific method has been described above

pressurized sterilization: commonly used for sterilization of meat products, medium acid and low acid canned food. The usual temperature is 100 ℃ ~ 121 ℃ (the absolute pressure is 0.2MPa). Of course, the sterilization temperature and time vary with the materials in the tank, shape, size, sterilization requirements and storage time. In the canning industry, D value and F value are commonly used to represent the sterilization temperature and time

d (DRT) value: refers to the time (minutes) required for bacteria to die by 90% (i.e. the number of viable bacteria decreases by one logarithmic cycle) at a certain temperature. The D (DRT) value of 121.1 ℃ (250 ℉) is often written as Dr. For example, Dr of Bacillus stearothermophilus = 4.0 ~ 4.5 minutes; A. Dr of Clostridium botulinum type B = 0.1 ~ 0.2 minutes

f value: refers to the time (minutes) required to kill a certain number of microorganisms by heating at 121.1 ℃ in a certain matrix. It is often used in cans, especially in canned meat. Due to the different types of cans, packaging specifications and formulas, the F value is also different, so the F value of each kind of can in production should be measured in advance

for canned food mixed with liquid or solid, rotating or shaking sterilization devices can be used. Although glass bottles and cans can also withstand high temperature, they are not suitable for high-temperature sterilization in pressure kettles. They must be soaked in hot water for cooking. Soft cans packed with composite film are usually sterilized by high-pressure boiling

ultra high temperature instantaneous sterilization: according to the influence law of temperature on bacteria and food nutrients, ultra high temperature instantaneous sterilization method, namely uhst (ultra high temperature for short times) sterilization, can be considered for food sensitive to heat treatment. This sterilization method can not only meet certain sterilization requirements, but also maintain food quality to the greatest extent

if milk is kept at high temperature for a long time, it is prone to some adverse chemical reactions. For example, Maillard reaction occurs between protein and lactose, resulting in browning of milk; Bad smell of H2S caused by protein decomposition; Sugars caramelize and produce peculiar smell; Whey protein denaturation, precipitation, etc. If ultra-high temperature instantaneous sterilization is adopted, it can not only facilitate the process conditions, meet the sterilization requirements, but also reduce the damage to the quality of milk

microwave sterilization: microwave (UHF) generally refers to electromagnetic waves with a frequency of MHz. At present, 915 MHz and 2450 MHz frequencies have been widely used in microwave heating. 915MHz, which can obtain a large penetration thickness, is suitable for heating foods with high water content, thickness or volume; 2450MHz should be selected for foods with low water content

the mechanism of microwave sterilization is based on thermal effect and non thermal biochemical effect. ① Thermal effect: Microwave acts on food, which absorbs microwave energy inside and outside at the same time, and the temperature rises. Under the action of microwave field, the molecules of contaminated microbial cells are polarized and vibrated at high frequency, resulting in thermal effect. The rapid rise of temperature changes their protein structure, resulting in the death of bacteria. ② Nonthermal biochemical effect: microwave can produce a large number of electrons and ions in the biochemical process of microorganisms, and change the physiological active substances of microorganisms; The electric field also changes the charge distribution near the cell membrane. 5. The relative humidity is no more than 80%, resulting in membrane dysfunction, so that the growth of microbial cells is inhibited, or even stop growing or dying. In addition, microwave can also cause the relaxation, breakage and recombination of hydrogen bonds in the molecular structure of cell DNA and RNA, and induce gene mutations

microwave sterilization and preservation of food is a new technology developed internationally in recent years. It has the characteristics of fast, energy saving and little impact on the quality of food. Therefore, it can retain more active substances and nutrients, which is suitable for the drying and sterilization of ginseng, shiitake mushrooms, Hericium erinaceus, pollen, Gastrodia elata, traditional Chinese medicine and Chinese patent medicine. Microwave can also be applied to the sterilization, enzyme killing, preservation and disinfection of a series of products such as meat and its products, poultry and its products, milk and its products, aquatic products, fruits, vegetables, cans, grains, pudding and bread, so as to extend the shelf life. In addition, microwave is used in the cooking of food, the thawing of frozen fish and frozen meat, the dehydration, drying, blanching, baking and expansion of food

at present, microwave milk sterilizers have appeared abroad, using high-temperature instantaneous sterilization technology. Under the frequency of 450MHz covering GB, ASTM, DIN, JIS, BS... Etc., they rise to 200 ℃ and maintain for 0.13 seconds. The total number of colonies and coliform group indicators of sterilized milk meet the requirements of sterilized milk, and the stability of milk has also been improved. The microwave sterilization device (2450MHz, 80kW) for bread developed by Swiss Karoli company irradiates for 1 ~ 2 minutes, the temperature rises from room temperature to 80 ℃, and the fresh-keeping period of bread slices is extended from the original 3 days to 30 ~ 40 days without mold growth

far infrared heating sterilization: far infrared refers to electromagnetic waves with a wavelength of 2.5 – 1000um. Many components of food have strong absorption of 3 ~ 10um far-infrared ray, so foods often choose this band of far-infrared heating

far infrared heating has high thermal emissivity; Less heat loss; Fast heating speed and high heat transfer efficiency; The food is heated evenly, and there will be no local overheating or entrainment; The loss of food nutrients is less

far infrared heating sterilization does not need to go through the heat medium, but irradiates on the items to be sterilized, and the heating directly penetrates from the surface to the interior. Therefore, far infrared heating has been widely used in the baking, drying and thawing of food, as well as the sterilization and enzyme killing of nut, powder, block and bagged food

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