Measures to prevent microbial contamination in milk
作者：admin 点击次数：122 发布时间：2025-06-06
Dairy products include fresh milk, sour milk, whole milk powder, evaporated milk, cream, cheese, cream, etc. To ensure the hygiene of dairy products, the following are required:
(1) The cow house and cattle body should be kept clean to prevent contamination of milk. (2) The first and second handfuls of milk that have been expressed, fetal milk and colostrum before and after calving, milk that has been treated with antibiotics for 5 days, mastitis milk and spoiled milk shall not be used for consumption or as raw milk.
(3) The milk must be cooled as soon as possible or processed in time, and the sterilized milk and sour milk should be stored in cold storage below 10 °C before leaving the warehouse, and the cream should be stored in cold storage below -15 °C to prevent deterioration. Freshly expressed milk contains lysozyme, which can inhibit the growth of bacteria, and the time that lysozyme is maintained is related to the number of original bacteria in the milk and the storage temperature, and the number of microorganisms in the milk varies with the length of storage time and temperature.
The expressed milk should be cooled in time (it is best to cool the milk below 10 °C) to maintain the ability of lysozyme to inhibit bacteria, which has a positive effect on the preservation of milk. Especially in summer, the higher the temperature, the more important it is to distribute the milk to each point of use as soon as possible and deliver it to consumers as soon as possible, which is of great significance to prevent microbial contamination. The antibacterial ability of lysozyme is also related to the number of bacteria in fresh milk, and the number of original bacteria is small, and the antibacterial time will be relatively prolonged.
With the passage of storage time, the temperature of fresh milk increases, lysozyme is gradually destroyed, the microbial sterilization and antibacterial effect in fresh milk is weakened, and the microorganisms in the milk are rapidly multiplied, at this time, the reproduction of some bacteria (Streptococcus lactis, lactobacillus, Escherichia coli and some protein-decomposing bacteria, etc.) dominates, and the properties of milk change, lactose decomposes, and lactic acid is produced. When the acidity rises to a certain limit, when the isoelectric point of protein is reached (the isoelectric point of protein and globulin is pH5.19, and the isoelectric point of casein is pH4.7), the protein in the milk begins to solidify, and the characteristics of the milk are flocculent precipitates, and there is a clear sour taste (the pH of milk with a sour taste is 6), and the spoilage of milk appears, and this milk cannot be used as raw milk.
(4) Purified milk should be purified and sterilized before it can be sold, and raw milk is prohibited from being marketed. Milk should not be mixed with water and no other substance should be added. The food additives used in all kinds of dairy products should comply with the current hygienic standards for the use of food additives.
The purpose of purification: to remove the contaminated non-soluble impurities in the fresh milk, because the impurities carry a certain number of microorganisms, after the impurities contaminate the milk, the microorganisms can diffuse into the emulsion, so the purification can reduce the number of microbial pollution.
Methods of purification: filtration and centrifugation. The effect of the filtration method depends on the pore size of the filter, which is generally filtered with 3~4 layers of gauze. Filter purifiers should pay attention to the cleaning and sterilization of filter cloths, unclean filter cloths are often the source of contamination of bacteria and impurities. Filter cloths should be thoroughly cleaned and sterilized when replaced. In general, dairies use two filters alternately. The filters are sterilized with flowing steam to ensure the hygienic quality of the milk.
The centrifugation method is in which the milk is subjected to strong centrifugal force in the centrifugal tank to purify the emulsion (impurities and bacteria to the inner wall of the separating bowl). The use of centrifugal milk purifiers can significantly improve the purification effect, which is conducive to improving the quality of dairy products.
It is worth mentioning that no matter which method of milk purification can not achieve the degree of complete sterilization, it can only reduce the content of microorganisms, which plays an auxiliary role in future disinfection.
(5) Milk disinfection refers to the sterilization with heat, which mainly kills pathogenic microorganisms and some other microorganisms in milk, but cannot kill all microorganisms. The time and temperature of milk sterilization are based on the need to guarantee maximum microbial elimination and maximum preservation of the nutrients and flavor of the milk. The requirements for the elimination of microorganisms are, first of all, the elimination of pathogenic bacteria, and Mycobacterium tuberculosis is one of the common pathogenic bacteria, and it is also a kind of heat-resistant bacillus, so to eliminate all pathogenic bacteria in milk, it is first necessary to ensure the elimination of Mycobacterium tuberculosis. At present, the sterilization methods of fresh milk mainly include low-temperature long-term disinfection (LTT), high-temperature short-term disinfection (HTST), and high-temperature instantaneous sterilization.
Sterilized fresh milk can cause deterioration due to the residual heat-resistant bacteria, especially the more contaminated fresh milk, the more bacteria remain after sterilization. Therefore, it is often the case that the pathogenic bacteria and coliform bacteria in the sterilized emulsion are not detectable, but the number of miscellaneous bacteria contained in the emulsion is quite high, often exceeding the number of bacteria specified in the sanitary standards.
For this reason, scientists in many countries have done a lot of experiments, and found that under the premise of ensuring the same sterilization effect, increasing the temperature is smaller than extending the sterilization time, so the current popular milk sterilization method is ultra-high temperature instantaneous sterilization. There are two types of UHT sterilization methods: one is direct steam heating, that is, high-temperature steam is injected into the milk, and the other is indirect steam heating, that is, milk is sprayed in high-temperature steam. The specific sterilization method is that the milk is preheated at 75~85 °C for 4~6min, and then passed through a high temperature of 136~150 °C for 2~5s. During the preheating process, most of the bacteria can be killed by ultra-high temperature instantaneous heating, mainly to kill the heat-resistant spore bacteria. Although the ultra-high temperature instantaneous sterilization method has a good effect on killing microorganisms, the emulsion will produce undesirable phenomena such as HS odor, whey protein denaturation, browning, etc., especially the denatured protein precipitate will be produced during storage. (6) The packaging and packaging materials of milk must meet the requirements of food hygiene, without any pollution, and should be protected from light, sealed and pressure-resistant. Before use, the packaging container should be sterilized by saturated steam, hydrogen peroxide, ultraviolet radiation and other methods to achieve no requirements. Sterilized milk should be filled using an aseptic filling system. In order to ensure the hygienic quality of milk, the packaging must be tight and complete, and the product name, factory name, production date, batch number, shelf life and consumption method must be indicated. The label of the food outside the package must match the content
Forged and counterfeit dairy products are strictly prohibited. (7) Storage and transportation of milk The storage temperature of pasteurized milk is 2~6 °C. Sterilized milk can be stored at room temperature, and the warehouse must be hygienic and dry, and shall not be stored in the same warehouse with harmful, toxic, peculiar smell, or items that have an adverse effect on the product. Refrigerated trucks should be used when transporting products, and the vehicles should be clean and hygienic, and special vehicles should be used to transport products in summer. Avoid violent shocks and high temperatures during transportation, and prevent dust and prevention, avoid sun and rain, and do not mix with harmful, toxic or odorous items.