Application of carbon dioxide bacteriostatic technology in keeping fresh of dairy products

Bacteriostatic preservation of dairy products is an important link in the stage of storage and sale of dairy products, which is related to the health of consumers, economic loss of production enterprises and corporate image Acidification and deterioration are the core factors that lead to the deterioration of the flavor of dairy products and the increase of the bag size, and they are also the biggest challenges faced by the technology of fresh-keeping and storage of dairy products The acidification of dairy products is not only a personal performance of spoilage group, but also a combination of spoilage group and extracellular enzyme released by spoilage group The main cause of deterioration of dairy products is rich nutrition of microbial dairy products, which is an ideal medium for broad-spectrum microbiota Most of the dairy products, such as pasteurized fresh milk and yoghurt, need to be stored at low temperature to maintain good quality and nutrients But low temperature storage does not inhibit all microorganisms, such as psychrophilic bacteria According to the literature, 25% of the shelf-life problems of dairy products are caused by thermophilic psychrophilic bacteria The growth of psychrophilic Gram-negative bacteria deteriorates the microorganisms in refrigerated raw materials and pasteurized milk, skimmed cheese and similar products Pseudomonas is a representative strain of psychrophilic bacteria Gram negative, facultative anaerobic and psychrophilic bacteria can not only grow in milk, but also release many extracellular enzymes that can degrade milk components, mainly protease and lipase, which can decompose protein and fat, causing sensory defects in milk These enzymes have heat-resistant properties, which affect the preservation quality of pasteurized milk Yeasts and molds (geotrichia, fan · mold, Mucor, Cladosporium, Penicillium) lead to flavor, texture and visible decay (separator) The growth of heat-resistant Lactococcus can reduce the pH value of pasteurized milk until clotting occurs Relevant research shows that there are still a certain number of microorganisms in fresh milk after pasteurization Generally, the growth of microorganisms is slow within 7 days after pasteurization of milk With the extension of storage time, the number of psychrophilic bacteria mainly composed of Pseudomonas increased rapidly When the storage time reached 10-12 days, the number of Pseudomonas reached the peak At the same time, Streptococcus lactis and so on began to breed rapidly, decomposing lactose to produce lactic acid, the acidity increased, and the pH value of milk continued to decline At this time, the growth of Pseudomonas was limited and a large number of Pseudomonas began to die 12 days later When pH dropped to 5, the growth of S.Lactis was also inhibited Due to the strong acid resistance of Lactobacillus, it gradually occupied the advantage of reproduction and continued to produce acid At this time, a large number of milk clots appeared in the milk, and whey precipitated The application of carbon dioxide bacteriostatic technology in dairy products carbon dioxide is the most oxidized state of carbon atom It exists in a colorless and tasteless gas state at room temperature and slightly soluble in water Carbon dioxide is a kind of natural antimicrobial agent, which can inhibit the growth of microorganisms Because carbon dioxide molecules are nonpolar and easily dissolve in lipid bilayer of microbial cell membrane, it will increase the fluidity of membrane, thus exposing the cytoplasm to toxic environment The cytoplasm of microorganism contains water, and the hydration reaction between carbon dioxide and water will generate carbonic acid, which will reduce the pH value of the cytoplasm, and this change will pressurize the cell Carbon dioxide accumulates in the cell, which needs more energy to be consumed by microorganisms as metabolic waste At the same time, the degradation reaction takes place in a small extent instead of oxygen, which disturbs the normal physiological balance of microorganisms There are two ways to apply CO2 antibacterial technology to the preservation of dairy products: adding CO2 directly and map controlled packaging technology It has been confirmed that carbon dioxide can inhibit the growth of gram-positive and Gram-negative microorganisms The effect of this inhibition mainly depends on the solubility of carbon dioxide in the product, so the more soluble liquid dairy products are more suitable to add carbon dioxide directly The method of adding carbon dioxide directly is to connect the steel cylinder containing liquefied and compressed carbon dioxide with the bottom of the liquid milk container through the pressure reducing valve and the conduit, and to fill the liquid milk with a proper amount of carbon dioxide at a lower pressure When this method is used, the shelf life of liquid milk can be significantly prolonged For example, the shelf life of pasteurized milk is generally about 15 days, and the shelf life can be extended by about 7 days after carbon dioxide is directly charged However, this beneficial effect on the shelf life of liquid milk is also affected by the barrier of packaging materials Packaging material is a barrier to isolate liquid milk from the external environment, focusing on oxygen, which accelerates the oxidation and deterioration of dairy products The performance is generally characterized by the oxygen permeability index of packaging materials The larger the oxygen permeability is, the more oxygen will pass through the same area of packaging materials in unit time As we all know, oxygen can promote the growth and reproduction of microorganisms, which leads to a great discount in the antibacterial effect of carbon dioxide and the effect of prolonging shelf life The author has used the vac-v2 differential pressure gas permeameter of Labthink brand to test the oxygen permeability of various liquid milk packaging materials under different storage temperatures, as shown in Table 1 It can be seen that the oxygen permeability of the composite membrane is significantly lower than that of the PE membrane, because the composite membrane combines the advantages of a variety of membrane materials, especially the composite membrane with aluminum foil, the oxygen permeability is only 0.05% ~ 0.16% of that of the PE membrane at the same temperature Table 1 also reflects the influence of temperature on the oxygen permeability index of packaging materials, that is, the higher the temperature is, the greater the oxygen permeability of the materials is, and the more unfavorable it is to block the oxygen Based on the test results in Table 1, it is recommended to select multi-layer plastic composite film, aluminum plastic composite film, paper aluminum plastic composite film, or aluminum-plated composite film for liquid milk with direct addition of carbon dioxide bacteriostasis method, which can produce synergistic bacteriostasis Relevant research shows that the shelf life of raw milk is increased by 97.9% when it is packed with high barrier materials and 65.6% when it is packed with low barrier materials Map, also known as modified atmosphere packaging, is a kind of packaging form which can adjust the gas content in the packaging to create a favorable atmosphere for bacteriostatic and fresh-keeping Because of the shape of powdered dairy products, map air-conditioned packaging technology is more suitable In fact, a certain proportion of nitrogen and carbon dioxide mixed gas needs to be filled into the packaging The inert gas nitrogen can reduce the oxygen residue in the packaging of milk powder to a minimum, while carbon dioxide can play a synergistic role in inhibiting the reproduction of aerobic bacteria in milk powder and extending the shelf life It should be noted that the antimicrobial effect of map is closely related to the stability of the gas content in the packaging If the packaging used for milk powder has poor gas barrier, or the packaging of milk powder is poorly sealed, the content of nitrogen or carbon dioxide in the packaging can be easily changed This is also the reason why the packaging materials with high gas barrier are required for the modified atmosphere packaging of milk powder, and the daily monitoring mechanism for the content of gas in the packaging can be established regularly The researcher of Jinan Languang packaging safety testing center, where the author works, used the class 650 headspace gas analyzer to test the headspace gas content of two kinds of milk powder samples, i.e canned milk powder and aluminum plastic composite membrane bag milk powder The results are shown in Table 2 During the test, the content of each gas component is relatively stable, which may be due to the excellent gas barrier of tinplate It can be seen from table 2 that the oxygen content increases slowly with the extension of storage time, and the nitrogen and carbon dioxide content decrease gradually This may be due to pinholes or creases on the surface of the aluminum-plastic composite bag, or leakage points, increasing the overall oxygen penetration of the package Conclusion: bacteriostasis is a long-standing topic in the field of dairy products storage and preservation The microflora mainly composed of psychrophilic bacteria are still active in the low temperature environment, which is the main group of spoilage bacteria in dairy products Carbon dioxide is a kind of natural anti microbial agent, which can inhibit the growth of microorganisms The method of adding carbon dioxide directly to dairy products and the map modified atmosphere packaging technology filled with carbon dioxide mixture can be used to achieve the purpose of anti microbial and fresh-keeping.