Effect of the hottest psychrophilic bacteria on lo

The influence of psychrophilic bacteria on long shelf-life dairy products

with the continuous expansion of processing scale, raw milk often needs to be stored for one to several days from collection to processing. In order to prevent the deterioration of raw milk, refrigeration methods are widely used. Using refrigeration methods to preserve raw milk leads to the dominance of psychrophilic bacteria in raw milk, which has become an important hazard factor affecting product quality

major psychrophilic bacteria in milk and dairy products

the International Dairy Federation proposed that all bacteria that can grow below 7 ℃ are called psychrophilic bacteria, while bacteria that can reproduce below 20 ℃ are called psychrophilic bacteria. The psychrophilic bacteria in milk include Pseudomonas, Achromobacter, Flavobacterium, alkalogenes and some coliforms. In addition, some lactic acid bacteria, Micrococcus, yeast and mold also belong to psychrophilic bacteria, In particular, mold prefers low-temperature environment. From 1960 to 1970, someone counted the distribution proportion of low-temperature bacteria in milk and dairy products, as shown in the following table:

main types and occurrence rates of low-temperature bacteria in milk and dairy products. Unit:%

samples of refrigerated sterilized raw milk, municipal milk, refrigerated sterilized milk Dairy researcher and time Thomas et al. (1963) Ogawa (1968) Shimizu et al. (1968) Dempster (1968) Zhongjiang (1970) Number of strains isolated Pseudomonas 36.1 for the convenience of customers 4790.743.052.0 Aeromonas 3.03.1 alkalogenes 8 total 8.04.6 colorless Bacillus 51.320.427.6 Flavobacterium 0.410.22.2 aerogenic Escherichia coli 2.15.611.64.0 Micrococcus 16.04.05.0 Streptococcus 3.03.3 Cladosporium 2.1 other 26.00.77.71.3

effect of psychrophilic bacteria on dairy products

uht E treatment of psychrophilic bacteria on effect of shelf life of dairy products

number of psychrophilic bacteria cfu/mla Protease B lipase c3.6 × 1068 weeks d6.0 × 1063 weeks and 10 weeks

most psychrophilic bacteria can produce thermostable extracellular degrading enzymes (mainly protease, lipase and alkaline phospholipase) during storage, which are basically unaffected in the pasteurization process. These thermostable extracellular protease and lipase can maintain partial activity even after UHT treatment, resulting in variations in the flavor and texture of milk and dairy products, mainly as follows:

1 After the protease decomposes milk protein, the product will be bitter. The amino acids released during the hydrolysis will aggravate the browning reaction, decompose k-casein, and cause protein gelation

2. After decomposing fat, it leads to the increase of free fatty acids and the deterioration of taste and flavor

detection and control of psychrophilic bacteria

at present, there are many methods for detecting the number of psychrophilic bacteria in raw milk. The most common method is to count after 10 days of culture at 5 ℃ - 7 ℃, or after the sample is cultured at 21 ℃ and cultured at 21 ℃ for 25 hours with selective medium, take G-bacilli in the sample as an indicator of psychrophilic bacteria pollution. These methods have serious defects in time and pertinence

most of the proteases produced by psychrophilic bacteria have very high thermal stability, and cannot be inactivated by a single heating treatment. Due to the phenomenon of low temperature inactivation (LTI), it may be the best choice to use pasteurization or UHT treatment combined with low temperature heating treatment to kill the proteases produced by psychrophilic bacteria in raw milk. The most effective method is to heat at 55 ℃ for 1 hour after UHT treatment, and the protease activity is only 17%, However, it is impossible to keep a large amount of raw milk or finished products at 55 ℃ for a long time, either in terms of equipment or economy. Therefore, the secretion of protease is mainly controlled by controlling the growth of psychrophilic bacteria in raw milk, and the number of psychrophilic bacteria is controlled by controlling the storage temperature of raw milk

carry out deep refrigeration on raw milk. They have also passed the certification of "Building Research Institute Environmental Assessment Method" (BREEAM), which is the main method to control psychrophilic bacteria. When the temperature is low, the growth of psychrophilic bacteria shows a long lag period and generation time (GT), and the average generation time of common psychrophilic bacteria in raw milk is 8-9 hours at 4 ℃

at the beginning of raw milk storage, the bacterial concentration has an important impact on its reproduction speed. When analyzing the changes of metal species and content of wear particles at 4 ℃, if the initial bacterial concentration is less than 5 × 104 cfu/ml, then it will reach 1 in 4 days × 105 cfu/ml, if the initial bacterial concentration is> 5 × 104 cfu/ml, then it will reach 5 in 4 days × 105 cfu/ml。 Therefore, the inhibitory effect of low temperature on psychrophilic bacteria depends on the initial pollution degree of raw milk. The storage temperature of raw milk has a significant impact on the quality of products. Compared with raw milk stored at 6 ℃, the shelf life of UHT milk produced from raw milk stored at 2 ℃ is much longer

psychrophilic bacteria pollution is the most common problem in the processing of modern dairy products. Most psychrophilic bacteria can produce heat-resistant enzymes such as protease and lipase when stored in 4-6. These enzymes can withstand pasteurization and even UHT treatment, resulting in the variation of the flavor and texture of milk and dairy products, shortening the shelf life, thus affecting the quality of products and the smooth progress of processing. Therefore, the pollution of psychrophilic bacteria in raw milk should be controlled, It is of great significance to improve product quality and extend shelf life

source: Xu Boqiang of Inner Mongolia Yili Group

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