Studies On The Changes And Control Of Psychrotrophic Bacteria In Raw Milk

Psychrotrophic bacteria is the primary putrefying bacteria in low-temperature storage rawmilk，increased by the storing time, psychrotrophic bacteria growth and reproduction quickly. Aftersterilization in production, psychrotrophic bacteria will be killed mostly, but the the protease andlipase they made are heat-resistant, they can even keep10%activities after UHT sterilization, inlow-temperature storage, the activities will be activated, the enzyme can resolve protein fat and partof lactose in raw milk, result in the bad quality of product such as coagulated, delamination andbecome bitter.In this article, we use the method recommend by IDF, psychrotrophic bacteria are cultivated in6.5℃for10days, the raw milk come from10different milk station, in the experiment, we shouldcount the number of psychrotrophic bacteria in milk after the raw milk stored at4℃by0hour,6hours,12hours,18hours,24hours,30hours,36hours,42hours,48hours,54hours,60hours,66hours,72hours.We can get the result that: the initial number of psychrotrophic bacteria in each milkstation are different (between103to105), but all less of105cfu/ml, when raw milk is stored under4℃, the number of psychrotrophic bacteria and initial number have positive correlation, althoughthere is a big difference on psychrotrophic bacteria number in different milk station, but when theyare stored under4℃, the number of psychrotrophic bacteria have small change from0hour to36hour, most of them come to logarithmic phase when stored42to54hour, that is the time begin toproduct enzyme. When store raw milk at milk station for14hours, the quantity of psychrotrophicbacteria can reach around store at factory for about30hours.In order to analysis the source of psychrotrophic bacteria in raw milk, we detect thepsychrotrophic bacteria in milk station’s air and the water they use to cleaning the station, and thepipelines and facilities milk contacted, we get the result that there are many psychrotrophic bacteriain the air, cattle and the facilities used during milking, there is little or no psychrotrophic bacteria inthe water they use.After separation and purification the psychrotrophic bacteria in raw milk, we can get12psychrotrophic bacteria strain which can product enzyme, most of them are pseudomona fluorscens, they have strong ability to product enzyme, so they have big influence on the quality of raw milk.In the test, we inoculate a certain amount of psychrotrophic bacteria, store at4℃, detect theactivities of protease and lipase and the quality of raw milk with time growing, in the other group,we inoculate different amount of psychrotrophic bacteria in raw milk, store at4℃, detect after acertain time the activities of protease and lipase and the quality of raw milk, analysis the relationbetween the amount of psychrotrophic bacteria and inoculate time and the activities of protease andlipase and the quality of raw milk. In the experiment, at the same time, use the milk compositionanalyzer detect the protein and fat in raw milk, the results prove that: when inoculate105cfu/100mlpsychrotrophic bacteria in the sterilization milk, store at4℃for40hours, the quality of raw milkhave evident change, detecting the protein and fat in raw milk we can found that the amount ofprotein and fat have obvious decrease, we can also found that the inoculate amount and he activitiesof protease and lipase have positive correlation, the more inoculate amount, the strongerdecomposition they have to protein and fat in raw milk.We can get the conclusion from the experiments:1.When raw milk in low-temperature storage in factory, psychrotrophic bacteria have smallchanges in42hours, around at42hours begin to product enzyme, store raw milk at milk station for14hours, the quantity of psychrotrophic bacteria can reach around store at factory for about30hours,think about the time in milk station, raw milk store in factory should less than18hours.2.The pipeline and air in milk station are the most important reasons for psychrotrophic bacteriain raw milk.3.The main strain is pseudomona fluorscens, there are little sealed bacillus genera andmicrococcus at the same time.When storage the same time, the quantity of psychrotrophic bacteria reach at106, the raw milkhas serious spoilage; when the quantity is105, raw milk will serious spoilage by storage40hours.