Study of the composition of the residual microflora of milk after pasteurization

Abstract

The article presents the results of studies of the composition of the residual microflora of pasteurized milk, depending on the bacterial landscape and the initial contamination of raw milk. The thermal stability of test  cultures of microorganisms that significantly affect the  quality and  storage capacity of fermented dairy products has been  studied. To study  the  composition of the  residual microflora of milk after  pasteurization, sterile milk was infected with  test  cultures of microorganisms at doses  from  101 CFU/cm3 to 107 CFU/cm3. After infection, the  milk was pasteurized at temperatures of (72 ± 1) °C and  (80 ± 1) °C with  a holding time  of 10–20  seconds. The detection and  enumeration of microorganisms was carried out  by standardized microbiological methods. Microorganisms were identified by visual assessment of dominant colonies and cell morphology in micropreparations. The thermal stability of microorganisms important for dairy products, in particular cheeses, the source of which  is raw milk, has been  studied. It has been  established that of the  coccal  forms,  the  greatest risks are associated with  enterococci. Escherichia coli at  infection doses  above 106 CFU/cm3 partially retains viability both  at low-temperature and  at high-temperature pasteurization. Pasteurization temperatures do not have  a lethal effect on spore  bacilli, their number in pasteurized milk does not decrease, regardless of the  initial dose of infection. Low-temperature pasteurization activates the process of clostridial spore  germination. The ability to reactivate cells after  thermal shock was observed in Escherichia coli, Staphylococcus aureus, Pseudomonas, and  mold  fungi.  Thus,  the  residual microflora of milk  subjected to  low-temperature pasteurization is represented by enterococci, thermophilic streptococci, micrococci, staphylococci, asporogenous bacilli  and  spore bacteria. The above microorganisms constitute  the  residual microflora of pasteurized milk and are involved in the  maturation of cheeses, determining their quality and  safety,  [as well as] affecting the  storage capacity of the finished product.