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.