Effect of heat treatment on the content of microorganisms in drinking milk

Abstract

The microflora of raw milk is in the center of constant attention at the enterprises where it is processed. Since the microbiota of pasteurized milk is determined by the percentage of heat-resistant bacteria that are present in the milk raw material. Defects during storage are associated with the development of residual microflora of pasteurized products. The aim of the work was to determine the quantitative changes in milk microflora using different pasteurization temperatures. Pasteurization of raw milk of the first and extra grades was carried out at t = +72 °С and t = +91 °С with a holding time of 15–20 s. In pasteurized and raw milk, the residual amount of microorganisms of different groups was determined: mesophilic, psychrotrophic, lactic acid, heat-resistant, and spore-forming. It was found that in raw milk before pasteurization, the main part of the microflora was psychrotrophic and mesophilic microorganisms up to 70%, the share of lactic acid microbiota was up to 25%, heat-resistant and spore-forming bacteria were 4% and 0.8%, respectively. The temperature regime of heat treatment (t = 72 °C exposure for 20 s) contributed to the reduction of mesophilic bacteria by 93.4% when using extra grade milk and by 91.5% when using first grade milk. That is, almost 6.4 times more bacteria remain in drinking milk when raw materials of lower quality are used. The intensity of death of heat-resistant microflora of milk under the regime of 72 °C with a holding time of 20 s was only 15.2 % when using raw extra milk and 4.2 % (first). Pasteurization at a temperature of 91 °C for 20 seconds had a much stronger effect on this microflora, as its efficiency was 52.9 % and 49.2 %. That is, the efficiency of the pasteurization mode was 3.5 and 11.7 times stronger, respectively, compared to the mode at a temperature of 72 °C. Therefore, in order to apply an effective pasteurization regime at the enterprise, it is necessary to know the quantitative and qualitative composition of the microflora of raw milk.