Quality and shelflife of milk pasteurized at 74 degrees C/19-22 s free from post-pasteurization contamination.

摘要

This study was conducted to determine the quality and shelf life of HTST pasteurized milk, free from post-pasteurization contamination and stored at 4 and 6 degrees C. The number of the following microorganisms was determined: total bacteria; thermoresistant bacteria; psychrotrophic bacteria; caseolytic bacteria; lipolytic bacteria; Bacillus cereus; coliform bacteria; and mesophilic aerobic bacterial spores. Moreover, the acidity and milk protein stability were determined, and a sensory evaluation was carried out. The bacteria most sensitive to the pasteurization parameters were coliform and psychrotrophic bacteria. Their absence in 1 cm3 of milk immediately after heat treatment also indicated the absence of microbiological post-pasteurization contamination. The degree of destruction of bacteria in pasteurized milk was confirmed to be dependent on the total number of bacteria and number of thermoresistant bacteria in raw milk. It was proven that the growth rate of microorganisms in pasteurized milk was slower and the shelf life of stored milk was longer at 4 degrees C than at 6 degrees C. Changes in the total bacterial count (mainly psychrotrophic bacteria and lipolytic bacteria) of milk were observed not earlier than 21 days of storage at 4 degrees C. Similar changes in the total bacterial count were not observed after 14 days of storage at 6 degrees C. Milk samples received a score of 5 in the sensory evaluation carried out immediately after heat treatment on a ranking scale from 1 (the worst) to 5 (the best). After 21 days of storage at 4 degrees C, the mean score was 4.5, whereas at 6 degrees C, it was 4.0. Changes observed in the acidity of stored milk samples were insignificant. Changes in milk protein stability were not detected in any of the milk samples. This study revealed that it was possible to produce HTST pasteurized milk with a 14-day shelf life in Poland..