EFFECT OF NISIN AND HIGH-TEMPERATURE PASTEURIZATION ON THE SHELF-LIFEOF WHOLE MILK

Two experiments were conducted to assess the effectiveness of nisin on the keeping quality of pasteurized whole milk. After pasteurization, milk samples were stored at 10 degrees C and samples were analysed at intervals for total plate count (TPC), Lactobacillus count, calcium io n concentration, pH and total acidity (TA). In the first experiment ni sin was added to milk samples in the range 0 to 50 IU ml(-1) prior to pasteurization at 72 degrees C for 15 s. All concentrations of nisin u sed were effective in controlling microbial growth. Milk containing 40 and 50 IU ml(-1) nisin had not spoiled after 41 days' storage compare d to spoilage time of 14 days for the control milk. In the second expe riment 40 IU ml(-1) of nisin was combined with three 'pasteurization' processes: 72 degrees C for 15 s, 90 degrees C for 15 a and 115 degree s C for 2 s. The milk processed at 72 degrees C for 15 s with nisin sh owed an increased keeping quality of about 7 days compared with the co ntrol and showed a significantly lower count for Lactobacillus. In con trast, nisin had a much gr-eater effect on TPC counts in the milk past eurized at 90 degrees C for 15 s, and after 28 days' storage at 10 deg rees C the milk was still acceptable. Milks treated both with and with out nisin at 115 degrees C for 2 s were microbiologically acceptable a fter 28 days, with counts less than 10 ml(-1). However, the milk with nisin was superior in flavour, as no noticeable off-flavours were appa rent after 32 days. All these results are consistent, as shown by the microbiological and chemical analyses. Addition of nisin to milk prior to pasteurization provides an opportunity to achieve extended shelf-l ife in regions with poor refrigeration.