The effect of nisin on the keeping quality of reduced heat-treated milks

Milk was subjected to a combination process involving reduced heat treatmen t (RHT) of 117 degreesC for 2 s and nisin (75 and 150 IU ml(-1)). The micro bial activity and other quality aspects were compared with a RHT control (w ithout nisin) and with a ultrahigh temperature (UHT) milk processed at 142 degreesC for 2 s. Nisin was found to inhibit microbial growth for products stored without refrigeration, and RHT-nisin samples stored at 30 degreesC s howed very low spoilage rates during 150 days, although not low enough to s atisfy requirements for commercial sterility. RHT-nisin samples could be di stinguished from and were preferred to the UHT control. Significant brownin g occurred during storage at 30 degreesC and above but was less in the RHT- nisin milk samples compared with the UHT milk. In RHT-nisin milk samples st ored at 20 and 10 degreesC, no microbial activity could be detected in most samples after storage for 1 year. The effectiveness of this combination of RHT, nisin, and low storage temperatures against gram-positive spore-formi ng bacteria suggests potential for use of nisin in extended shelf life prod ucts.