Challenge testing of the lactoperoxidase system in pasteurized milk

Aims: To determine the role of lactoperoxidase (LP) in inhibiting the growt h of microorganisms in pasteurised milk. Methods and Results: Four micro-organisms of importance in the spoilage of pasteurized milk were challenged in lactoperoxidase (LP)-enriched ultra-hea t treated (UHT) milk after subsequent pasteurization. Milk samples were sto red at the optimum temperatures for growth of the individual bacteria. Past eurization was carried out at 72 degreesC/15 s and 80 degreesC/15 s to dete rmine the effect of the LP system on the micro-organisms. An active LP syst em was found to greatly increase the keeping quality (KQ) of milks inoculat ed with Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus the rmophilus and pasteurized at 72 degreesC, but had little or no effect in mi lks heated at 80 degreesC, presumably due to virtual inactivation of LP at 80 degreesC. However, pasteurization temperature had no effect on the KQ of milks challenged with Bacillus cereus spores. Conclusions: This study suggests that the LP system, rather than heat-shock ing of spores, is responsible for the greater KQ of milk pasteurized at 72 degreesC/15 s compared with 80 degreesC/15 s. Significance and Impact of Study: The study emphasizes the care required in selecting pasteurization temperatures in commercial practice and to avoid the temptation to compensate for inferior quality of raw milk by increasing pasteurization temperature.