Heat Resistant Bacterial Lipases and Ultra-High Temperature Sterilization of Dairy Products

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Abstract
Properties of a heat-resistant lipase produced by Pseudomonas spp. MC50, a psychrotroph isolated from raw milk, were studied to determine if such enzymes could interfere with successful production of ultra-high temperature sterilized dairy products. In milk MC50 lipase was extremely heat-resistant at 100-150.degree. C and would be expected to suffer little inactivation during recommended ultra-high temperature processes of 121-149.degree. C for 0.5-8 s. The optimum temperature for MC50 lipase activity in butter oil emulsion was 40.degree. C, and activity at 20-25.degree. C was 27-41% of maximum. The optimum pH was pH 8.5 with only 8.3% of maximum activity at pH 6.5 but 43% of maximum activity at pH 7. Lipase activity was greatest in 5% butter oil emulsions, but activity was never < 65% of maximum over the range of 1-10% butter oil. The pH may be the most critical factor in influencing lipase activity in ultra-high temperature sterilized dairy products. Of 5 batches of normal whole milk and 4 batches of normal 10% half-and-half subjected to 1 of 3 ultra-high temperature sterilization treatments, all exhibited evidence of lipase activity during subsequent storage at room temperature and 40.degree. C. Changes in acid degree values during storage suggested that hydrolytic rancidity could develop in these products in 1-7 mo. depending on the ultra-high temperature process and storage temperature. Heat-resistant lipases may be common to raw milk and will survive readily most ultra-high temperature sterilization treatments. Although pH may be inhibitory, other factors are suitable to allow lipase activity in dairy products leading to development of rancidity.