Heat Resistant Bacterial Lipases and Ultra-High Temperature Sterilization of Dairy Products1,2

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 to 150°C and would be expected to suffer little inactivation during recommended ultra-high temperature processes of 121 to 149°C for .5 to 8s. The optimum temperature for MC50 lipase activity in butter oil emulsion was 40°C, and activity at 20 to 25°C was 27 to 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 less than 65% of maximum over the range of 1 to 10% butter oil. The pH may be the most critical factor in influencing lipase activity in ultra-high temperature sterilized dairy products. Of five batches of normal whole milk and four batches of normal 10% half-and-half subjected to one of three ultra-high temperature sterilization treatments, all exhibited evidence of lipase activity during subsequent storage at room temperature and 40°C. Changes in acid degree values during storage suggested that hydrolytic rancidity could develop in these products in 1 to 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.