The effect of heat stable bacterial lipase (S) on the flavor and fat degradation of pasteurized milk cheddar cheese

Two species of psychrophilic bacteria, Pseudomonas fragi 169 and Pseudomonas fluorescens 31, chosen for their high production of heat stable lipase(s) in a milk medium, were each inoculated into five lots of raw milk. The samples were incubated until counts were from 3 to 5 million cells per ml. as determined by the Direct Microscopic Count and were then pasteurized at 63°C for 30 minutes. Lots of the same milk not inoculated were pasteurized at 63°C for 30 minutes and used as controls. After pasteurization, inoculated and control lots of milk were made into Cheddar cheese. Acid Degree Values were determined on extracted fat after two weeks and again at successive three month intervals for one year. Flavor differences between treated and control samples were determined by a triangle taste test. The Acid Degree Values of fat from inoculated lots of cheese increased significantly over those from controls in the first three months of curing. Significant differences in Acid Degree Values were noted at every curing period. The Acid Degree Values produced by the lipases of Pseudomonas fragi 169 were significantly higher than those produced by Pseudomonas fluorescens 31. No significant difference in the flavor of cheese made from milk inoculated with Pseudomonas fragi 169 or its controls was noted until after nine months of curing. A significant difference was noted between cheese lots made from milk inoculated with Pseudomonas fluorescens 31 and control lots after twelve months curing. Inoculated and control lots of cheese scored very close in flavor.