ACTION OF LIPASES OF STAPHYLOCOCCUS AUREUS ON MILK FAT.

The activity of the lipase(s) of two strains of coagulase-positive Staphylococcus aureus was determined in milk fat incubated at 15, 22, and 30 C for 8 days. Total fat hydrolysis was measured by acid degree values (ADV). Neutral lipids were separated into component groups on a Florisil column. Free fatty acids were determined by temperature-programmed gas-liquid chromatography. The ADV were 25 to 50% greater at 22 than at 15 C and 4 to 7 times greater at 30 than at 22 C. The lipases liberated as much as 0.48 g of fatty acids per gram of fat during 8 days at 30 C. The enzyme showed a predilection for the palmitic acid-glycerol bond. Addition of fatty acids C(14) to C(18) inclusive to inoculated sterile skim milk caused inhibition of S. aureus as follows: (i) complete at 0.05 and 0.10% concentration of C(10) and (ii) partial at 0.05 and complete at 0.10% concentration of C(8). The samples showing inhibition were negative for peptonization, coagulase, and change in pH. Addition of oleic and stearic acid to sterile skim milk inoculated with S. aureus resulted in an increase in nonprotein nitrogen, and the C(4) to C(12) acids caused a decrease in protease activity.