Many psychrotrophic bacteria contaminating raw milk produce phospholipase
that withstands pasteurization and UHT treatments. This enzyme acts on the
milk fat globule membrane and exposes triacylglycerides to the action of lipase.
Phospholipase production by various isolates of Pseudomonas was investigated. The
isolates were cultured aerobically at 8 °C in nutrient broth, McKellar's minimal salts
medium, Chrisope's medium, and skim milk. Each strain produced phospholipase
during the 50 h incubation. Enzyme production varied significantly (P < 0·001) with
strain and growth medium. Strains varied significantly (P < 0·001) in their enzyme
production in each medium and during the incubation time as well. Strain,
incubation time, and the growth medium significantly influenced (P < 0·001) heat
stability of the enzyme activity. Pasteurization reduced the activity, but did not
eliminate it in skim milk.