The presence of yeasts in milk may cause physical and chemical changes limiting the durability and compromising the quality of the product. Moreover, milk and dairy products contaminated by yeasts may be a potential means of transmission of these microorganisms to man and animals causing several kinds of infections. This study aimed to determine whether different species of yeasts isolated from bovine raw milk had the ability to develop at 37°C and/or under refrigeration temperature. Proteinase and phospholipase activities resulting from these yeasts were also monitored at different temperatures. Five genera of yeasts (Aureobasidium sp., Candida spp., Geotrichum spp., Trichosporon spp. and Rhodotorula spp.) isolated from bovine raw milk samples were evaluated. All strains showed one or a combination of characteristics: growth at 37°C (99·09% of the strains), psychrotrophic behaviour (50·9%), proteinase production (16·81% of the strains at 37°C and 4·09% under refrigeration) and phospholipase production (36·36% of the isolates at 37°C and 10·9% under refrigeration), and all these factors may compromise the quality of the product. Proteinase production was similar for strains incubated at 37°C (16·81% of the isolates) and room temperature (17·27%) but there was less amount of phospholipase-producing strains at room temperature (15·45% of the isolates were positive) when compared with incubation at 37°C (36·36%). Enzymes production at 37°C by yeasts isolated from milk confirmed their pathogenic potential. The refrigeration temperature was found to be most efficient to inhibit enzymes production and consequently ensure better quality of milk. The viability of yeasts and the activity of their enzymes at different temperatures are worrying because this can compromise the quality of dairy products at all stages of production and/or storage, and represent a risk to the consumer.

The asymmetric distribution of phospholipids in the erythrocyte membrane during the intracellular development of the human malaria parasite Plasmodium falciparum was studied. Infected cells of high parasitaemia were treated with phospholipase A2 or sphingomyelinase C, followed by isolation of the host red cell membrane using the Affigel (731) bead method. Additionally, phosphatidylserine on the surface of infected cells was probed using a phosphatidylserine-sensitive prothrombinase assay. Trophozoite-infected cells showed an increase in phosphatidylethanolamine and phosphatidylserine and a decrease in phosphatidylcholine in the outer leaflet. In addition to the changes already present in trophozoite-infected cells, schizont-infected cells showed a decrease in sphingomyelin as well as a further increase in phosphatidylserine in the outer leaflet. The results are discussed with respect to possible mechanisms and consequences of these changes.

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.