J. JIANG, L. BJÖ RCK AND R. FONDÉ N. 1998. Nineteen different strains of lactobacilli, lactococci, streptococci and propionibacteria commonly used as dairy starter cultures were tested for their ability to produce conjugated linoleic acid (CLA) from free linoleic acid in vitro. Two strains of Propionibacterium freudenreichii ssp. freudenreichii and one strain of P. freudenreichii ssp. sheramnii were found to be capable of converting free linoleic acid to extracellular CLA. The highest level of CLA formed in the media was 265 mg ml −1 . Of the different isomers, cis-and trans-9,11-octadecadienoic acid represented more than 70% of the total CLA formed. The inhibitory effect of linoleic acid on the growth of the bacteria and its conversion to CLA in different media by propionibacteria are discussed.
Cooling strategies for pumping of raw milk were evaluated. Milk was pumped for 450 s at 31 degrees C, or pumped after cooling to 4 degrees C and subsequently subjected to various incubation times. Two types of milk were used; i.e. milk from cows fed a diet high in saturated fat supplements resulting in significantly larger milk fat globules than the other type of milk which comes from cows fed a low-fat diet that stimulates high de novo fat synthesis. The content of liquid fat was determined by low-field 1H NMR, which showed that milk from cows given the saturated fat diet also contained less liquid fat at both 4 degrees and 31 degrees C than the other type of milk. This can be ascribed to the differences in the fatty acid composition of the milk as a result of the fatty acid composition of the diets. After pumping of the milk at 31 degrees C, measurement of fat globule size distribution revealed a significant coalescence of milk fat globules in the milk obtained from the saturated fat diet due to pumping. Pumping at 4 degrees C or pumping the other type of milk did not result in coalescence of milk fat globules. Formation of free fatty acids increased significantly in both types of milk by pumping at 31 degrees C. Cooling the milk to 4 degrees C immediately before pumping inhibited an increased content of free fatty acids. However, when the milk was incubated at 4 degrees C for 60 min after cooling and then subjected to pumping, a significant increase in the formation of free fatty acids was observed in both types of milk. It is suggested that this increase in free fatty acids is caused by transition of polymorphic crystal forms or higher level of attached lipoprotein lipases to the milk fat globule before pumping.
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