Various oil‐accumulating yeasts were tested for their ability to produce lipase and live on fats and oils as carbon sources. Of these,Candida lipolytica seemed most promising, and the possibility was explored of modifying fats and oils by fermenting them withC. lipolytica and extracting the modified oil deposited in the yeast cells. Oxygen was required for the growth of yeast on fats and oils, but unless the oxygen level was controlled at a low value after cell populations peaked, most of the substrate oil was converted to citrates rather than accumulating as oil. Oil accumulation byC. lipolytica from a corn oil substrate was slightly depressed by excess nitrogen in the medium. The yeasts were able to use about 18 g/l of oil in 72 hr. At substrate oil levels greater than 18 g/l, the dry yeasts were 60% oil, and about 45–57% of the substrate oil was recovered as yeast oil. The fatty acid composition of the yeast oil was quite similar to that of the substrate oil under optimum conditions of deposition. Sterols, but not tocopherols, were transferred from the substrate to the yeast oil.Candida lipolytica oil was high in free fatty acids. The greatest potential for biomodification by fermentation withC. lipolytica seems to be in altering glyceride structure.

Propionibacterium thoenii strain P127, which produces the bacteriocin propionicin PLG-1, was grown in a skim milk medium and produced bacteriocin in that medium. No bacteriocin activity was detected in skim milk medium in which strain P127-1, a bacteriocin-negative variant of strain P127, had been grown. Five psychrotrophic spoilage or pathogenic organisms (one strain each of Listeria monocytogenes, Pseudomonas fluorescens, Vibrio parahaemolyticus, Yersinia enterocolitica, and one strain of Corynebacterium sp.) were incubated for 24 h in laboratory medium, nonfermented skim milk, and skim milk that had been fermented by strain P127 or P127-1. Strains were inhibited only in the skim milk fermented by strain P127, as evidenced by loss in numbers of viable cells after 24 h at 10 degrees C and less growth than in other media after 24 h at optimal growth temperatures. Growth of selected strains was delayed or slowed during prolonged incubation (21 d) at 10 degrees C. Propionicin PLG-1 shows promise as a preservative for food products.

The effect of probiotic bacteria on the formation of conjugated linoleic acid (CLA), microbial growth, and organoleptic attributes (acidity, texture, and flavor) of fermented milk products was determined. Four probiotic bacteria, Lactobacillus rhamnosus, Propionibacterium freudenreichii subsp. shermanii 56, P. freudenreichii subsp. shermanii 51, and P. freudenreichii subsp. freudenreichii 23, were evaluated individually or in coculture with traditional yogurt cultures (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus). The lipid source was hydrolyzed soy oil. L. rhamnosus, in coculture with yogurt culture, resulted in the highest content of CLA. Growth and CLA formation of propionibacteria were enhanced in the presence of yogurt cultures. Texture and flavor attributes of fermented milks produced with propionibacteria were significantly different than the fermented milks processed with yogurt cultures. The fermented milks processed with probiotic bacteria in coculture with yogurt cultures demonstrated similar acidity, texture, and flavor as the fermented milk produced with yogurt cultures.

The objective of this research was to study the effects of probiotic bacteria, lipid source, and fermentation time on the CLA content of a milk model system. The evaluation of 11 probiotic bacteria showed that they were able to produce CLA from linoleic acid in a model system containing hydrolyzed soy oil (1%) emulsified in milk, but not in model systems of unhydrolyzed soy oil (1%) emulsified in milk or 1% fat milk. Propionibacterium freudenreichii subsp. shermanii 56, P. freudenreichii subsp. shermanii 51, and P. freudenreichii subsp. freudenreichii 23 demonstrated the greatest increase in CLA content. Propionibacterium freudenreichii subsp. shermanii 51, produced the highest cis-9,trans-11 CLA content and also produced the greatest increase in trans-10,cis-12 CLA content as fermentation time was increased from 24 to 48 h. The fermentation of probiotic bacteria for 24 h was often most effective in increasing the CLA content. Viable counts of probiotic bacteria increased significantly from 0 to 24 h. These results demonstrated that the content of CLA during fermentation was primarily dependent on the strain of probiotic bacteria and the lipid source in the milk model system. This research suggests an efficient approach to produce CLA-enriched cultured dairy products.