Graduate Thesis Or Dissertation
 

Studies on the inhibition of spoilage organisms in Cottage cheese by lactic streptococcus organisms

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  • A report in the literature suggesting that the use of cottage cheese dressing cultured with S. diacetilactis would afford enhanced flavor and shelf life in the finished product prompted this study to examine reasons for its effectiveness. The method used involved addition of a culture of S. diacetilactis 18-16 to 12 percent cream at the rate of one percent with incubation at 21° C. for six hours; this dressing was then used to cream dry Cottage cheese curd. It was found that the dressing was effective in suppressing the growth of organisms in the Bacillus, Pseudomonas, Proteus, Alcaligenes, Aerobacter, and Staphylococcus genera when the dressing was used on Cottage cheese curd that had been heavily contaminated with these organisms. The disc assay method was used to determine what effect various culture fractions would have against the growth of these spoilage organisms. The culture fractions tested were: cell suspension of S. diacetilactis (2 x 10¹⁰ cells/ml.); sonicated cell suspension (2 x 10⁸ viable cells/ml.); broth supernatant of 24 hour culture; broth supernatant of 24 hour culture concentrated ten times by lyophilization; this same concentrate, dialyzed to remove salts and sugars; uninoculated broth, single strength, concentrated, and concentrated-dialyzed; 24 hour milk culture; 24 hour broth culture. Buffered lactic acid at pH 4.5, 5.0, 5.5 or 7.0 was also tested. In all cases, there was no inhibition from these culture fractions except where there were viable cells of the lactic organisms present. Cross streaking with S. diacetilactis 18-16 against the various spoilage organisms also showed inhibition of the bacteria at the point of intersection; not all organisms were inhibited to the same degree. Competitive growth studies were also conducted between S. diacetilactis and S. aureus and between S. diacetilactis and P. fragi in milk and broth. Growth repression of P. fragi ranged from one to ten percent of the control growth in the presence of viable cells of S. diacetilactis; there was no inhibition in the presence of heat-killed cells. Similar results were found with S. aureus grown in the presence of S. diacetilactis. Maintenance of the pH at 7.0 did not alter the inhibitory effect, suggesting that the lowered pH caused by acid produced by the lactic organisms was not a contributing factor. The minimum growth temperature of S. diacetilactis 18-16 was found to be approximately 7.5° C.; the cells were still metabolicly active at this temperature. Inhibition of P. fragi did occur when grown with S. diacetilactis at 7.5° C., suggesting that active metabolism and not cell growth per se was responsible for the inhibitory effect. Experiments were conducted with Cottage cheese curd that had been heavily contaminated with spoilage organisms and creamed with cultured S. diacetilactis dressing or plain cream. Results showed that the cultured dressing was effective in retarding loss of aroma and slimy curd defect when the cheese was held at 7.5° C. for 30 days, or for two weeks at 10° C. and even when held at 21° C. for as long as six days.
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