Graduate Thesis Or Dissertation
 

Production of carbonyl compounds by lactic streptococci : relation to Cheddar cheese flavor

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  • In an attempt to investigate the probable roles of lactic starter flora in the development of fruitiness and slit-openness in Cheddar cheese, several experimental lots of cheese were made with different commercial lyophilized starters until the defect could be consistently duplicated. From many such trials, two cultures, B and C, which repeatedly provided the defect, and one, designated as A, tiat yielded high-grade product, were selected for further experiments. With starter culture as the only variable, "defective" and "normal" cheeses were made under uniform operating conditions in adjacent vats from portions of the same bulk milk to compare the microbiological and chemical changes occurring within the respective cheeses. Attempts to correlate differences between "normal" and "defective" starters in the rates of cheese - sugar and -protein degradation to the development of fruity flavor and slit-openness in cheeses were unsuccessful; no significant differences were noted in this regard between starters A, B, and C. Distinct differences throughout the ripening period, however, were observed in the starter population trends in the cheeses made with the "normal" and the "defective" cultures. Starter A exhibited a rapid decline in numbers of the starter population; cultures B and C persisted at very high population levels for prolonged periods. As a result, the progressive replacement of starter flora by succeeding lactobacilli, considered necessary for normal flavor development, was considerably delayed in the "defective" cheeses. In the "normal" cheeses, replacement occurred within six to eight weeks. Taxonomic studies on starters A, B, and C showed that the "normal" culture was almost entirely made up of S. cremoris strains, and the "defective" starters contained S. lactis and S. diacetilactis strains in addition to S. cremoris strains. Of these, the S. lactis and S. diacetilactis strains were generally found to produce greater concentrations of carbonyl compounds in milk cultures than S. cremoris strains, and as such the carbonyl concentrations in milk cultures of B and C were considerably greater than in A. Similar trends were observed when known strains of S. cremoris, S. lactis, and S. diacetilactis were compared for total carbonyl production in milk cultures. No striking differences in the types or numbers of carbonyl compounds were found in milk cultures of starters A and C. However, in a given amount of culture, the latter had higher concentrations of formaldehyde, acetaldehyde, diacetyl, and possibly pyruvic acid. The effect of the "defective" starter strains on the final flavor of Cheddar cheese was, therefore, reasoned to be two-fold. One was indirect and concerned with the sequential predominance of different microbial flora that occurred within the ripening cheese. The other was direct and related to the starter metabolic activity, and the accumulation within the cheese of certain carbonyl compounds important in flavor impairment.
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