Effect of sucrose on the production of flavor compounds by yogurt culture bacteria Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dv13zw87q

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  • Organoleptically, flavored yogurts appear to contain lower levels of acetaldehyde than plain yogurt. This study was undertaken to seek the reasons for this phenomenon by focusing on the analyses of acetaldehyde production by yogurt culture bacteria in yogurt base containing added sucrose and by performing other analyses such as the determination of cell numbers, pH, titratable acidity, volatile acidity and average flavor threshold (AFT) values of acetaldehyde in various media. The volatile compounds produced by yogurt culture bacteria were trapped and chromatographed by a gas entrainment on-column trapping gas-liquid chromatographic technique. Acetaldehyde was identified by coincidence of retention time with that of the authentic compound and quantitated using methyl acetate as an internal standard. Acetaldehyde production by mixed cultures was first detectably inhibited by 8% sucrose. However, acid production and cell counts of both species, grown together in the mixed culture, were inhibited by concentrations of sucrose of 4% and higher. A rapid production of acetaldehyde at concentrations between 0% and 8% sucrose occurred between 2 and 6 hr incubation times. The level of this compound then decreased up to 15 hr incubation and leveled off with continued incubation up to 24 hr. By itself, Lactobacillus bulgaricus was stimulated by 4%, 8% and 12% sucrose resulting in the high production of acetaldehyde and lactic acid and increased cell numbers in comparison with cultures grown in media containing no added sucrose. A level of 8% sucrose was most effective. The rod continuously produced acetaldehyde up to 24 hr, suggesting it is responsible for the production of high concentrations of acetaldehyde in yogurt. Acetaldehyde production by Streptococcus thermophilus varied from strain to strain. In general, however, less acid was produced and microbial numbers were lower, as sucrose content increased. The coccus produced much lower amounts of acetaldehyde at equivalent incubation times as compared with the rod. During refrigerated storage of from 1 to 14 days, acetaldehyde concentration greatly decreased in media containing both 0% and 8% sucrose. Values for volatile acidity of yogurts containing 0% to 8% sucrose were low. There was little or no difference in volatile acidities between yogurts containing 0% and 8% sucrose. Average flavor threshold values for acetaldehyde were slightly higher in 2%-fat milk with 8% added sucrose than in plain 2%-fat milk. With the addition of both strawberry flavor and 8% sucrose, the AFT of acetaldehyde was much higher. Since there were no great differences in the levels of acetaldehyde found in yogurts containing 0%, 4% or 8% sucrose (8% being the amount ordinarily added to flavored yogurt), it is suggested that the strong masking effect exerted by fruit, fruit flavor and sucrose, as demonstrated by AFT values, is largely responsible for the organoleptic sensation of lower levels of acetaldehyde in flavored yogurt.
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