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. / Graduation date: 1972
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/26912 |
| Date | 27 July 1971 |
| Creators | Yang, Chang-su |
| Contributors | Bills, Donald D. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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