|Abstract or Summary
- During the course of ripening, Cheddar cheese frequently develops
a flavor defect described as fruity. Recent work has indicated
that the use of certain starter cultures ultimately results in the
development of the defect as the cheese ages. The flavor compounds
responsible for the defect, however, have not been elaborated. The
purpose of this investigation was to isolate and identify the components
responsible for the fruity flavor defect and to evaluate the role
of certain cheese starter cultures in the development of the defect.
Since the fruity character of the defect is apparent in the aroma
of the cheese, the compounds responsible for the defect were expected
to be reasonably volatile. Volatile constituents were isolated
by a distillation technique from fat expressed from a typically fruity
cheese by centrifugation. The volatile constituents were then separated
by gas-liquid chromatography. By monitoring the odor of the
effluent stream of the column, it was possible to determine which components had fruity odors, and these were subsequently identified
by mass spectral analysis and coincidence of retention time with the
authentic compounds. Ethyl butyrate, ethyl hexanoate, and ethyl
octanoate were found to be the only compounds with detectable fruity
The volatiles from the fat of four cheeses possessing varying
degrees of the defect and their matching non-fruity controls were
analyzed by a gas entrainment, on-column trapping, gas-liquid chromatographic technique. The manufacturing and curing conditions of
each fruity cheese and its matching control were identical, except
for the use of different starter cultures. Ethanol, ethyl butyrate,
and ethyl hexanoate were more abundant in each of the fruity samples.
The approximate concentration range of these compounds was
as follows: In fruity cheese; ethanol 400 to 2,040 ppm, ethyl butyrate
1.6 to 24 ppm, ethyl hexanoate 0.9 to 25 ppm. In non-fruity cheese;
ethanol 36 to 320 ppm, ethyl butyrate 0.7 to 4.7 ppm, ethyl hexanoate
0.3 to 2.2 ppm. In ten commercial Cheddar cheeses selected at
random from the market, the concentration of ethanol ranged from
5.5 to 620 ppm.
Single-strain cultures of Streptococcus lactis, Streptococcus
diacetilactis, and Streptocococcus cremoris as well as three mixedstrain
commercial cultures were evaluated for ethanol and acetaldehyde
production in non-fat milk medium. Among the single-strain cultures there appeared to be no correlation between ethanol production and
species, although considerable variation was noted for strains within
a species. The mixed-strain cultures were designated A, B, and C.
Cultures B and C had been implicated in the development of the fruity
flavor defect in Cheddar cheese, while culture A produced normal
cheese of good quality. Cultures B and C produced approximately 40
times more ethanol than culture A when incubated in non-fat milk
medium for one month at 7°C.
Certain single-strain cultures and the three mixed-strain cultures
were tested for their ability to reduce acetaldehyde and propanal,
and to catalyze the formation of ethyl butyrate when ethanol and
butyric acid were provided as substrates. Acetaldehyde and propanal
were reduced to the corresponding alcohols by all cultures, but the
formation of ethyl butyrate was not observed in any culture.
A good correlation between high levels of ethanol and high levels
of ethyl butyrate and ethyl hexanoate in the fruity cheeses suggests
that the quantity of ethanol present in the cheese may determine the
amount of ester formed. Further, starters resulting in the defect
produced considerably more ethanol than cultures resulting in normal
cheese when incubated at 7°C, a normal temperature for curing Cheddar
cheese. This observation adds weight to the hypothesis that certain
cultures are directly responsible for the defect.