- Stale flavor development has been recognized as a defect of
stored dry milk powders for several years. Recently, stale flavor
development has been found to occur upon storage of sterilized concentrated
milk, and is recognized as the principal limiting factor to
commercial utilization of this process (Seibert, 1963).
Some attempts have been made to identify the volatile flavor
components of sterilized concentrated milk (Patel et al, 1963;
Bingham, 1964). The flavor components responsible for the stale
flavor defect as it occurs in sterilized concentrated milk have not
been identified, however.
The purpose of this work was to identify the compounds responsible
for the stale flavor defect of sterilized concentrated milk.
It was felt that this information was essential to an understanding
of the staling phenomenon, which in turn might eventually lead to
correction of the defect.
Commercial samples of sterilized concentrated milk were obtained.
Stale flavor development was hastened in some of the samples
by storing them at 21°C. Subjective flavor panel evaluation of stored
and fresh samples revealed significant differences between the two.
Gas chromatographic analysis of the volatile head space components
by the procedure described by Morgan and Day (1965) revealed
only minor differences between the fresh and stale samples.
It was reasoned, therefore, that the compounds responsible for the
stale flavor defect were primarily of a less volatile nature,
A technique for isolating the higher-boiling flavor components
was developed. This technique consisted of lyophilization of the
sterile concentrated milk, uniform wetting of the lyophilized milk
with water to 10% moisture, solvent extraction of the fat and flavor
components from the moistened milk powder, and reducedtemperature,
reduced-pressure steam distillation of the flavor components
from the extracted fat. The resulting flavor extract was
studied by gas chromatography in conjunction with mass spectrometry
A base-treated pre-column was used in front of the regular gas
chromatography column to remove fatty acid peaks from the chromatograms.
A technique, which consisted of repeatedly trapping (from
several successive chromatograms) particular regions of the effluent
from a non-polar column onto a short section of packed column and
re-chromatographing the trapped components on a polar column, was developed to build up the concentration of flavor components and to
improve the separation of components for mass spectral analysis.
The following compounds were positively identified in the flavor
extract from stale sterile concentrated milk: 2-heptanone, 2-nonan.one,
2-undecanone, 2-tridecanone, benzaldehyde, napthalene, a dichlorobenzene,
L-decalactone, benzothiazole, and o-aminoacetophenone.
Acetophenone was tentatively identified. Of these compounds, 2-
heptanone and the dichlorobenzene were positively identified in the
extract from fresh sterile concentrated milk, and L-decalactone
was thought to be present.
The ketones and L-decalactone undoubtedly make some contribution
to the stale flavor defect (USDA, 1964). The identification
of o-aminoacetophenone in stale sterilized concentrated milk supplements
its identification in stale nonfat dry milk powder (Parks,
Schwartz and Keeney, 1965), and further implicates it as an important
compound in the stale flavor defect. This compound possesses
a characteristic "grape-like" odor. Benzothiazole has not previously
been identified in milk products. It possesses a characteristic
"rubber-like" odor. Its possible significance in the stale flavor
defect will require further study.