- Increasing interest has been shown in the irradiation sterilization
and irradiation pasteurization of foods, but problems of off-flavors
and odors are still unsolved, especially in the case of dairy products.
From the flavor chemistry point of view, milk lipids are very highly
susceptible to irradiation effects. Therefore, this investigation was
designed to study some irradiation induced reactions involving flavor
changes in the milk fat and to identify the volatile components produced
in the milk fat upon irradiation.
Milk fat, prepared from raw sweet cream and washed free of
phospholipids, was first irradiated in the presence of air and under
vacuum in glass vials at 4.5 Mrad with gamma rays from cobalt-60.
The irradiation resulted in increase in TBA number, peroxide value,
total monocarbonyls, bleaching of color, slightly rancid and typical
candle-like off-flavors. Free fatty acids were also produced upon
irradiation. The changes were more drastic in air along with production
of a slight oxidized flavor. The monocarbonyls identified by
column and paper chromatographic methods in irradiated milk fat include: C₁ through C₁₂, C₁₄ , and C₁₆ n-alkanals; C₃ through C₉,
C₁₁, C₁₃ and C₁₅ alk-2-ones with only traces of C₆ and C₈ alk-2-
ones; and C₅, C₆, C₉, and C₁₂ alk-2-enals.
Irradiation of milk fat that had been dried over calcium hydride
also caused free fatty acid production, especially short chain fatty
acids. Methyl octanoate treated with calcium hydride and irradiated
at 1.5, 3.0, 4.5, and 6.0 Mrad yielded small quantities of free
octanoic acid, confirming that irradiation caused fission of the ester
linkage even when traces of water were removed. The quantities of
octanoic acid formed increased with increasing dose of irradiation.
For identification of volatile components, the milk fat was irradiated
in 307x409 'C' enameled cans under vacuum. The headspace
analysis showed some air still left in the cans. Irradiation resulted
in consumption of oxygen and production of hydrogen, carbon
monoxide, carbon dioxide, and methane as identified in the headspace
gases. The volatiles were isolated from the irradiated and
control milk fats by low temperature, vacuum steam distillation at
40°C and 1-2 mm Hg. The volatile components were then extracted
from the aqueous distillate with ethyl ether. The ethyl ether extract
exhibited the typical candle-like defect. The ethyl ether concentrate
was analyzed by combination of GLC and fast-scan mass spectrometric
techniques. Identification of various components was achieved
on the basis of mass spectral data and coincidence of gas chromatographic retention times. In the case of the components
for which only GLC t[subscript r]/t[subscript r] evidence was available or the mass spectra
obtained were not satisfactory, the identity assigned was only tentative.
The volatile compounds that were positively identified to be
present in irradiated milk fat are given below:
n-Alkanes C₅ through C₁₇
1-Alkenes C₅, C₇ through C₁₇
Fatty acids C₄, C₆, C₈ and C₁₀
n-Alkanals C₅ through C₁₁
Others γ-decalactone, δ-decalactone, 2-heptanone,
benzene, ethyl acetate, chloroform, and
The tentative identification was obtained for the following
γ-lactones C₆ and C₈
δ-lactones C₆, C₈, C₁₁, and C₁₂
1, ?-alkadienes C₁₀, C₁₁, C₁₂, C₁₆ and C₁₇
iso-alkanes C₁₀, C₁₁, C₁₂, and C₁₃
Others methyl hexanoate, 2-hexanone, 4-heptanone
The compounds present in unirradiated control milk fat included:
short chain fatty acids (C₄, C₆, C₈, and C₁₀), C₈, C₁₀,
and C₁₂ δ-lactones, 2-heptanone, chloroform, dichlorobenzene,
benzene, toluene, and ethyl-benzene. Only tentative identity was
established for most of these components in control milk fat.
Possible reaction mechanisms are presented for the formation of
the compounds in irradiated milk fat.