3.8 Unsaponifiable Constituents 225(3.99)The dose used to decontaminate meat and spices
is in the range 3–30 kGy. Therefore, the chem-
ical reactions mentioned here are to be expected.
Considerably lower doses are used for the treat-
ment of fruit and vegetables as shown in the fol-
lowing examples: disinfection of fruit and nuts
Fig. 3.40.Increase in the concentration of hydrocar-
bons relative to the radiation dose during irradiation
of chicken meat (according toNawaret al., 1990)
(0.15–0.5 kGy), inhibition of the germination of
potatoes, onions and garlic (0.2–0.15 kGy), de-
layed ripening of bananas, mangoes and papayas
(0.12–0.75 kGy). Consequently, the chemical de-
tection of the irradiation of these foods is quite
difficult.3.7.6 Microbial Degradation of Acyl Lipids to Methyl Ketones
Fatty acids of short and medium chain lengths
present in milk fat, coconut and palm oils are de-
graded to methyl ketones by some fungi. A num-
ber ofPenicilliumandAspergillusspecies, as well
as severalAscomycetes, Phycomycetesandfungi
imperfectiareabletodothis.
The microorganisms first hydrolyze the trigly-
cerides enzymatically (cf. 3.7.1) and then they
degrade the free acids by aβ-oxidation pathway
(Fig. 3.41). The fatty acids<C 14 are transformed
to methyl ketones, the C-skeletons of which have
one C-atom less than those of the fatty acids.
Apparently, the thiohydrolase activity of these
fungi is higher than theβ-ketothiolase activity.
Hence, ester hydrolysis occurs instead of thio-
clastic cleavage of the thioester of aβ-keto acid
(see a textbook of biochemistry). Theβ-keto acid
released is rapidly decarboxylated enzymatically;
a portion of the methyl ketones is reduced to the
corresponding secondary alcohols.
The odor threshold values for methyl ketones
are substantially higher than those for aldehydes
(cf. Tables 3.32 and 3.47). Nevertheless, they
act as aroma constituents, particularly in flavors
of mold-ripened cheese (cf. 10.2.8.3). However,
methyl ketones in coconut or palm oil or in
milk fat provide an undesirable, unpleasant odor
denoted as“perfume rancidity”.3.8 Unsaponifiable Constituents*
Disregarding a few exceptions, fats and oils
contain an average of 0.2–1.5% unsaponifiable
compounds (Table 3.48). They are isolated from*The free higher alcohols described under 3.6.2 and the
deacylated alkoxy-lipids belong to this class.