2009_Book_FoodChemistry

3.8 Unsaponifiable Constituents 225
(0.15–0.5 kGy), inhibition of the germination of
potatoes, onions and garlic (0.2–0.15 kGy), delayed
ripening of bananas, mangoes and papayas
(0.12–0.75 kGy). Consequently, the chemical detection
of the irradiation of these foods is quite
difficult.
3.7.6 Microbial Degradation of Acyl Lipids
to Methyl Ketones
(3.99)
The dose used to decontaminate meat and spices
is in the range 3–30 kGy. Therefore, the chemical
reactions mentioned here are to be expected.
Considerably lower doses are used for the treatment
of fruit and vegetables as shown in the following
examples: disinfection of fruit and nuts
Fatty acids of short and medium chain lengths
present in milk fat, coconut and palm oils are degraded
to methyl ketones by some fungi. A number
of Penicillium and Aspergillus species, as well
as several Ascomycetes, Phycomycetes and fungi
imperfecti areabletodothis.
The microorganisms first hydrolyze the triglycerides
enzymatically (cf. 3.7.1) and then they
degrade the free acids by a β-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 the β-ketothiolase activity.
Hence, ester hydrolysis occurs instead of thioclastic
cleavage of the thioester of a β-keto acid
(see a textbook of biochemistry). The β-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
Fig. 3.40. Increase in the concentration of hydrocarbons
relative to the radiation dose during irradiation
of chicken meat (according to Nawar et al., 1990)
* The free higher alcohols described under 3.6.2 and the
deacylated alkoxy-lipids belong to this class.