Microbial Degradation of Acyl Lipids to Methyl Ketones

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 degraded to methyl ketones by some fungi. A number of Penicillium ind Aspergillus species, as well as Ascomycetes, Phycomycetes 2ir d fungi 

The microorganisms first hydrolyze the triglycerides enzymatically (cf. 3.7.1) and then they degrade the free acids by a P-oxidation pathway (Fig. 3.41). The fatty acids Ci4 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 p-ketothiolase activity. Hence, ester hydrolysis occurs instead of thio-clastic cleavage of the thioester of a P-keto acid (see a textbook of biochemistry). The P-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 . 

Disregarding a few exceptions, fats and oils contain an average of 0.2-1.5% unsaponifiable compounds (Table 3.48). They are isolated from 

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