Triglycerides keto acids

The metabolism of hexoses proceeds according to the bod/s requirements. This results in (1) energy production by conversion to carbon dioxide and water, (2) storage as glycogen in the hver or triglyceride in adipose tissue, or (3) conversion to keto acids, amino acids, or protein. 

Reversed phase partition TLC may be applied to fractionate alcohols [78, 86], aldehydes and ketones [10, 125], fatty acids [4, 80, 84, 124] and their methyl esters [114, 213], keto-acids, hydroxy-acids and lactones [82]. More complete separations of these compounds or of suitable derivatives, are, however, obtained with gas chromatography and it is appreciably more sensitive than TLC it is moreover more suitable for quantitative determinations. Reversed phase partition TLC offers substantial advantages only in the analysis of lipids of relatively high molecular weight, like wax esters [86], steryl esters [79] (see also Chapter L), carotenoid esters (see Chapter K) and triglycerides [4, 81, 89, 124] some derivatives of naturally occurring lipids are also best fractionated on hydrophobic layers [21, 130, 131, 165]. 

Carbonyls are quite important to the flavor of cheeses. The TNO-CIVO compilation of volatiles in foods lists 29 carbonyls as having been identified in Cheddar cheese at that time. Carbonyls (methyl ketones) may arise in fermented prodncts iiutiaUy via lipase activity of the starter cnlture. Dairy products contain a significant quantity of a-keto acids which are readily hydrolyzed from the triglyceride by microbial lipases and then decarboxylated to form odd carbon nnmber methyl ketones. 

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 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. 

Beer contains trace amounts of lipids. A Swedish beer (12° Plato) was found to contain (mg/1) triglycerides, 0 1-0 2 diglycerides, 0 1 monoglycerides, 0 1-0 3 sterol esters, 0 01 free sterols, 0 01-0 02 and free fatty acids, C4-C10, 10-15 Ci2-Cx8, 0-0 5 [42]. Similar data were found for American beers. The role that lipids play in head formation is discussed later. The free fatty acids are volatile and more detailed values are given below. Whereas unsubstituted fatty acids are volatile, the addition of a further substituent usually results in loss of volatility. Data for hydroxy-, keto, di-, and tri-basic acids are given in Table 22.7. 

The regulation of ketogenesis is related to the regulation of the tissue concentration of free fatty acids, the main keto-genic precursors (Krebs and Hems, 1970 Mayes, 1970). This concentration, in turn, depends mainly of the rate at which fatty acids reach the liver from the triglyceride stores of... 

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