Di- and Mono-Acylglycerols

Fatty acids released by lipases can be determined quantitatively by TLC, GC and HPLC. TLC methods are used in conjunction with densitometric methods or autoradiographic methods using radiolabelled TAG. Many GC and HPLC methods that have been outlined earlier are widely used to isolate and quantify FFAs in lipolytic assays. Additionally a method using p-nitrophenyllaurate as a substrate was described by Maurich et al. (1991) who quantified activity by the release of p-nitrophenol. Veeraragavan (1990) used a RP-HPLC method with triolein as the substrate. Triolein was emulsified in buffer with the aid of a surface active agent and the lipase added under controlled conditions. Lipolytic activity was measured by the release of oleic acid and quantified by absorbance at 208 nm. 

Patel et al. (1996) produced trimethylsilyl derivatives of FFAs, MAGs, DAGs and TAGs in butterfat hydrolyzed by different lipases. GC was used to quantify these compounds the method enabled FFAs of acyl carbon numbers from 4 to 18, MAGs with acyl carbon numbers of 12-18, DAGs with acyl carbon numbers of 16-36 and TAGs with acyl carbon numbers of 30-46 to be identified. 

Aydemir, S., Kogak, C., Yildiz, M.C. 2003. Changes of free fatty acid contents and sensory properties of white pickled cheese during ripening. Food Chem. 80, 77-83. 

Andrikopoulos, N.K. 2002. Chromatographic and spectroscopic methods in the analysis of triacylglycerol species and regiospecific isomers of oils and fats. Crit. Rev. Food Sci. Nutr. 42, 473-505. 

Milk lipids. In, Utilizations of Milk Fat, Bulletin, 260, International Dairy Federation, Brussels, pp. 3-6. 

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Aside from volatile components (e.g., free fatty acids, secondary oxidation products, tocopherols, sterols, etc.), the deodorizer distillate also contains some neutral oil (tri-, di-, and mono-acylglycerols). With the exception of the more volatile monoacylglycerols, this neutral oil is present mainly as a result of mechanical entrainment by the stripping steam and is therefore considered as a direct refining loss. 

Hepatic lipase is involved in the metabolism of high-density lipoproteins and intermediate density lipoproteins (IDLs), converting the HDL2 fraction to HDL3 and generating LDLs from IDLs. The enzyme appears to have broad specificity it hydrolyzes tri-, di-, and mono-acylglycerols, acyl-CoA thioesters, and even phospholipids. hHL is secreted by the liver parenchymal cells and does not require any cofactors for its activity. 

Direct synthesis of diacylglycerols from fatty acids and glycerol was achieved by Hoq et al. (1984) on a hydrophobic membrane reactor, yielding 80-90% mixtures of di- and mono-acylglycerols. The product composition was dependent on the type of lipase used (Hoq et al., 1985). 

Fatty acid (FA) residues constitute 90 wt.% of milk lipids. They are bound in acylglycerols (tri-, di-, and mono-), phospholipids (PL), glycolipids, cholesteryl esters, and FA may occur in free form. 

Included in this class of compounds are the mono-, di- and tri-acylglycerols in which the hydroxyl groups of the glycerol molecule... 

Fig. 1.13. Separation of a product of partial transesterificaiion of rapeseed oil with methanol using combined RPC and NARPC gradient elution. Column Separon SGX Cm, 7 pm, 150 x 3 mm i.d. Ternary gradient from 30% A + 70% B to 100% B in lO min and to 50% B -t- 507r C in 20 min. followed by isocratic elution with the final mobile phase composition for 5 min, at I ml/min. Injection volume 10 pi. UV detection at 205 nm. Notation of sample compounds Ln. L, O and G are used for linolenic acid, linoleic acid, oleic acid, gadoleic acid, respectively, and for their acid parts in mono-, di- aixl tri-acylglycerols and methyl esters Me means methyl in methyl esters.

FFA, 14.5% acylglycerols (mono-, di-, and triacylglycerols), 0.4% phytosterols (campesterol, p-sitosterol, stigmasterol, and cholesterol), and 1.5% hydrocarbons (mostly squalene). 

Glycerides (acylglycerols). General name for fatty acid esters of glycerol. Depending on the number of fatty acids esterified with one molecule of glycerol the G. are classified as mono-, di-, and triglycerides. 

The triacylglycerols (Figure 14—6) are esters of the tri-hydric alcohol glycerol and fatty acids. Mono- and di-acylglycerols wherein one or two fatty acids are esteri-fied with glycerol are also found in the tissues. These are of particular significance in the synthesis and hydrolysis of triacylglycerols. 

This is an important method of preparing mono- and di-acylglycerols. The composition of the equilibrium mixture of all four components depends on the relative amount of triacylglycerol and of glycerol dissolved in the lipid phase. 

It is important to appreciate that, in stereochemical terms, the positions occupied by the acid chains are not identical. Under the stereospecific numbering system the positions are designated sn-1, sn-2 and sn-3, as shown.They are readily distinguished by enzymes and this may lead to preferential reactivity at one or more of the positions. Phosphorylation, for example, always takes place at carbon atom sn-3 rather than at carbon atom sn-1. Although triacylglycerols are predominant, mono- and di-acylglycerols do occur naturally, but in much smaller amounts. 

Staling of bakery products is retarded by lipase, presumably through the release of mono- and di-acylglycerols (cf. 15.4.4). The defatting of bones, which has to be carried out under mild conditions in the production of gelatin, is facilitated by using lipase-catalyzed hydrolysis. 

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