Triacylglycerols alcoholysis

Figure 4. Exchange reactions at the carboxyl group (1) hydrolysis (Chapter xx), (2) esterification (Chapter xx), (3) acidolysis (Chapter xx), (4) alcoholysis (Chapter xx), and (5) glycerolysis (Chapterxx). The starting ester RCOOR will often be a triacylglycerol. MAG—monoacylglycer-ol DAG—diacylglycerol TAG—triacylglycerol.

The fatty acid or alcohol groups present in an ester can be exchanged in a number of ways by reaction with an excess of other fatty acids (acidolysis), alcohols (alcoholysis), or other esters (interesterification). Generally, the starting point will be a triacylglycerol, and these reactions provide routes by which the composition and properties of oils and fats can be modified. 

Alcoholysis. Methanolysis of triacylglycerols is used to prepare methyl esters for fatty acid analysis, a process frequently referred to as transesterification. This can be acid-or base-catalyzed, the method being chosen to avoid modifying acid-or base-sensitive fatty acids and to minimize reaction times. Sterol esters of fatty acids react more slowly than triacylglycerols, and samples containing them require more vigorous reaction conditions. The preparation of methyl esters from oils and fats for GC and GC-MS analysis has been extensively reviewed (19, 22, 23). 

Production of Oleochemicals Fatty acid esters, which have traditionally been used as flavors, plasticizers, food preservatives, emulsifiers, and lubricants, are receiving renewed interest as an alternative energy source as biodiesel. They can be produced by the esterification of fatty acids and alcohols or by the alcoholysis of vegetable oils/triacylglycerols. 

Lipases (triacylglycerol hydrolases, EC 3.1.1.3) are enzymes that catalyze reactions such as hydrolysis, interesterification, esterification, alcoholysis, acidolysis, and aminolysis [1]. There is an increasing interest in the development of lipase applications to oleochemical transformations to obtain esters of long-chain fatty acids, as monoalkyl esters of fatty acids [2]. Utilization of lipase as a catalyst for the production of biodiesel, defined as a mixture of monoalkyl esters, is a clean technology due to its nontoxic and environmental fnendly nature, requiring mild operating conditions compared with chemical method [3]. 

Alcoholysis is widely employed to convert lipids directly to methyl esters without first isolating the free acids and to prepare partial glycerides through interaction of a triacylglycerol and free glycerol. For methanolysis the fat or oil is dissolved in excess of methanol containing a catalyst along with benzene, toluene or dichloromethane as co-solvent. 

The synthesis by route C is an alcoholysis reaction. The reaction proceeds as an emulsion due to the immiscibility of triacylglycerol and glycerol. Under this emulsified condition, the equilibrium concentration of the monoacylglycerol is 30% (Yamane et al., 1986). However, when the reaction is run at temperatures slightly below the melting point of the fat used, the concentration is favourably increased to 70-90% (McNeill et al., 1990 McNeill and Yamane, 1991). The reaction rate is affected by moisture content. Relatively high moisture leads to high initial reaction rates (McNeill et al., 1991) but excessive levels result in hydrolysis and production of free fatty acids. Compared to the synthetic pathways of A and B, route C seems a cheaper alternative since the primary reactive components do not include free fatty acids reactants that otherwise would have to be produced by other methods to make them available for the process. 

The reaction of triacylglycerols with alcohol is called alcoholysis. It can be catalysed by acids (sulfuric acid or 4-toluenesulfonic acid), and then an interesterification reaction, which is a parallel reaction to esterification of free fatty acids occurring in the fat. Alcoholysis is kinetically similar to esterification. The base catalysed reaction employs methoxides, hydroxides and carbonates of alkali metals. The most commonly used catalysts are sodium methoxide, sodium hydroxide or potassium hydroxide. Industrially produced alcoholysis products are methyl, ethyl or propyl esters (Figure 3.82) of fatty acids. Analogous to hydrolysis, mixing of fat (oil) with alcohol gives a two-phase system... 

Despite the lack of selectivity, some structured Upid-hke products have been synthesized by chemicals methods. The catalyst most often used for chemical interesterification of triacylglycerols or for alcoholysis between triacylglycerols and fatty acids is sodium methoxide, which is more active than other base, metal or acid catalysts. In general, sodium alkoxides are easy to use as catalysts. They are inexpensive, active at relatively low temperatures (50-90 ), and are effective at low concentrations. Although no regioselectivity or fatty acid selectivity occurs, the chemically synthesized commercial products listed in Table 2 contain sTAG with the desired nutritional function. 

The key to the successful development of the two-step synthesis of sTAG was the discovery that high yields of 2-MAG can be obtained by alcoholysis of triacylglycerols in aprotic solvents such as methyl tert-bulyl ether (MTBE) plus ethanol at controlled water activity (Millqvist Fureby et al, 1994,1996a, 1996b). 

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