After the very early demonstration by Chevreul in 1823 that the fusion temperature of variously unsaturated fatty acids depends on the number of double-bonds per molecule, it is now well known that the texture of margarine may be altered through the composition of its triglyceride molecular basis. This is easily achieved by modifying the distribution of the fatty chains on the glycerol molecule via an important catalytic reaction, interesterification.
While the first published mention of the
synthesis of a triglyceride (esterification of glycerol by butyric acid) was by Pelouze in
1844 (Pelouze J et al Ann Chim Phys 1844, 10, 434), the alcoholysis reaction
between tristearin (and tripalmitin) and ethanol (and
amylic alcohol) was described in 1852 (Duffy PJ, J Chem Soc 1852, 5,
Thus, fatty acid methyl esters can be conveniently prepared by displacing the glycerol from the trigyceride molecule, in heating a mixture of triglycerides with methanol (or another alcohol). If the alcoholysis reaction is conducted at high temperature, it can be made to go in the reverse direction with the formation of glycerides from esters of the volatile alcohol. If a small proportion of glycerol is used, the migration and interchange of the fatty acid radicals lead to the formation of triglycerides of new composition. It was found that the use of glycerol was not necessary, the use of alkaline catalysts is sufficient.
It appears that Normann W (who also patented earlier the catalytic hydrogenation of fatty acids) was the first to patent the chemical interesterification of edible lipids in 1920 (German Patent 417, 215 with Firma Oelwerke Germania GmbH). Van Loon C of Anton Jurgens Company patented the industrial process of glyceride rearrangement in England in 1924 and in Netherlands in 1927, Grün A (US Patent 1, 505, 560 in 1924) and Barsky G (US Patent 2, 182, 332 in 1939) in the United States. This process was viable in the food industry since the 1940s to improve the spreadability and baking properties of lard but was largely developped in the 1970s as a hydrogenation replacement for the manufacture of margarines without trans fatty acids. More recently, interesterification was used in the production of low calorie fat replacers (Olestra, Salatrim...)
At the beginning, fats were heated at 100-200°C with 0.1% sodium ethylate or stannium hydroxyde until the needed randomization degree was reached. Now, K glycerolate or Na methylate is used, any traces of catalyzer being removed by washing with water.
The figure below shows how are obtained several randomized heterogeneous triglycerides by interesterification of a mixture of tristearine (SSS) and triolein (OOO).
The build up of triglycerides with determined structure is realized through the use of enzymes (lipases, esterases) as transferases in 3 types of reactions :
transesterification : exchange
of carbonyl groups between esters
acidolyse : exchange of carbonyl
group between esters and carboxylic acids (fatty acids)
alcoholyse : exchange of alcohol
between esters and alcohols.
Acidolysis is the most commonly used
process to enrich vegetal oils in one component (linoleic, oleic or
eicosapentaenoic acid). Lipases (e.g. from Rhizopus) are used to
catalyze this reaction thus exchanging fatty acids in the 1 and 3 positions with
free fatty acids in the medium. The secret of specific syntheses has been the
selection of lipases with the correct specificity and chemical environment (use
of aqueous/organic biphasic systems).
Advantages of enzymes over conventional chemical catalysts include substrate specificity, lower energy costs and a high quality end-product.
It is thus easy to manufacture artificial or "structured" triglycerides enriched with EPA from fish oil (beneficial to cardiovascular health), short-chain or medium-chain fatty acids to prevent obesity or hypercholesterolemia.
The short-chain fatty acids (C2-C4) may be acetic, propionic, butyric or a combination of all three, while the long-chain fatty acid (C16-C22) is derived from hydrogenated vegetal or animal oils. Randomized fats have diverse applications, both in the food industry and in clinical applications. Interesterified fats and oils differ from their native products in their melting and crystallisation properties since that process confers new rheological properties to the products. In clinical applications, randomization provides energy-rich substrates for parenteral, enteral and infant feeding which are well-absorbed. Industries have also made products with low energy and fat content (fat replacer). Benefat (Danisco trade name for Salatrim) is the most known blend of structured triglycerides being recognized as safe (GPAS) by the US FDA. It is proposed to consumers to manage their energy intakes without sacrificing desirable taste ans texture and interfering with the absorption of fat soluble vitamins. Salatrim is made primarily of 18:0 (50%) and 16:0 (6.6%), combined with the short-chain organic acids : acetic (C2:0, 21.7%) and butyric (C4:0, 2.6%) acids. These short-chain compounds may be treated nutritionally as carbohydrates. Thus, replacing 10g of fat with an equal amount of Salatrim would reduce the amount of actual fat available to 5.5g.
Medium-chain triglycerides containing caprylic, capric and lauric acids are manufactured as nutritional supplements, carriers for flavorings and colorings, or release agents for baked goods.
Interesterification is used by the food industry to alter the melting and crystallization characteristics of fats to change certain functionality, such as spreadability or a particular melting point. Interesterification influences the physical properties of fats due to differences in the individual melting and crystalline properties of molecular species. The overall effect is the generation of a hard fat from a soft fat.
As well as achieving suitable melting properties, interesterification also optimises crystallisation behaviour to generate more stable crystalline forms. Saturated triacylglycerols can exist in more than one crystalline form (polymorphs), which results in different patterns of molecular packing in the crystals and multiple melting points. The three basic polymorphs are referred to as a, b' and b. The a form is the least stable with the lowest melting point, and b the most stable with the highest melting point. For example, the melting point of the a, b' and b polymorphic forms of the species stearic/oleic/stearic (SOS) are 22.4, 36.5 and 41.7° C, respectively. During interesterification, the b' form is commonly generated, thus improving the stability and granularity of the fat. For example, the proportion of solid fat at body temperature (37° C) in the native cocoa butter and randomly interesterified cocoa butter is 1 and 37% respectively (Berry SEE, Nutr Res Rev 2009, 22,3).