PRINCIPLES OF FAT MODIFICATION TECHNIQUES

2.1 Fractionation

Fractionation by crystallization is widely used to separate fat with harder and softer fractions. Oil and fats are mixtures of triglycerides. Because of their different fatty acid composition, the oil and fats have melting point spanning from –50 to +80°C. Every oil has its own melting range. Milk fat exhibits a wide melting range from –30°C to about +37°C. This provides the possibility of crystallizing out a series of glycerides fractions at temperature below their melting points. This is called fractionation by crystallization from the melt or dry fractionation. This is basically a thermo-mechanical process by which raw material is separated into two portions by crystallization. The process consists of three distinct stages: supercooling of melt, formation of crystal nuclei or nucleation and crystal growth. The crystals are then separated by low or high pressure filters. Different fractionation methods used for fats & oils include solvent fractionation, detergent fractionation and dry fractionation.

Currently, dry fractionation of anhydrous milk fat is performed by two conventional systems. Tirtianx and De Smet, both from Belgium, which are bulk crystallization processes. The widely used Tirtianx dry fractionation process enables one-or up to five step

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1975). The milk fat fractions thus obtained can be either used as such or the fractions can be blended in various proportions for use as ingredients in various food-fat formulations. The major shortcoming inherent in this system is long residence time (8-12 hours) for nucleation and crystal growth.Some other fractionation techniques to improve resources, purity with speed are: fractionation by short-path distillation (Forss and Holloway, 1967); fraction by supercritical fluids (Kaufman et al, 1982; Arul, et al 1987), etc. These processes are high energy and capital intensive and not used commercially for fractionation. Another method of fractionation by crystallization using solvent has some advantages of rapid crystallization of crystals due to low.

2.2 Hydrogenation

The triglyceride of naturally occurring oils composed of unsaturated and saturated fatty acids. The unsaturated fatty acids contain from one to six double bonds. The number double bonds in the carbon chain of fatty acid and its position in triglyceride molecule are responsible for susceptibility to oxidation and physical state of oil and fats at a given temperature. The susceptibility of double bonds to oxidation (Autooxidation) can be decreased by saturating the double bonds with external pure hydrogen under specified conditions. When hydrogen is added to fatty acid double bond, it becomes saturated with constant increase in the oxidative stability and melting point of oil .The process is commonly known as “Hardening” of fat. The beauty of the process is that it can be stopped at any point up to complete saturation. Hence, it is possible to obtain fat of various physical and rheological characteristics by altering the level of hydrogenation. The process is selective and starts from fatty acid having more number of double bond (linolenic Linoleic Oleic stearic). This preferential hydrogenation of polyunsaturated fatty acid is required for improving oxidative stability. Usually, nickel metal is used as catalyst for the process. During the process, isomerization takes place due to the movement of double bonds to new positions to form trans-isomers. The trans fatty acids have higher melting points and thus contribute to increase in the melting point of fat. A similar saturation of double bond is enzymically catalyzed by microorganisms in rumen of cattle or buffaloes (Gurr, 1981).

2.3 Dehydrogenation

Current research interests in the United States are focusing on desaturation of fatty acids using lipase activity. If successful, this can lead not only to a healthier more unsaturated milk fat, but also a more spreadable butter. The anhydrous milk fat could also be used readily in more challenging applications such as mayonnaise and salad dressings without the need of fractionation.However some scientist have major objection to this approach of desaturation. As all know, that desaturase enzymes specific for conversion of 18.0 cis require the free acid as substate. Thus it would be necessary to hydrolyse the triglyceride to some extent, allow the 18:0 component of these acids to be desaturated and finally to re-esterify the free acids. It is quite possible that they do not return to their original positions in the triglyceride moiety, hence the relation of final material to milkfat would be somewhat tenuous.

2.4 Interesterification

The nature of fatty acids esterified in triglyceride is not only factor influencing the physical properties of fat. Another important influence is the distribution of the free fatty acids among the different positions of glycerol molecule. Natural fats tend to have specific

asymmetrical distributions of fatty acids in the molecule. Interesterification is a method of altering the melting point of a fat by randomizing the positions of fatty acids. The positions may be exchanged between fatty acids of the same triglyceride molecule (intra molecular exchange) or between fatty acids of different molecules (inter molecular exchange). The fat is heated in presence of catalyst (usually sodium, sodium methoxide, sodium ethoxide) to a temperature of 110-160°C.

The interesterified fat is used for the manufacture of margarines, shortenings and confectionery fats (Srinivasan, 1978). An extension of this process is the use of microbial lipases as catalysts for the reaction. There are three type of lipases available to catalyse the process of interesterification. These are, non specific (randomized); 1:3, positional specific o the triglyceride and fatty acid specific. In US, this new area of research related to non- aqueous lipase interesterification is getting a lot of interest and funds, to modify milkfat. An exciting, further extension to this is to separate the modified fat into various fractions using superficial carbondioxide extraction.

This lipase-catalyzed interesterification of milk fat improves the nutritional properties and butter flavor. And it was found to be the better fat for infant formulae. (Gregt-W-de et

al., 1995). Bystrom and Hartel (1994) used this technique for producing Cocoa butter

replacer from milk fat.

Christorphe et al (1978) found that milk fat randomized with chemical catalyst does not raise the blood serum cholesterol level. Randomized milk fat appears to be more rapidly digested in vivo than i.e. untreated milk fat. By the process of interesterification, not only the physical properties but also the metabolic effects of the fat can be changed (Gurr, 1984).