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Interesterified fats are oils where the fatty acids have merely been moved from one triglyceride molecule to another. Interesterification does not alter the fatty acids. This is generally done in order to modify the melting point, slow rancidification and create an oil more suitable for deep frying or making margarine with good taste and low content of saturated fatty acids. This is unlike partial hydrogenation which produces trans fatty acids.

Chemistry
Fats such as soybean oil consist mainly of various triglycerides which are made up of a glycerol backbone esterified to three fatty acid molecules. The triglycerides contain a mixture of saturated, monounsaturated and polyunsaturated fatty acids. Interesterification is carried out by blending the desired oils and than rearranging the fatty acids over the glycerol backbone with, for instance the help of Lipase enzymes. Polysunsaturated fatty acids decrease the melting point of fats significantly. A triglyceride containing three saturated fatty acids is generally solid at room temperature and not very desirable for many applications. Rearranging these tryglycerides with oils containing unsaturated fatty acids lowers the melting point and creates fats with properties better suited for high quality products. In addition blending interesterified oils with liquid oils allows the reduction in saturated fats in many trans fatty free food products. The interesterified fats can be separated through controlled crystallization or fractionation.



In polyunsaturated fats, the PUFA is commonly found at the middle position (sn2) on the glycerol. Stearic acid is not usually found at sn2 in vegetable oils used in the human diet.

Health effects
In most vegetable dietary fats palmitic (C16:0) and stearic acids (C18:0) mainly occupy the 1- and 3-positions of the triacylglycerol molecule, whereas an unsaturated fatty acid such as oleic acid or linoleic acid usually occupies the 2-position. In animal fats this is not the case. Interesterification of vegetable oils will enhance the amount of saturated fatty acids at the 2-position. Fatty acids at the 2-position are biologically different from fatty acids at the 1 and 3 position because they are handled differently during digestion and metabolism and a major scientific question is whether there are health effects following from this. As consequence the area has been well researched.

In studies addressing the health effects of interesterification as such, a diet high in interesterified fat should be compared with a diet high in a non-interesterified fat with the same fatty acid composition. If the two diets show similar changes in the resulting blood lipid profiles (i.e. not different from each other), this indicates that interesterification has no effect on metabolism or biological effects. Conversely, effects of interesterification can not be properly addressed if the interesterified fat and the non-interesterified fat being compared have different fatty acid compositions.

Zock et al compared the effects of an IE test fat with 40% C16:0 on the 2-position with a non-interesterified test fat with only 6.5 % C16:0 on the 2-position in a 3-week diet study. Despite the very high intakes and the marked difference in positional distribution, no statistically significant effects on fasting blood lipids were observed in the group as a whole. Nestel et al examined the effects of an IE fat blend with 25% C16:0 on the 2-position with a native fat blend with only 9% C16:0 on the 2-position. Again, despite a high intake level and the clear difference in positional distribution of the fats fed, no effects were observed on fasting blood lipids. Meijer and Weststrate examined the effects of interesterification, using a ‘real’ hardstock as applied in foods. The control was the same fat blend with a similar fatty acid composition, but not interesterified. The IE fat blend contained more C16:0 on the 2-position (18%) than the control blend (7%). None of the fasting levels of blood lipids measured after 3 weeks showed any change related to treatment of the fat blend. Fasting glucose level was also not affected.

In 1970 Grande et al used interesterification to prepare a blend of fats and oils mimicking the fatty acid composition of cocoa butter. No difference between the interesterified fat blend and cocoa butter was observed in levels of total cholesterol in fasting blood. Recently, Sundram et al. compared the effects of three types of fat: native palm olein, a blend with partially hydrogenated soybean oil and an interesterified mixture of oils. They concluded that both the IE blend and the partially hydrogenated fat blend decrease the fasting LDL/HDL-cholesterol ratio, indicating an adverse effect on CVD risk. Sundram et al also found that fasting plasma glucose levels were higher after 4 weeks on the interesterified fat than after the other diets. As was pointed out in a letter to the Editor by Destaillats et al a major limitation of the Sundram study is that the diets differed in overall fatty acid composition.The interesterified fat had 30% more saturated and 57% less mono-unsaturated fatty acids than the untreated palm olein. The direction of the effects on blood lipids are in line with what can be predicted based on these differences in fatty acid content between the study diets (Mensink 2003) Another recent study by Berry et al compared shea butter (3% C18:0 on the 2-position) and interesterified shea butter (23% C18:0 on the 2-position), while keeping overall fatty acid composition of the diets constant. This study found no effects of interesterification on fasting levels of blood lipids, glucose and insulin. This is line with a number of other human intervention studies.

Christophe et al have studied the effect of interesterification of butter oil. In a small pilot study, they observed an 11% lower blood total cholesterol level after interesterification. In a larger, better designed study, no effect was seen of substituting enzymatically interesterified butter oil for butter.

In summary almost all longer term diet studies and acute post meal studies do not show any adverse health effects attributable to interesterification.