Interesterified fat is a type of oil where the fatty acids have been moved from one triglyceride molecule to another. This is generally done to modify the melting point, slow rancidification, and create an oil more suitable for deep frying or making palatable margarine with low saturated fat content. It is not the same as partial hydrogenation which produces trans fatty acids, but interesterified fats used in the food industry can come from hydrogenated fat,[clarification needed] for simplicity and frugality.
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 then rearranging the fatty acids over the glycerol backbone with, for instance, the help of catalysts or lipase enzymes. Polyunsaturated fatty acids (PUFAs) 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 triglycerides with oils containing unsaturated fatty acids lowers the melting point and creates fats with properties better suited for target food products. In addition, blending interesterified oils with liquid oils allows the reduction in saturated fatty acids in many trans fatty acid free food products. The interesterified fats can be separated through controlled crystallization, also called fractionation.
In vegetable polyunsaturated oils, the PUFA is commonly found at the middle position (sn2) on the glycerol. For instance, stearic acid (a saturated fatty acid, C18:0) is not usually found at the sn2 position in vegetable oils used in the human diet.
In most vegetable dietary fats, the palmitic (C16:0) and stearic acid (C18:0) saturated fats mainly occupy the 1- and 3-positions of the triacylglycerol molecule, whereas an unsaturated fatty acid such as oleic acid (18:1ω–9) or linoleic acid (18:2ω–6) 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[clarification needed] during digestion and metabolism, and a relevant scientific question is whether there are health effects following from this. Although this question has received relatively little attention in dietary fats and health research, there are a number of good controlled human intervention studies that have addressed it.
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 noninteresterified 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 interesterification has no effect on metabolism or biological effects. Conversely, effects of interesterification cannot be properly addressed if the interesterified fat and the noninteresterified fat being compared have different fatty acid compositions.
Zock et al. compared the effects of an IE (interesterfied) test fat with 40% C16:0 on the 2-position with a noninteresterified 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.
A 2007 study funded by the Malaysian Palm Oil Board, 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 both the IE blend and the partially hydrogenated fat blend increased the fasting ratio of LDL to HDL cholesterol, 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. For the postprandial study the glucose incremental area under the curve (IAUC) following the IE meal was 40% greater than after either other meal (p<0.001), and was linked to relatively depressed insulin and C-peptide (p<0.05).
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 monounsaturated 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).
A 2007 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. However, in a larger, better designed study the same authors could not reproduce the cholesterol-lowering effects.