The main tools that we find at our disposal for the fabrication of suitable fats for the substitution of partially hydrogenated fats are: hydrogenation, preferentially executed to iodine values close to zero; chemical or enzymatic interesterification; fractionation; and the search for new raw materials, possibly executed via modern plant breeding (e.g. Bockisch, 1993).
A very low maximum TFA level can only be achieved by either no hydrogenation or by full hydrogenation. Full hydrogenation delivers fats with very high SFC levels. Tropical oils and fats are in their natural state already relatively saturated and therefore high in solids. They might be subjected to fractionation, see Section 15.3.3, in order to further increase their level of SFA. These two sources of concentrated SFA, hydrogenated fats or tropical fats, form the starting point for the manufacture of hardstocks, the structuring fats in a fat composition, by interesterification. Liquid oils contain hardly any solids. Therefore use of interesterification and/or frac-tionation alone as a means of reducing TFA in food compositions based on liquid oils used for hardstock production is not an option. Use of full hydrogenation is an option to generate a starting fat that is rich in SFA. As fully hydrogenated fats are, for reasons discussed above (melting profile and mouthfeel), only suited to a limited number of applications they are normally subject to further oil modification.
Factory-scale hydrogenation processes under normal conditions do not achieve complete saturation of all double bonds. Thus, the residual iodine value (IV) of a fully hydrogenated product is normally around 1-2. The IV is a measure of the degree of unsaturation of an oil or a fat, it gives the amount of iodine (in grams) that reacts with 100 g of oil or fat. A residual IV of 1 corresponds to 1.15% remaining monounsaturated fatty acids in the fat composition. For 'fully hydrogenated' fats (IV = 1-2), the thermodynamic equilibrium ratio between the cis and trans configuration of unsaturated fatty acids is reached (28/72). Combining these two pieces of evidence one finds that each IV point of the fully hydrogenated fat corresponds to 0.85% TFA. To obtain a product with a trans level below 1.25%, the hydrogenation should be continued until an IV below 1.5 is reached (the 'practical' specification of a fully hydrogenated product).
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