Food Lipids: Chemistry, Nutrition, and Biotechnology

saturated fatty acids are hydrogenated before the less unsaturated fatty acids have
become hydrogenated (Fig. 22). High selectivity gives high oxidative stability for a
given IV. For many applications one wants as much oxidative stability as is consistent
with producing oil having no solid fat crystals at normal usage temperatures. Selectivity
is always relative because perfect selectivity [i.e., all linolenic acid (18:3)
converted to linoleic acid (18:2) before any linoleic acid is converted to oleic acid
(18:1)] has not been achieved. However, producing catalysts with ever-increasing
selectivity is a goal of catalyst suppliers.
3. Isomerization
Two types of isomerization spontaneously occur during hydrogenation: geometrical
and positional. Only the extent to which isomerization occurs can be affected by
processing conditions and catalyst selection. Geometrical isomerization refers to conversion
of only cis double bonds to trans double bonds. Most unhydrogenated plant
oils have only cis double bonds in their constituent fatty acids; however, milk fat
and animal depot fats may have modest amounts of natural trans fatty acids, usually
attributed biological hydrogenation by rumen bacteria.
Positional isomerization refers to the shift of double-bond position within the
chain. If hydrogen atoms exist on the catalyst surface, the double bond opens, and
hydrogen atoms add to the carbons atoms at either end [63]. When the catalyst has
sufficient hydrogen atoms, another hydrogen atom is added such that the original
double bond is converted to a saturated, single bond between carbons. When there
are not sufficient hydrogen atoms to cover the catalyst, a hydrogen atom may be
removed from either side of the partially saturated bond. This produces a new double
Figure 22 SFI properties of soybean oil hydrogenated under selective and nonselective
conditions. (Redrawn from Ref. 53.)
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.