When naturally occurring unsaturated fatty acids are altered
by partial hydrogenation, they are converted to saturated fatty
acids, which have the effect of straightening the chains and
changing the physical properties.
Also during partial hydrogenation, some of the unsaturated
fatty acids, which are normally found as the cis isomer about
the double bonds, are changed to a trans double bond and remain
unsaturated. Trans fatty acids of the same length and weight
as the original cis fatty acids, still have the same number of
carbons, hydrogens, and oxygens but they are now shaped in a
more linear form, as opposed to the bent forms of the cis isomers.
See graphic on the left.
See the definitions of cis-trans
Although the trans fatty acids are chemically "monounsaturated"
or "polyunsaturated" they are considered so different
from the cis monounsaturated or polyunsaturated fatty acids that
they can not be legally designated as unsaturated for purposes
of labeling. Most of the trans fatty acids (although chemically
still unsaturated) produced by the partial hydrogenation process
are now classified in the same category as saturated fats.
Trans fat has both the benefits and drawbacks of a saturated
fat. On the plus side, it has a longer shelf life than regular
vegetable fat and is solid at room temperature. The major negative
is that trans fat tends to raise "bad" LDL- cholesterol
and lower "good" HDL-cholesterol, although not as much
as saturated fat. Trans fat is found in margarine, baked goods
such as doughnuts and Danish pastry, deep-fried foods like fried
chicken and French-fried potatoes, snack chips, imitation cheese,
and confectionary fats.