Food Lipids: Chemistry, Nutrition, and Biotechnology

life at 100�C of 12 hours. These investigators concluded that in organic solvents,
porcine pancreatic lipase remains rigid and cannot undergo partial unfolding, which
causes inactivation. The heat stability of a lipase also depends on whether a substrate
is present. This is because substrates remove excess water from the immediate vicinity
of the enzyme, thus restricting its overall conformational mobility (81).
Most lipases in nonimmobilized form are optimally active between 30 and 40�C
(82). Immobilization confers additional stability to the lipase compared to nonimmobilized
lipase. Excellent reviews on the immobilization procedures and bioreactors
for lipase catalysis were published recently (48,83,84). The immobilization support
must possess the following properties: high surface area to allow maximum contact
with enzyme, high porosity to allow good flow properties, high physical strength,
solvent resistance, high flow properties, and chemical and microbiological inertness
(85,86).
Other Factors. Other factors that affect yield of products are substrate molar
ratio; enzyme source, activity, and load; incubation time; specificity of the enzyme
to substrate type and chain length; and regiospecificity.
f. Chemical Versus Enzymatic Synthesis. The most useful property of lipases
is their regio- and stereospecificity, which result in products with better defined and
more predictable chemical composition and structure than those obtained by chemical
catalysis. Potential advantages of using enzymes over chemical procedures may be
found in the specificity of enzymes and the mild reaction conditions under which
enzymes operate (87). Enzymes form products that are more easily purified and
produce less waste, and thus make it easier to meet environmental requirements (87).
Chemical catalysts randomize fatty acids in triacylglycerol mixtures and do not lead
to the formation of specialty products with desired physicochemical characteristics
(51). The specificities of lipase have classically been divided into five major types:
lipid class, positional, fatty acid, stereochemical, and combinations thereof (81). Enzymes
have high turnover numbers and are well suited for the production of chiral
compounds important to the pharmaceutical industry.
Transesterification using sn-1,3 specific lipase results in SL products with fatty
acid at the sn-2 fatty acids remaining almost intact. This is significant from a nutritional
point of view because the 2-MAGs produced by pancreatic lipase digestion
are the main carriers of fatty acids through the intestinal wall (88). Fatty acids
esterified at the sn-2 position are therefore more efficiently absorbed than those at
the sn-1 and sn-3-positions. A TAG containing an essential fatty acid at the sn-2
position and short or medium chain fatty acid in the sn-1 and sn-3 positions has the
advantage of efficiently providing an EFA and a quick energy source (89).
Some studies have shown that the rate of autoxidation and melting properties
of TAGs can be affected by the position of unsaturated fatty acids on the glycerol
molecule (90,91). TAGs having unsaturated fatty acids at the 2-position of glycerol
are more stable toward oxidation than those linked at the 1- and 3-positions.
The energy saved and minimizations of thermal degradation are probably
among the greatest attractions in replacing the current chemical technology with
enzyme biotechnology (51). Table 6 shows some of the potential advantages of the
enzymatic approach to structured lipid design. Potential food applications of SL are
listed in Table 7.
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