Food Lipids: Chemistry, Nutrition, and Biotechnology

osmarinic acids) being most effective in bulk oils and the less polar compounds
(carnosol) being more effective in emulsified lipids [18]. Similarly, the antioxidant
activity of carnosic acid was improved in emulsified corn oil when it was made
nonpolar by methylation [95].
The charge of dispersed lipids also influences oxidation rates, especially in the
presence of transition metals. Since iron and other transition metals are common
contaminants in most water systems, their ability to catalyze oxidation at the oil–
water interface of dispersed lipids could be important. When the surface charge of
dispersed lipids as either micelles [96,97] or phospholipid vesicles [98] is negative,
iron-catalyzed lipid oxidation rates are much higher than they are at positively
charged interfaces. This effect presumably exists because iron can bind to the interface
of the dispersed lipid. The inhibitory effect of positively charged lipid micelles
can be partially overcome by nitrilotriacetic acid, which forms negatively charged
iron chelates, and by addition of lipid-soluble peroxides [96,97]. Positively charged
emulsifiers are uncommon in foods. However, proteins at pHs below their pIs could
produce positively charged lipid emulsion droplets. Unfortunately, little is known
about the ability of proteins to influence oxidation by modifying the charges that
govern metal attraction/repulsion. The charge of dispersed lipid droplets can also be
important for the activity of antioxidants. Negatively charged ascorbic acid is a more
effective antioxidant in the presence of a positively charged lipid dispersion [99],
while spermine (positively charged) is more effective when the lipid interface is
positive [100].
Lipid oxidation can be inhibited by encapsulation. Potential mechanisms of
inhibition include physical inhibition of oxygen diffusion into the lipid, chemical
(e.g., free radical scavenging) and physical (e.g., chelation) antioxidant properties of
the encapsulating agents, and possibly interaction of lipid oxidation products with
the encapsulating material. Both protein and carbohydrate encapsulating agents have
been found to retard oxidation rates. The effectiveness of these encapsulating agents
depends on factors such as concentration of the encapsulating agent [101], method
of encapsulation (which affects the porosity of the encapsulating layer) [102], and
the environmental relative humidity under which the encapsulated lipid is stored
[103].
Some evidence indicates that encapsulation does inhibit oxygen diffusion into
the lipid, but the same research also indicates that oxygen diffusion is not the only
mechanism by which encapsulation inhibits oxidation [104]. More research is needed
to determine the antioxidant mechanisms of encapsulation, since this technique could
be an effective way to increase the stability of oxidatively labile lipids, thereby
increasing their incorporation into foods.
Another factor that may influence oxidation rates is the physical state of the
lipid. Lipids in foods often exist as a combination of both liquid and crystalline
states, a condition that depends on both fatty acid composition and temperature. The
influence of liquid fat concentration on oxidation rates was investigated in liposomes,
where arachidonic acid oxidation rates were found to be greater at temperatures
below the solid–liquid phase transition temperature of the host lipid [105]. The
increase in oxidation rates was attributed to phase separation of the most unsaturated
fatty acids, which increases the concentrations of oxidizable substrate into localized
domains [106]. Little is known about how transition temperatures influence oxidation
rates in food lipids.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.