Food Lipids: Chemistry, Nutrition, and Biotechnology

erols to generate semisolid or plastic fats more suitable for specific applications, it
may also be applied to phospholipid fractions. Hydrogenated lecithins are more stable
and more easily bleached to a light color, and therefore are more useful as emulsifiers
than the natural, highly unsaturated lecithin from soybean oil. These advantages are
exemplified by a report that hydrogenated lecithin functions well as an emulsifer and
as an inhibitor of fat bloom in chocolate [32].
In practice, hydrogenation involves the mixing of the lipid with a suitable
catalyst (usually nickel), heating, and then exposing the mixture to hydrogen at high
pressures during agitation. Phospholipids are not as easily hydrogenated as triacylglycerols;
as a result, their presence decreases the catalyst activity toward triacylglycerols
[33]. In this situation, phosphatidic acid was the most potent poisoning
agent; however, fine-grained nickel catalyst was more resistant to the poisoning effect
of phospholipids than moderate-grained catalyst. In any event, hydrogenation of
phospholipids requires higher temperatures and higher hydrogenation pressures. For
example, hydrogenation of lecithin is carried out at 75–80�C in at least 70 atm
pressure and in the presence of a flaked nickel catalyst [34]. In chlorinated solvents
or in mixtures of these solvents with alcohol, much lower temperatures and pressures
can be used for hydrogenation, particularly when a palladium catalyst is used [35].
VIII. HYDROXYLATION
Hydroxylation of the double bonds in the unsaturated fatty acids of lecithin improves
the stability of the lecithin and its dispersibility in water and aqueous media. Total
hydroxylating agents for lecithin include hydrogen peroxide in glacial acetic acid
and sulfuric acid [36]. Such products have been advocated as useful in candy manufacture
in which sharp moldings can be obtained when the hydroxylated product is
used with starch molds.
IX. HYDRATION
The amount of water absorbed by phospholipids has been measured by a number of
different methods, including gravimetry, X-ray diffraction, neutron diffraction, NMR,
and DSC [37]. For any measurement, however, Klose et al. [38] cautioned that the
morphology and method of sample preparation can induce the formation of defects
in and between the bilayers, and therefore will influence the water content of lamellar
phospholipids.
The electrical charge on the phospholipid head group does not in itself determine
the nature of the water binding. However, it does affect the amount of water
bound. The amount of water absorbed by PC from the vapor phase increased monotonically
from 0 water molecules per PC molecule at 0% humidity to between 14
[39] and 20 [40] water molecules per lipid molecule at 100% relative humidity.
Observed differences may be due to the difficulty of exerting accurate control over
relative humidities near 100% when temperature gradients in the system are present.
The results of X-ray diffraction studies indicated that when directly mixed with bulk
water PC imbibed up to 34 water molecules [41,42]. Considerably less water was
imbibed by PE, with a maximum of about 18 water molecules per lipid [43]. From
the saturated vapor phase, however, liquid crystalline egg yolk PE only absorbed
about 10 water molecules per lipid molecule [39], whereas for charged phospholipids,
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