Food Lipids: Chemistry, Nutrition, and Biotechnology

vessel (hydration tank) to allow the phosphatides to become fully hydrated and to
coalesce. Hydration of the phosphatides is not instantaneous, and adequate time must
be allowed. Higher temperatures solubilize more phosphatides, and lower temperatures
increase oil viscosity; either one reduces the efficacy of degumming [40]. The
gums, being more dense than oil, can be removed by settling or filtering; more often,
however, they are centrifuged out. The wet degummed oil is either dried (as described
later) or immediately neutralized. Usually, about 90% of the phosphatides are removed
by this process. The gums typically contain 25% water and 75% oil-soluble
substances (of which one-third is neutral oil).
There are both hydratable and nonhydratable phosphatides. Of the 1–3% phosphatides
in soybean oil, 0.2–0.8% are generally regarded nonhydratable. The phosphatides
are composed of phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine,
and phosphatidic acid. The first two are always hydratable; but, the
latter two can complex with divalent metal ions, rendering them nonhydratable. Acid
degumming and superdegumming make more of the phosphatides hydratable. Nonhydratable
phosphatides remain oil soluble. The nonhydratable phosphatides are believed
to be calcium and magnesium salts of phosphatidylethanolamine and, especially,
phosphatidic acid, that arise from the enzymatic action of phospholipases when
the cellular structure of the seed is damaged [41]. Nonhydratable phosphatides are
particularly problematic in soybean oil.
Acid degumming is an improvement over conventional degumming described
above and has become the usual practice in the U.S. soybean industry. A small
amount (0.05–0.2%) of concentrated phosphoric acid (75%) is added to warm oil
(70�C) followed by stirring for 5–30 minutes and degumming as described in connection
with conventional degumming. Longer mixing times are often substituted
for lower reaction temperatures. Phosphoric acid is added to make the phosphatides
more hydratable by binding calcium and magnesium ions before adding water. Phosphoric
acid pretreatment also partially removes chlorophyll from the oil.
Phosphatide content varies widely in vegetable oils but is highest in crude
soybean oil (Table 2) [42–46]. Soybean oil is the only oil that is regularly degummed.
The use of expanders in preparing soybeans for extraction almost doubles
the usual phosphatide content of soybean oil. Only about half the phosphatide content
of soybeans is extracted with hexane when preparing soybeans by flaking alone.
Sometimes, the degumming operation is conducted at the mill so that the gums may
be added back to the meal. Gums contribute digestible energy to livestock. The
available U.S. supply of soy lecithin is about twice the volume that can be economically
sold. The gums for lecithin production are dried and may be further purified
and/or bleached. Soybean lecithin is a mixture of about 40% phosphatides (16%
phosphatidylcholine, 14% phosphatidylethanolamine, and 10% phosphatidylinositol),
35% oil, 17% phytoglycolipids, 7% carbohydrate, and 1% moisture [49].
Recently superdegumming processes have been developed in which more of
the phosphatides are rendered hydratable. A strong solution of citric acid is added
to warm oil (70�C), and the mixture is stirred and cooled to 25�C to precondition
the gums. Then water is added with stirring for an additional 3 hours to hydrate the
gums. This process causes the phosphatides to form liquid phospholipid crystals,
which are easily removed during centrifugation.
Another variation of degumming, dry degumming, is occasionally applied to
oils relatively low in phosphatide content, such as palm, coconut, and peanut oils.
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