Biomedical Engineering Reference
In-Depth Information
65 to 70% fats, which contain 65% triacylglycerols, 30% phospholipids, and 4%
cholesterol.
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Of the 30% phospholipids in egg yolk, the percentages are PC 73; PE,
15.5; PI, 0.6; lysophosphatidylcholine, 5.8; sphingomyelin, 2.5; lysophosphatidyle-
thanolamine, 2.1; plamalogen, 0.9.
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These lipoproteins can be separated by high
speed centrifugation of egg yolk into the sediment, called granules, and the super-
natant fraction, called plasma. Granules represent 19 to 23% of the yolk solids,
whereas plasma represents 78% of the liquid phase. On a dry weight basis, granules
contain around 34% lipids, of which 37% are phospholipids, mainly PC (82%) and
PE (15%). Plasma contains a low-density lipoprotein (LDL) fraction that is com-
prised of 84 to 89% lipid, of which 26% is phospholipid (71 to 76% PC, 16 to 20%
PE, and 8 to 9% sphingomyelin and lysophospholipid).
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Owing to the good emulsifying properties of egg yolk lipoproteins, oil can be
dispersed in other food ingredients so that the lipoproteins can contribute to the
consistency of mayonnaise and salad dressing and to the structure of cream puff
shells. Whole eggs are used in rolls, sponge and layer cakes, and bread, while yolks
are used in salad dressings, mayonnaise, doughnuts, sweet goods, and cakes that
require more yellow color. Because of the high protein content, egg white is used
in angelfood cakes, puff pastry, white pound cakes, layer cakes, cupcakes, meringue
toppings, and candies, as well as in a number of premixed products.
Natural Emulsifiers in Milk
Few of these have yet to be used commercially. Besides soybean lecithins the only
other natural source of lecithin used extensively in foods is that obtained from eggs
or that found in milk. Milk is one of the naturally occurring emulsions found in
foods. Cow's milk contains milk fat (3 to 6%), protein (3 to 4%), lactose (5%), and
ash (<1%). Of the milk fat, the phospholipid fraction is generally not larger than
1% and triacylglycerols comprise 97 to 98% of the total milk fat. Other lipid soluble
substances found include sterols, carotenoids, and fat-soluble vitamins which all aid
emulsification.
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PHOSPHOLIPIDS AS LIPOSOMES FOR FOODS
Liposomes are tiny hollow bodies made up of phospholipids and filled with water.
Their diameter is in the region of 200 to 500 nm. They were originally made to
imitate cells, so that the transport of substances into and out of the cell could be
studied. Liposomes form compartments without which the cell would be unable to
function. Together with water, phospholipids form double layers in a structure that
enables them to act as “cell walls”, dividing the compartments from each other
which has great technical benefit.
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Water-soluble substances can be stored in the
liposome core surrounded by the structural bilayer membrane which has two useful
effects. First, encapsulated substances can be distributed in food more homoge-
neously, and second, encapsulated substances are immobilized because of the phys-
ical stability of the liposomes. The encapsulated substances do not easily migrate,
and take longer to be released from the product. For example, a lemon-flavored cake
stays fresh longer and retains the flavor.
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