Composition Of The

The egg is composed of approximately 10% shell, 30% yolk, and 60% white or albumen. The egg is a very good source of high-quality protein and many minerals and vitamins. The exact levels of the minerals and vitamins is determined by the level of each in the diet of the hen. An average chemical composition of eggs, including shells, is summarized in Table 3.

The protein of egg white is complete; it contains all of the essential amino acids in well-balanced proportions. The white is made up mainly of the proteins ovalbumin, conalbumin or ovotransferrin, ovomucin, ovoglobulins, ovomucoid, and lysozyme. The structure of the thick white is the result of a complex of ovomucin and lysozyme.

The important yolk proteins are ovovitellin and ovoliv-etin. The lipid materials in the egg are all in the yolk. The fatty acid composition can easily be modified by changing the fatty acid makeup in the feed. In most commercial eggs where the hens are fed a corn and soybean meal diet the fatty acids are about one-third saturated and two-thirds mono- and polyunsaturated. Yolk color is controlled by the level of pigments, mostly xanthophyll, in the feed of the hens.

Some elements of the medical profession have emphasized the role of dietary cholesterol in human cardiovascular problems. The negative effect on egg consumption that this emphasis on cholesterol has had led to a number of research attempts to reduce the cholesterol content of eggs. Genetically it has been possible to achieve only slight reductions. Selection of hens for small yolk size has met with some success. A review on altering cholesterol by feeding (5) concluded that dietary modifications resulted in only minor changes in the cholesterol content of egg yolk. Another report (6) states that including 1.5 to 3.0% of menhaden oil in the laying hen's ration results in a temporary reduction of about 50% in the cholesterol concentration in the yolk. The fatty acid composition of the eggs was also modified in that the eicosopentenoic acid and do-cosahexanoic acid (omega-3 fatty acids) content of the yolks was significantly increased when the fish oil was included in the hen's ration. Levels offish oil in excess of 3% in the ration resulted in fishy flavored eggs.

The relative ease with which egg composition can be modified has led to the development of a variety of "designer eggs." It has been reported that all vitamins and minerals content of eggs, except for choline and zinc, can be adjusted significantly by dietary modification for the hens (7).

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