| MadSci Network: General Biology |
John,
One source (Nutritional Biochemistry, Brody, 1994) states that an average large egg weighs 50 grams and to that, protein contributes 6 g (12%). These 6 g are divided about equally between yolk and egg white. However, note that for a 50 g egg, 33 g are contributed by the white and 17 g are contributed by yolk. That means that the white is only 9% protein while the yolk is 18% protein. These values are fairly consistent with an old (1975) documentation of the nutritional value of eggs. The Dictionary of Nutrition and Food Technology (Bender, 1975) provides the following breakdown:
Whole egg (*per 100 g): 12 g protein 12.5 g fat 57 mg calcium 2.5 mg iron 0.330 mg vitamin A 0.1 mg vitamin B1 0.36 mg vitamin B2 0.07 mg nicotinic acid 0.0045 mg vitamin D 470 mg cholesterol Egg white (*per 100 g): 11 g protein: ovomucin ovalbumin ovomucoid ovoglobulin conalbumin 87.8 g water Egg yolk (*per 100 g): 17 g protein 31 g fat 130 mg calcium 6 mg iron 1 mg vitamin A 0.32 mg vitamin B1 0.36 mg vitamin B2 0.02 mg nicotinic acid 0.0125 mg vitamin D 1.8 g cholesterolEggs are not the most protein dense food: about 12% of the egg is protein while salmon is 27% protein! However, this criterion for nutritional value is not necessarily the best one. For growing animals, only 9 amino acids are considered indispensable. Thus, a food would be considered ideal if it provided these 9 amino acids. The "chemical score" describes this criterion. To calculate the chemical score, you determine the quantities of each of the 9 indispensable amino acids. You then compare the amount of the limiting amino acid (the amino acid of lowest quantity) to the amount in a reference food. The percentage would be the chemical score. As it turns out, the egg is considered nutritionally complete and is the food to which all other foods are compared! It contains all 9 of the indispensable amino acids and has a chemical score of 100.
As an example, salmon (remember, it is more protein dense than eggs) has a chemical score of 71. That means that the amino acid (of the 9) that it has the least of, lets say it's tryptophan, is 71% of the amount of tryptophan found in eggs. For more details, see the first source listed above.
Julianne
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