The Amino Acid Content of Hen's Egg in Relation to Dietary Protein
Intake, Breed and Environment¹

P. Lunven and C. Le Clément de St. Marcq
Protein Food Development Group
Nutrition Division

In 1963 the Joint FAO/WHO Expert Group on Protein Requirements decided to adopt the essential amino acid pattern of either egg or human milk for reference purposes in preference to the FAO provisional pattern proposed in 1957.

The data, however, were not sufficiently precise to provide a basis for choosing between the essential amino acid pattern of these two protein sources.

The Group suggested therefore that further research be conducted in the field of protein requirements and indicated that more work was needed on the pattern of essential amino acids and, in general, on the quantity of amino acids in foods.

Since the publication of the FAO/WHO report on this meeting in 1965 (2), much progress has been made in the techniques of amino acid assay with the development of high-speed automatic amino acid analysers and with the introduction of spheric polystyrene sulfonated resins of high resolving power.

In spite of the extension of these new laboratory facilities, it is surprising to see how poor is the international scientific literature in recent analytical work on the amino acid composition of hen's egg.

Similarly, we have found very few references on the amino acid composition of hen's egg in relation to the protein content of the diet and to other factors such as breed and environmental conditions which may have an influence on the amino acid pattern of the egg and, therefore, introduce another variable in the assessment of protein quality of foods and in the study of protein requirements where whole hen's egg is used as reference protein.

An exhaustive compilation of available data carried out in the FAO Nutrition Division (3) on the amino acid content of foods shows that analyses of egg are small in number and the results often variable. Most of them have been made by microbiological methods which are known to be less accurate than ion-exchange chromatography and which, furthermore, are all previous to 1965.

For these reasons we have carried out a study of the hen's egg amino acid content by ion-exchange chromatography, using the most recent automatic equipment, including an electronic curve integrator.

Methodology

Two breeds of hen have been used: 40 pullets New Hampshire (N.H.) and 20 pullets White Leghorn var. Livornese (W.L.) which were fed on two isocaloric diets with a protein content (N × 6.25) respectively of 20% and 11% and a balanced amino acid pattern (see Annex 1). The experiment started with 3 month-old animals and the collection of eggs lasted one full year.

One group of animals was fed in individual cages and the other was free-running in an open-air enclosure in order to check possible differences in egg protein composition between animals bred under controlled conditions and those subject to climatic fluctuations. The number of eggs laid, their weight, relative weights of yolk, white and shell were recorded and the protein and amino acid content of whole egg, yolk and egg-white were determined taking into account the various parameters: protein intake, breed, enclosure or cage.

Analytical techniques

The analytical techniques used are as follows:

Total Nitrogen: microkjeldahl (4)

Amino acids ion-exchange chromatography (5)

- Beckman Model 120 C amino acid analyser

- Resin PA 28 for neutral and basic amino acids PA 35 for acid amino acids

Cystine as cysteic acid (6)

Tryptophan by ion-exchange chromatography after alkaline hydrolysis (7)

Experiment

The animals were distributed among 6 groups of 10 pullets (2 W.L., 4 N.H.). of these 6 groups, 3 were fed on the lower protein feed (11%) and 3 on the higher protein feed (20%). Two groups of N.H. were free-running birds. All animals were fed ad libitum and all eggs were identified with a number corresponding to each hen in special laying cages.

Ten eggs from each group were pooled at random every fortnight. These eggs were homogeneized with a turmix and lyophilized in 24 hours in an Edwards ELPTC freeze dryer. The egg powder was homogeneized once more and an aliquot was taken from the total sample used for the various assays.

Results

This study was carried out on about 2000 eggs. The samples were analyzed always in duplicate and often in triplicate for the individual determination of protein, amino acids, cystine and tryptophan respectively.

Protein content of egg

A significant difference was noted in both breeds as to the quantity of protein in egg, in relation to the protein level in the feeds (Table 1).

Table 1

Protein Content of Egg (in % - dry weight)

Hens given a high protein feed lay eggs with 1 to 2% more protein than in the other group. Moreover, as might be expected, the total number of eggs laid by the first group was higher than by the second.

In addition, it appears that breed has an influence on the protein content of the egg; although the W.L. hens lay eggs weighing about 5% less than the N.H., their protein content, regardless of diet, is 5% higher.

This is probably due to the fact that the yolk in W.L. is relatively bigger than in N.H. and being richer in protein (16% versus 10.9%) than egg-white, it makes a significant difference to the comparative protein content of the whole egg.

These results were also found in the free-range group.

Amino acid content of egg

There is practically no difference between the amino acid patterns of hens fed both diets and between each breed. The remarkable stability of hen's egg amino acid content should therefore be underlined.

In general, our results show slightly lower values than previously published data for all the amino acids except for cystine, lysine and tryptophan. The cystine content, in particular, is much higher. It should also be noted that the isoleucine content is considerably (about 20%) lower than currently accepted figures.

Our data are compared to previously published data and are expressed respectively in mg per gram of total nitrogen (Table 2), in mg per gram of total essential amino acids (A/E ratio) (Table 3).

In Table 4 we have tabulated the amino acid composition of eggs produced by the two strains fed respectively the 11% and 20% protein diets over a period of one year. It is obvious from this table that the amino acid content of hen's egg is very stable regardless strain and protein intake. In addition, our detailed data, which do not appear in this report, indicate that there is no significant seasonal variation in hens under controlled conditions nor in those living in an open-air enclosure.

Table 2

Essential amino acids in egg protein and in human milk compared to
1957 FAO provisional pattern (mg per g of total nitrogen)

1 Protein requirements - FAO Nutr. Studies No. 16, 1957

2 Protein requirements - FAO Nutr. Meetings Report Series No. 37, 1965

3 Mitchell, H.H., R.J. Block - J. Biol. Chem., 163, 599 - 620 (1946)

4 Protein requirements - FAO Nutr. Meetings Report Series No. 37, 1965

5 Amino acid content of foods - FAO Nutr. Studies No. 24, 1970

6 The amino acid composition of hen's egg in relation to the protein content of poultry feed - C. Le Clément de St. Marcq, E. Carnovale, A. Fratoni and P. Lunven (to be published)

Table 3

Selected essential amino acid patterns (A/E)
A/E ratio: mg per g of total essential amino acids

Table 4

Amino acid content of hen's egg (mg/gN) in relation
to dietary protein level and breed

Discussion

The 1965 Joint FAO/WHO Expert Committee report on protein requirements describes, in p. 35 and following, the various considerations which led them to adopt the whole hen's egg as reference protein.

In particular:

- The Committee expressed the view that the FAO provisional pattern contained too much tryptophan, sulfur-containing amino acids and lysine. This view was supported by observations on man and experimental animals.

- The Committee therefore recommended that these amino acids be present in lesser quantities in the reference protein (see Table 3) and considering that the new levels assigned to them were closer to those of hen's egg amino acid pattern when expressed in terms of A/E ratio, decided to adopt the latter as reference protein (8).

(At this point, it might be interesting to note that the correlation between the chemical score calculated by the A/E ratio method and the data on biological value is poorer than when the score is expressed in terms of mg of individual essential amino acid per gram of total nitrogen (Mitchell and Block method)) (9).

- In the light of the new data provided in this report, it appears that the hen's egg pattern (1971) expressed in terms of A/E ratio resemble much more closely the 1957 FAO provisional pattern than the 1965 hen's egg and human milk amino acid patterns.

- One of the immediate implications of these new data on the composition of the reference protein is that the disparity between an improved reference amino acid pattern as suggested by the Committee in 1965 and the whole hen's egg amino acid patterns. becomes even greater.

- This disparity is greatest in the case of lysine, tryptophan and sulfur-containing amino acids when they are expressed in mg per g of total nitrogen but is still important when the essential amino acid patterns are calculated as A/E.

- It should also be noted that the isoleucine content of hen's egg is considerably lower than in previously published data. This is particularly important because the frequency of isoleucine as the limiting amino acid seems to have been over-estimated in chemical score determinations based on the 1965 hen's egg and human milk amino acid patterns.

The Committee therefore, taking into account all the works which have been done on man and on animals, might wish to take this opportunity to design a synthetic amino acid reference pattern which would give better correlation between the chemical score of dietary proteins and their biological assessment.

Summary

1. Protein intake and breed seem to have no effect on the amino acid composition of hen's egg although there are some small changes in total protein content.
2. Lysine, tryptophan and especially cystine are present in greater proportion than in previously published data whereas isoleucine is much lower than the currently accepted figures.
3. These results imply that the questions raised by the Joint FAO/WHO Expert Group on Protein Requirements in 1965 regarding the excess of tryptophan, sulfur-containing amino acids and lysine in the 1957 FAO provisional pattern are also valid for the whole egg protein which contains even higher quantities of these essential amino acids.

ANNEX 1

Composition of poultry feeds

ANNEX 1 (Cont'd)

Amino acid composition of poultry feeds

(1) Research work carried out in the National Nutrition Institute, Rome, Italy. (Director, Prof. S. Visco).

(2) FAO. Protein Requirements. Report of a Joint FAO/WHO Expert Group. FAO Nutrition Meetings Report Series No. 37, 1965.

(3) FAO. Amino Acid Content of Foods. FAO Nutritional Studies No. 24, 1970.

(4) A.O.A.C. No. 2044 and 16001, 1960.

(5) Moore, S. and W.H. Stein. J. Biol. Chem., 192, 663, 1951.

(6) Schram, E., S. Moore and E.J. Bigwood. Biochem. J., 57, 33, 1954.

(7) Lunven, P. Thèse Fac. Pharm. Paris, 1968.

(8) FAO Nutrition Meetings Report Series No. 37. p. 38 (1965)

(9) Cresta, M., J. Périssé, M. Autret and E. Lombardo. Ann. Nutr. Alim. 1971 (in press).