343. Studies of the Biological Value of the Proteins (Nitrogen) of Dried Skim Milk: Value As An Addition to Bread; Tests With Young and Olderrats; Effects of Storage

Abstract

A considerable amount of work has been done in recent years on the improvement of the nutritive value of bread by the addition of small amounts of dried skim milk to the flour (i, 2, 3, 4, 5). It is generally agreed that this improvement is largely due to the protein, calcium and riboflavin supplied by the milk. The experiments now to be described deal further with the protein side of the problem.Our earlier experiments (3) showed that the addition of 2 or 6 % milk to white bread resulted in an increase in the biological value of the proteins of the mixture, but no evidence was obtained that, at these levels of feeding, the milk proteins exerted a supplementary effect on those of bread. At the time it was not feasible to make a simultaneous determination on the milk itself. It was suggested that this lack of a supplementary effect was probably due to the relatively low level of the milk additions. At the same time it was pointed out that, during the baking of bread, the proteins in the crust suffer some injury (6, 7), and that it was possible that the milk proteins might be similarly affected by baking to a degree sufficient to inhibit any supplementary effect. To test this point an experiment was planned in which milk was added to bread either before or after baking.We thought that it might be opportune to inquire at the same time into the effect of age of the rat on the assessment of the biological value of proteins, a problem on which the literature is far from unanimous. French & Mattill(8), for example, have reported biological values for white bread, containing an unstated quantity of milk, of 74 and 83 for young and mature rats respectively. Sumner(9), on the other hand, obtained lower values for dried milk and dried whole egg with 12-months old rats than with rats 40-50 days old.We decided to carry out the present experiment according to our normal procedure with young actively growing rats and to repeat it with the same animals when active growth had ceased. As a result of the findings of this experiment certain other problems arose, and the results of tests to solve some of them are also included in this report. The term 'biological value of the protein' recurs frequently throughout this paper and to economize space the abbreviation b.v. will be used. EXPERIMENTAL National wheatmeal bread made from 85 % extraction flour was used in the experiments described below. In all experiments the diets were made to contain about 8 % protein; the b.v. and true digestibility of the proteins were determined on rats by the method of Mitchell & Carman(io,11). In each experiment three litters of four female rats were used; they were 24-29 days old and weighed 49-68 g. at the beginning of the tests. The cages

Section D. Nutritive value of milk and milk products

“…As Carlson et al (666) do not quote the biological value of the proteins of the sample of milk used by them it is impossible to say whether the improvement in the biological value of white flour proteins found after the addition of 3 and 6 % milk was due to an additive or a supplementary effect. Baking into bread did not affect the biological value of the proteins of skim milk powder added to flour (656). Ice cream made from whole milk and cream is a highly nutritious food, as can be judged from analyses of the Canadian product (676,677), and by the very similar results obtained in the United States (13,678,679) which include values for calcium, vitamin A, vitamin ^ and riboflavin.…”

“…Henry et oL (656) and Henry & Kon (657) noticed a gradual deterioration from 89 to 71 in the biological value of the proteins of skim milk powder during 3 years' storage at room temperature in a barrel exposed to air. Further tests showed (419,420) that the loss in biological value was due to inactivation of lysine by a Maillard reaction between its free amino-group and the aldehyde group of lactose, that the reaction took place only above a critical moisture content of 3 % and that it did not depend on the presence of oxygen, though with oxygen certain additional deteriorative changes took place.…”

“…Jones & Divine (664) found by the rat-growth method that the proteins of white flour were markedly and progressively improved by the addition of 5,10 and 15% of dried skim milk; Hove et al{683) made similar observations with additions of 3, 6 and 20% milk, and Carlson, Hafner & Hayward(666) with 3 and 6%. Henry et al (656,667) and Henry & Kon (668) found by the balance-sheet method only additive effects when the milk supplied less than 20 % of the proteins of the mixtures but supplementary effects were observed when its contribution was higher. As Carlson et al (666) do not quote the biological value of the proteins of the sample of milk used by them it is impossible to say whether the improvement in the biological value of white flour proteins found after the addition of 3 and 6 % milk was due to an additive or a supplementary effect.…”

“…Henry & Kon (655) found no difference in the biological value of the proteins of dried whole milk whether preheated to 165° F. or to 190° F. before spray-drying. Henry, Kon & Rowland (656) examined a commercial sample of over-neutralized, and another of over-heated, roller-dried Bkim milk together with samples of good quality roller-and spray-dried milk, but found no statistically significant differences in biological value or true digestibility. On the other hand, another sample purchased as 'best skim powder' proved on investigation to be both over-heated and over-neutralized and had the very low biological value of 55.…”