Milk when heated shows interesting physico-chemical alterations. All the samples of milk boiled for one hour tested by Magee and Harvey showed a greater viscosity than corresponding fresh samples, and almost all the samples of milk pasteurized for thirty minutes hosed slightly lower viscosities than the fresh samples. Heating made a difference in the volume of the curd. The increase in swelling of the curd prepared from 20 cc. of milk amounted to 0.07 cc. after pasteurizing and to 1.70 cc. after boiling.
The quantity of colloidal ferric hydroxide necessary to precipitate the proteins of milk varied with the type of milk. Fresh milk (5 cc.) required 1.04 cc. more than pasteurized milk, and 1.5 cc. more than boiled milk. They also found that the boiled samples of milk invariably gave a smaller depression in freezing point than the fresh samples. It is evident that heat progressively reduces the number of free ions and molecules in solution of milk. In 1915, Milroy18 reported that the pH of milk is lowered by boiling. Fresh milk gave a pH of 6.73, boiled milk, a pH of 6.59. The figures of Magee and Harvey are pH 6.69 for fresh milk, and pH 6.55 for boiled milk.
Professor of Biological Chemistry and Nutrition, School of Medicine, Creighton University, Omaha, Nebraska.
As for the effect of heat-treated milk on the biologic welfare of the offspring, Lane-Claypon  states that infants fed on heated milk thrive better than on raw milk. On the other hand it must be stated that Daniels and Stuessy  observed that heating milk renders it inadequate, and that rats fed boiled milk grew only to one-half their size. The addition of protein in the form of casein or egg yolk permitted the rats of resume growth. Daniels and Loughlin21 attributed the favorable results obtained on the addition of these foods to their calcium content. As has already been pointed out, the calcium salts are rendered less soluble when milk is heated. In the insoluble form they may be lost owing to the fact that some of the precipitated material adheres to the sides and bottom of the container. (Concluded in Trade Winds.).
The effect of boiling and pasteurizing on the retention of calcium, phosphorus and nitrogen has been studied by Daniels and Sterns22 and also by Magee and Harvey.23 The former workers used two types of milk in their experiments, milk pasteurized at 145 F. for thirty minutes and milk quickly boiled, the application of heat lasting about eight minutes. By analyzing the urine and feces of infants, they found that for calcium, phosphorus and nitrogen the retention was greater for the boiled milk than for the pasteurized milk.
They concluded that the longer heat is applied, the greater are the alternations which some of the milk components undergo, and that a baby fed pasteurized milk over a long period of time receives too little calcium for its needs. Lane-claypon admits that there is a loss of calcium on heating cows milk, but maintains that in view of the higher per cent. of this element in cows milk in comparison with womans milk, the loss on precipitation is very insignificant. Moreover, the calcium precipitated by boiling can be mixed up again in the milk and there need be no loss in total calcium content, although this element may now be present in different form.
Magee and Harvey made studies similar to Daniels and Stearns. Their test animal was the young pig. On a diet of cereals and cows milk the retention of calcium, phosphorus and nitrogen was lower with heated milk than with fresh milk. The addition of soluble calcium to the ration containing heated milk raised the retention level of calcium, phosphorus and nitrogen. They suggested that heat has a detrimental effect on the nutritive value of milk and that one of the important factors contributing to the loss in dietary efficiency is the reduction in the amount of soluble calcium.
It is unfortunate that Magee and Harvey used the pig as the experimental animal, for the milk of the cow is poorer in calcium and phosphorus than the milk of the sow. Magee and harvey themselves state that after thirty days on a diet of cereal and cows milk the young pig developed signs of rickets, while addition of soluble calcium salt to this diet enabled it to remain in a thriving condition for sixty days. It is evident from the results of Daniels and Stearns and of Magee and Harvey that much experimental work should be done to ascertain the cause of the lowering of the nutritive capacity of heated milk in the hope that some way may be found of preventing the loss or compensating for the loss in biologic value.
In connection with the topic of this paper it is important to mention the work of Friedberger with reference to the influence of heat on the nutritive value of foods. This investigator has applied the biologic method to the evaluation of cooked foods. His results are intensely interesting. He has shown that cooking alters the biologic value of a mixed diet to such an extent that its nutritive value is markedly diminished. Animals fed raw food eat only one-third as much as those fed the same food cooked. Animals fed raw food, however, gain about twice as rapidly as do animals subsisting on cooked food.
Friedberger24 claims that alteration in nutritive value does not take place in foods like oats, that predominate in carbohydrate, or in foods like butter, that predominate in fat. The differences observed in cooked and uncooked foods were not due to vitamin destruction. None of the animals showed signs of avitaminosis. The addition of vitamins to cooked foods did not change the biological effects of the food mixture. Animals on cooked food plus vitamin reacted like those not receiving any extra vitamins.
Friedberger believes that the diminution in the nutritive value of food as a result of the application of heat comes largely from changes in the protein components. In this regard we must mention the experiments of Morgan and King,25 who also observed retardation in growth as a result of the feeding of cereal proteins subjected to heat.
The effect of heat on the nutritive value of hens eggs is also worthy of mention. Friedberger26 found that he could maintain rats on a diet of eggs alone. Rats nourished with raw eggs gained 140 grams in weight in two and a half months, while rats from the same litter on a diet of boiled eggs gained only 88 grams in the same period of time. The ingestion of heated eggs also caused falling out of hair.
It is surprising why we have not more fully investigated the importance and necessity of pure raw milk. The necessity of a pure raw milk has been pointed out by Chapin.27 That raw milk may turn out to be of great benefit in treatment and prevention is reflected in the findings of Kessel and KE – Kang.28 These investigators studied the effect of an exclusive raw milk diet on intestinal amoeba. They observed that the feeding of an exclusive raw milk diet to monkeys and to infants caused a reduction in the number of intestinal amoeba. They were able to certain instances to clear the intestinal tract of E. dysenteriae and other forms, and to keep it clear for a period of three months, following the convulsion of their experiments.
When the artificial feeding is resorted to, the infant is compelled to subsist wholly on one food, pasteurized milk. The obvious deficiencies of pasteurized milk in vitamin C are supplemented by orange juice. But there may be deficiencies that are unrecognized as yet. In view of the recent findings with references to the pathological effects of dead bacteria or sterile bacterial filtrates and with reference to the profound deterioration resulting from the application of heat, it is very imperative to re-evaluate the nutritive value of milk, not merely on the basis of freedom from living micro-organisms, but on the basis of its ultimate influence on the growth and maintenance of the infant.
In pasteurizing milk we have found safety in one direction by a method which may show decided harm in other directions. The greater morbidity and the greater mortality among infants artificially fed render the re-evaluation suggested a very much needed and very urgent procedure.
Certified milk is the safest and purest raw milk obtainable, being drawn by healthy men from healthy cows under the supervision of a Medical Milk Commission and produced strictly in accordance with the methods and standards of the American Association of Medical Milk commission.