Shrader and Swenarton8 have published an interesting report on the situation in Baltimore. They found that in the summer of 1922 the peaks in the curves for bacterial count and mortality coincided. In that year the average bacterial count rose to 10 to 12 million per cubic centimeter; in 1923 it was not over 8 million, and in 1924 it never rose above 2,200,000. In 1922 the mortality rate in Baltimore for gastroenteritis was 58 per cent. above the average rate for other American cities, but in 1923 it was 30 per cent., and in 1924 it was 39 per cent. above the average rate.
The facts cited above serve as convincing proof of that pasteurization may at times prove to be sanitation at the wrong end. Wherever necessary the law in regard to pasteurization should be amended so that only milk with a definite limit as to its bacterial population could be legally submitted to pasteurization. Besides the ever-present danger of the re- inoculation of pasteurized milk in the home, there is the additional danger in the careless handling of pre-pasteurized milk provided by the false sense of safety engendered by the subsequent treatment by heat. In the summer of 1922 the average bacterial content of the milk entering Baltimore was higher than the preceding year, but this fact at first occasioned no particular concern, because all the milk was to be effectively pasteurized.
Pasteurization came into being long before we suspected that heat could produce any serious changes in the various food factors, such as vitamins, proteins, and inorganic compounds, or in the physico-chemical nature of the components of a food mixture.
In 1912 Frohlich9 showed that milk heated at 98 c. for ten minutes loses its protective action against ginea pig scurvy, while heating at 70 C. for thirty minutes gives uncertain results. The thermolability of the antiscorbutic substance in milk has also been demonstrated by Hess,10 Hess and Fish11, Chick, Hume and Skeleton12, and Hart, Steenbock and Smith.13 The addition of antiscorbutic to the milk diet of the infant is an attempt to compensate for the loss in vitamin C through the application of heat.
Under the influence of heat the proteins of milk undergo modification. The production of hydrogen sulphide and of ammonia points to decomposition of these proteins. The whey proteins, albumin and globulin, suffer coagulation. When heated at 80 C. for fifteen minutes, caseinogen is so modified that the time required for coagulation by rennin is prolonged. According to Soldner,14 the calcium salts are altered by heat to the extent that they become unsuitable for rennet coagulation and for absorption. Rona and Michaelis15 observed in 1909 that colloidal ferric hydroxide splits up the casein-calcium compound of milk.
When calcium is determined quantitatively and recorded in terms of calcium oxide, precipitation by collateral ferric hydroxide results in the appearance of 70 to 80 per cent. of the calcium oxide in soluble form, most of the phosphorus calculated as phosphorus pentoxide being carried down by absorption. Magee and Harvey16 compared the amounts of calcium oxide found in solution after precipitation of protein from fresh and heated milk with colloidal ferric hydroxide.
In the case of whole raw milk, 85 per cent. was found in solution; in the case of whole milk heated for thirty minutes at 65, only 71 per cent. Magee and Harvey also observed that the percentage of loss by dialysis of calcium calculated as calcium oxide is greater for pasteurized milk and still greater for boiled milk. That there is a loss in the soluble calcium and also in the soluble phosphorus compounds of milk has also been reported by Bell17 and others. The extent of the loss depends upon the temperature to which the milk has been heated.
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 [19] 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 [20] 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.
