Outhouse et al. 1928 give a summary of the literature pointing out the differing results of various workers according to the diet and environmental conditions of the cow on which the work was done .sx It may , however , be accepted that the value of the cow's milk in fat-soluble vitamins is at its maximum when the cow is on pasture in summer .sx It has been shown that the vitamin A and vitamin D contents of winter milk can be enhanced by feeding the cow with cod-liver oil Golding , Soames and Zilva , 1926 , and the vitamin D content by administering irradiated yeast Steenbock , Hart , Hanning and Humphrey , 1930 ; Wachtel , 1929 .sx Difference of opinion exists as to the effect on the antirachitic potency of the milk of irradiating a cow with ultra-violet rays from an artificial source .sx The inference from all that has been said is , however , that the application of ultra-violet rays to the skin of the cow would be expected to result in increasing the vitamin D value of the milk .sx Such a result has been obtained by many workers , but Steenbock , Hart , Riising et al. 1930 , where the literature is set out and discussed , although finding that the milk of goats could be so affected , were unable to demonstrate an increased antirachitic value in cow's milk by artificial irradiation of the animal .sx In one of their experiments the milk of cows exposed to sun without change of diet from May 26 to June 16 showed no improvement as the result of insolation , but the milk of the same cows at the end of a summer on pasture was markedly superior .sx The vitamin B and vitamin C contents of cow's milk appear also to vary with the diet of the animal .sx Kennedy and Dutcher 1922 found that 10 c.c. of milk from a cow on an adequate ration would supply an adequate daily provision of B-vitamins for a rat , while 15 c.c. from a cow on an inadequate diet would not .sx The anti-scorbutic value was found to vary more widely ; in experiments with guinea-pigs , 20 c.c. of summer milk were found superior in nutritive value and in antiscorbutic potency to 60 c.c. of winter milk Dutcher et al. , 1920-21 .sx A similar difference was found by Hess , Unger and Supplee 1920-21 .sx It must be recognized that cow's milk is a variable foodstuff , of which the vitamin value in any given instance cannot be assumed .sx Its use in infant-feeding must be specially safeguarded , not only by the use of milk products for the purpose of modification , but by the addition to the child's diet of the same supplements as are recommended for the breast-fed infant ( p. 274) .sx Treated Cow's Milk ( Irradiated , Heated , Dried , or Condensed) .sx IRRADIATED COW'S MILK .sx The vitamin D value of cow's milk , whether raw or dried , can be much enhanced by ultra-violet irradiation ; such milk may be of service prophylactically , in protecting children from rickets , but in the present state of knowledge it would be unwise to rely on it to the exclusion of cod-liver oil .sx Cowell 1925 ; see also Daniels , Pyle and Brooks , 1925-26 showed that children could rapidly be cured of rickets by giving them every .sx day a pint of milk which had been irradiated for 20 minutes .sx So long an irradiation in presence of oxygen is liable to impart to the milk an unpleasant flavour , which has been ascribed to the oxidation of proteins Schultz , 1929 , and may also damage the vitamin A it contains Scheer , 1928 .sx Types of apparatus have now been devised in which the irradiation of milk can be accomplished in the absence of air or in an atmosphere of carbon dioxide see Scheer , 1928 .sx Milk irradiated in this way has been shown to be a reliable prophylactic or curative agent for rickets and to be free from any objectionable taste or smell .sx Coward 1929 , 2 , as the result of numerous tests , has found great variation in the antirachitic value of fresh cow's milk after irradiation for periods up to 30 seconds .sx Such a variation is not surprising and is to be expected , since the ergosterol ( provitamin ) content of milk is subject to considerable variations Hentschel and Fischer , 1930 .sx The antirachitic value of milk thus irradiated was found to have been increased 5-50 times , and it was calculated that when irradiation was effective about 3 pints were equal in antirachitic value to one teaspoonful of a good cod-liver oil .sx So short an irradiation would not have the drawbacks alluded to above .sx HEATED COW'S MILK .sx A question of great moment is the influence of heat upon the nutritive value of cow's milk , and in certain respects the evidence on this question is conflicting .sx The matter was discussed at some length by Lane-Claypon 1916 in whose book are many references to authorities who consider that there is no proof of any inferiority of heated or pasteurized , as compared with raw , milk .sx Since then more information upon the effect of heat upon the vitamins and other constituents of milk has accumulated .sx The evidence as to any deleterious effect of careful pasteurization on the nutritive properties of cow's milk is not decisive .sx It is in any case at present advisable to pasteurize the ordinary milk supply to prevent the spread of milk-borne diseases .sx Owing to the variability of the vitamin content of milk , whether heated or raw , it is wise in all cases to supplement a diet of milk with vitamin additions .sx Of the vitamins present in milk , vitamin C is the most sensitive to the combined effects of heat and oxidation and evidence of the connexion between the use of pasteurized or heated milk and the incidence of infantile scurvy is presented on p. 264 .sx Effect of Heating on the Vitamin C of Milk .sx Pasteurizing .sx Direct comparison , by a modified form of the comparative method of Sherman , La Mer and Campbell ( p. 203 ) , of fresh milk with the same milk after aerobic pasteurization at 60 C. for 30 minutes Schwartze , Murphy and Cox , 1931-32 showed that the degree of diminution of the vitamin C content varied with the metal with which the milk came in contact during the process , being 20-40 per cent .sx ( i.e. hardly perceptible ) with aluminium , slightly greater with tinned copper , and 80-90 per cent .sx with copper .sx Boiling .sx It was found by Barnes and Hume 1919 , 1 , 2 that when .sx milk was scalded i.e. quickly heated to boiling in an enamelled pan and then allowed to cool in the air , it suffered comparatively little loss not more than 25 per cent .sx of its antiscorbutic power .sx A similar result was obtained by Schwartze , Murphy and Hann 1929-30 for milk boiled in glass or aluminium vessels .sx Sterilizing .sx As regards the effect of higher degrees of heat , i.e. temperatures above 100 C. , it would appear that these cause a rapid destruction of the antiscorbutic vitamin .sx Milk heated to 120 C. , for ten minutes to one hour , has been examined by several investigators Chick , Hume and Skelton , 1918 , 1 , 2 ; Hart , Steenbock and Smith , 1919 .sx No accurate estimate of its antiscorbutic power has been made , but after having been heated at 120 C. for one hour it can be used with impunity as a constituent of the basal diet of guinea-pigs for experiments with antiscorbutics ( ) .sx Effect of Heating on the Calcium of Milk .sx Evidence has been forthcoming that the calcium supply in cow's milk is affected by heating .sx Daniels and Loughlin 1920 studied the problem on rats and found that the animals grew less well on pasteurized milk than on milk just brought to the boil , and that this was due to an insufficiency of calcium in the pasteurized milk .sx The calcium was found to be thrown out of solution by the more prolonged heating and the precipitate thus formed was mechanically lost .sx When milk was brought to a temperature of 145-50 F. , maintained at it for 30 minutes and tested 5 hours later , the loss in diffusible calcium was computed to be about 2 per cent .sx ; at higher temperatures , up to 209 F. , the loss rose to 2.5-3.6 per cent .sx Mattick and Hallett , 1929 .sx From the Rowett Institute also comes evidence that when milk is heated there is a reduction in the diffusible calcium Magee and Harvey , 1926 , 1 .sx Young pigs on a diet of cereals and milk showed a lower retention of calcium , phosphorus .sx and nitrogen when the milk was heated than when fresh or sour Magee and Harvey , 1926 , 2 .sx When groups of calves were fed respectively on fresh milk , on pasteurized milk , and on pasteurized milk to which calcium lactate had been added , it was found that the increase in weight in the group on pasteurized milk , without any addition , was distinctly inferior to that in the other two groups .sx In one experiment the calves in the group receiving pasteurized milk actually contracted rickets Orr et al. , 1926 .sx It was also found that heat caused the disappearance by volatilization of 20 per cent .sx or more , according to the severity of the heating , of the total iodine of separated milk Magee and Glennie , 1928 .sx Although it cannot be disputed that the calcium supply in milk is damaged by heating , yet it might well be argued that in comparison with human milk , cow's milk contains so much calcium ( 38 mg .sx of CaO per 100 g. in human milk to 175 mg .sx in cow's milk Schall and Heisler,1927 ) that a heavy loss might occur before the human infant fed on it would suffer any deprivation .sx If , however , it be assumed that for this reason young infants would not suffer from diminution .sx of available calcium in pasteurized milk , it must be remembered that milk is also one of the most important sources of calcium for children older than those who are wholly milk-fed , and any impoverishment of the calcium supply in diets which may already contain insufficient of this element is much to be deprecated .sx Evidence , however , exists that heating cow's milk may render the available calcium insufficient even for babies wholly fed on it .sx Daniels and Stearns 1924 found that a series of infants maintained on pasteurized milk to which antiscorbutic had been added failed to grow , and in some cases actually showed a negative calcium balance , while those which received milk that had simply been brought to the boil grew better , and , except in one case , remained in positive calcium balance , though not apparently retaining the optimal amount of calcium .sx The faecal calcium in the periods during which pasteurized milk was given was greater than in the periods in which boiled milk was given , suggesting that the calcium was not merely lost mechanically but that a portion of it was in a form in which it was incapable of utilization .sx These experimental results are so important that they demand confirmation and extension .sx This matter is a very important one in view of the ever increasing spread of milk pasteurization in the large cities , and cannot be lightly dismissed .sx The evidence indicates that heating may be detrimental to the mineral content of milk and that the duration of the heating is more important than the temperature , if this does not exceed the boiling point .sx There is , however , little doubt that regular addition of vitamin D as cod-liver oil would , to a large extent , correct any calcium deficiency by securing the optimum utilization of the calcium presented to the organism .sx Raw , sterile milk is of course the ideal to be aimed at , but as such milk is at present unobtainable , there seem to be two alternatives , either to use pasteurized milk or to bring raw milk to the boil shortly before drinking .sx In the latter case the heating should not be prolonged .sx DRIED COW'S MILK .sx On general grounds , it would appear likely that much that has been said about heated milk would be applicable also to dried milk , in so far as the processes used in drying involve heating .sx The majority of dried milks are prepared by one or other of two processes , which may be termed respectively the drum and the ' spray ' processes Rep .sx L. G. Board , 1918 .sx