234 MILK AND ITS HYGIENIC RELATIONS 



present in the different samples of milk, and that it varied from 

 1*2 to 2 gms. per litre in raw milk. The loss by heating was 

 investigated when milk was boiled on the open fire for five, ten, 

 and fifteen minutes respectively. The loss in citrates obtained 

 by heating for five minutes varied from 12-29-31-86 per cent, 

 on the total, and from 5-01-29-78 per cent, when heated for 

 fifteen minutes. When heated for longer periods of half an hour 

 an apparent gain occurs which he believed to be due to loss of 

 water and increased concentration. The increase, however, shows 

 such wide deviations, that it is somewhat difficult to believe that 

 sufficient concentration would take place to account for a gain, 

 instead of a loss of approximately 30 per cent. When milk was 

 heated on the water-bath, the following approximate results were 

 obtained : 



Heated for 15 min. at 75 C. gave a loss of 4-13 per cent. 



30 75 C. 3-44 

 60 75 C. showed a gain. 



30 100 C. gave a loss varying from 18-30 per cent. 



60 100 C. 17-29 



At higher temperatures, e.g. at 120 C., there was a loss of 22 per 

 cent. 



Splittgerber investigated the effect of heat upon various 

 substances in milk, notably on the citric acid. He took known 

 solutions of mono-, bi-, and tri-calcium citrate, heated these to 

 dryness, and then heated further for prolonged periods. He found 

 that the tri-citrate heated by this method showed a slight loss of 

 approximately I mgm. on 181 after five hours' drying. The mono- 

 and bi-citrates showed a slight increase in weight, which he believed 

 might be accounted for by the holding of water of crystallisation. 

 These results of Splittgerber's are of considerable interest when 

 taken in conjunction with the work of several authors upon the 

 effect of heating on the calcium content of milk dealt with on 

 p. 231. It was there mentioned that the diminution in calcium 

 as a result of heating milk was believed to be due to a change from 

 mono- or bi-calcium phosphate to the fully saturated form, which last 

 becomes insoluble when heated. Splittgerber's work does not sug- 

 gest the occurrence of this change in the form of the citrate, although 

 this has been suggested by other writers. It is possible, however, 

 that the effect of heating in a complex solution such as milk may be 

 different from that which occurs when heated in aqueous solution. 



Bosworth and Prucha showed that citric acid disappears from 

 milk which has been allowed to go sour. 



The work above described does not suggest that the citric acid 

 content of cows' milk as a food for infants is of any great importance. 

 Such evidence as there is seems to show that even after boiling 

 the content is probably as high as in the infant's natural food 

 namely, human milk. Further, the same remarks apply u as for the 



