512 J. HOWARD BROWN AND PAUL E. HOWE 



of oxalate or sulphate results in the formation of insoluble cal- 

 cium oxalate or sulphate which may be removed by centrifuga- 

 tion. By the addition of sodium citrate, however, the calcium 

 of milk is not precipitated (Arthus, 1902), but remains in solu- 

 tion in a non-effective form in so far as its ability to combine 

 with casein or to react with the para-casein formed as a result 

 of the action of rennin are concerned. We have not found an 

 adequate explanation of the nature of the calcium-citrate com- 

 bination. That it is in solution is indicated by the work of 

 Arthus and of Bosworth and Van Slyke. We ha\-e evidence of 

 this in the fact that durinp; each sterilization there is formed a 

 heavy precipitate of a calcium salt which we assume to be cal- 

 cium citrate. It has the property of calcium citrate of being 

 precipitated when heated and redissolving upon cooling. On 

 the other hand, the calcium in solution in the clarified milk must 

 be sufficient to form the insoluble para-casein compound for 

 there is enough calcium in milk to do so and in the preparation 

 of transparent citrated milk none need be removed. Sabbatani 

 (1902) states that the effect of the citrate in blood is due to a re- 

 duction in the number of calcium ions in solution. He also 

 shows that the ratio of citrate to calcium for the prevention of 

 coagulation is three to one. Arthus holds that there is a specific 

 effect of the citrate ion which inhibits flocculation, in addition 

 to any changes which may take place in solubility. 



In the preparation of transparent milk as a bacteriological 

 medium we have diluted 1 part of skim milk with 2 parts of 

 distilled water and then added 0.4 per cent of sodium citrate. 

 After standing for about an liour the clarified milk may be filtered 

 through paper though this is hardly necessary if the mixture is 

 allowed to stand sufficiently long. To avoid caramelization of 

 the milk sugar during sterilization the reaction of the medium is 

 adjusted to about pH (i.S. The medium is then tubed and steri- 

 lized fractionally. During each steaming in the Arnold steri- 

 lizer a heavy precipitate is thrown down l)ut redissolves as the 

 medium cools. There finally results an almost water-clear me- 

 dium without any precipitate whatever. Oxalated milk may be 

 prepared in the same maimer excei)t that the fine precipitate of 



