110 THE COAGULATION OF CONDENSED MILK, 



A tin of Pestle's milk was heated at 60^ for two hours, then 

 the lid was punctured and the milk infected with a blob from a 

 coagulating milk. The small orifice was sealed with |)arafiin and 

 the tin was incubated for a month at 37'^. Upon opening the 

 tin it was found that the milk in the neighbourhood of the 

 orifice had become coagulated. The infecting blob had remained 

 at the point of infection and had not mixed with the bulk of the 

 milk. 



Another experiment was made with a tin of pasteurised 

 Nestle's milk, but in this case the milk was infected by means of 

 small capillary tubes containing pure cultures of the micrococcus; 

 the thin tubes were pushed right into the milk and were then 

 broken off. The small hole in the tin was then sealed with 

 paraffin as before. Upon opening the tin a month later, the 

 contents were found to be very stiff and lumpy, a signal evidence 

 of the action of the micrococcus. A control-tin of milk which 

 had been opened and sealed at the same time was unaltered. 



It is clear from these experiments that it is the micrococcus 

 which is responsible for the coagulation of the condensed milk. 



The causative micrococcus is probably by no means rare, and^ 

 as I have separated it from a sample of Nestle's milk which is 

 prepared in Switzerland or Norway, it would appear to be of 

 universal occurrence. The Nestle's coccus was identical morpho- 

 logically and culturally, and it produced the same characteristic 

 coagulation and lumpiness in test-flasks. 



Since the coccus is found in milks which keep perfectly, there 

 must be some condition which is necessary in order that the 

 change may occur. The most feasible is the presence of a quan- 

 tity of air in the tins. The coccus is aerobic, and in the experi- 

 mental flasks the thickening begins where the film of milk is 

 thinnest and most completely aerated, that is, at the margin of 

 the surface. In the tins in which the affected milk was con- 

 tained, there was a considerable space filled with' air, while in 

 the tins of Nestle's milk the air-spaces were very small. In the 

 latter case it is possible that the gas is inert, and, if so, the con- 

 dition for the growth of the coccus would be so unfavourable 

 that no alteration would occur. 



