88 DAIRY BACTERIOLOGY 



been formed in the milk the concentration of soluble calcium salts 

 is further increased. The worst conditions occur if rennet-forming 

 bacteria are present in the milk in addition to acid-forming bacteria, 

 as the amount of calcium necessary to precipitate the paracasein 

 is only half that necessary to precipitate the casein. It is 

 therefore obvious that only milk of the very best quality can be 

 used for condensing, and such milk is of course also the easiest to 

 sterilise with satisfactory results. According to Hunziker's investi- 

 gations l , the bacteria which are found in incompletely sterilised 

 evaporated milk are not so often hay and potato bacilli as a 

 slender peptonising, non-acid-forming, non-sporing rod bacterium. 

 This organism presumably belongs to the group of microbacteria 

 which the author points out as being able to stand comparatively 

 high temperatures for non-sporing bacteria. If the tins of evaporated 

 milk bulge out, this is always due to anaerobic spore formers, i.e., 

 butyric acid bacilli as well as anaerobic putrefactive bacteria. In 

 leaky tins in which the milk has been infected after sterilisation 

 one may, of course, find lactic acid bacteria as well as many other 

 stray organisms. In order that the bad tins may be picked out, 

 the manufacturers should always keep the condensed milk for a 

 short time before sending it out. Fermentation in sweetened 

 milk will always show itself in the course of about a week at 

 20 to 25 C., while the " incubation " time for sterilised un- 

 sweetened milk often lasts two to three weeks, even at a 

 temperature of 30 C. 



Milk is dried on drums in vacuo (Eckenberg's method), on drums 

 heated to 140 C. (Hatmaker's method), or by finely dividing it 

 and causing it to meet a current of air at 120 C., whereby it is 

 instantaneously converted into powder (spray process). Of these 

 methods, Hatmaker's is the easiest and cheapest, but the powder 

 which is _ produced is difficultly soluble even if alkali has been 

 added. The dried milk made by the spray process is far better, 

 being completely soluble when freshly prepared. This is due to 

 the fact that no denaturisation of the proteins takes place, as the 

 current of warm air is cooled right down to 60 C. immediately on 

 meeting the sprayed milk, owing to heat being absorbed in the 

 process of evaporation. All the methods can of course be com- 

 bined with an initial condensation of the milk. Only dried milk 

 made by Hatmaker's process is sterile. The dried milk made by 

 the other processes can of course be made more or less free from 

 fijerms by pasteurising the milk more or less thoroughly at the 



1 Otto F. Hunziker, " Condensed Milk and Milk Powder," Illinois, 1920. 

 This work, which is based on wide experience, is strongly recommended to 

 any one wishing to obtain information on these special branches of dairy 

 practice. See also C. Porcher, " Le Lait Desseche," Lyon, 1912. 

 Porcher strongly recommends dried milk for infant feeding. 



