PRESERVATION OF MILK AND ITS TREATMENT 87 



the formation of large crystals of the difficultly soluble milk sugar, 

 and finally filled into steamed (preferably sterile) vessels. Milk 

 which has been treated in this manner is by no means sterile, but 

 the high sugar content inhibits the development of microorganisms, 

 as in jam. Quite a number of orange and white micrococci can, 

 however, always be found in the milk, and occasionally also 

 yeast, red as well as white Torulse, of which latter a few ferment 

 saccharose even in these high concentrations, and, therefore, can 

 make the tins bulge out 1 . If the milk has not been thoroughly 

 aerated after it has been in the vacuum pans, it is only in the top 

 end of the tins that alcohol is formed. In this end moulds may 

 develop in addition to yeasts, but their vitality is cut short as soon 

 as all the oxygen present in the tin has been used up. This stage, 

 however, need not necessarily mark the cessation of the harmful 

 effect of the moulds, as the proteolytic enzymes contained in the 

 moulds, after having digested the mycelium itself, may continue to 

 act on the surrounding medium ; Rogers, Dahlberg and Evans have 

 thus shown 2 that certain reddish brown lumps which are now and 

 again found in old condensed milk, and the origin of which has 

 hitherto been a mystery because no cells were found therein, are 

 formed by Aspergillus repens in the way described above. 



As the unsweetened condensed milk is not so viscous, it must 

 be homogenised to prevent the separation of cream ; it must be 

 sterilised after having been tinned, in order that it may keep. As 

 highly concentrated milk coagulates to a gelatinous mass at the 

 high temperatures used in sterilisation, the unsweetened milk 

 cannot be evaporated so far as that which has been sweetened ; 

 the author was the first to show 3 that the soluble calcium salts 

 present in the milk are the cause of this phenomenon, their con- 

 centration naturally increasing with that of the milk ; if acid has 



1 Hammer has named this yeast Torula lactis condensi (Iowa Agric. 

 Exp. Station, 1919, Bull. 54, pp. 211 220. According to the author's 

 researches, the micrococci as well as the yeasts come in the majority of 

 cases from the sugar which is used, it is therefore very necessary that the 

 sugar should be heated to a sufficiently high temperature after having been 

 dissolved. The large amount oi ! cane sugar in condensed milk inhibits the 

 development of the ordinary cane-sugar-fermenting Saccharomycetes ; only 

 Zygosaccharomycetes (i.e., certain sexually differentiated Saccharomycetes) 

 will be able to grow in the concentration of cane sugar in question, but these 

 are, so far as the author is aware, never found in milk products. 

 " Journal of Dairy Science," 1920, vol. 3, p. 122. 



3 Hoppe Seylefs " Zeitschrift f. physiol. Chemie," 1914, Bd.. 93, pp. 299, 

 300. In this work it is shown that the coagulation of faintly acid milk on 

 boiling, which is a well-known phenomenon in cookery, is not directly due 

 to the slight amount of acid, but to the soluble lime salts formed by it. 

 The conditions causing the coagulation of normal (not acid and not con- 

 dciised) milk on -sterilisation at 130 to 140 C. are more complicated, as in 

 this case a proteolytic decomposition also takes place. For further informa- 

 tion regarding these conditions, see the author's work in " Landwirtschaft- 

 liches Jahrbuch der Schweiz," 1905, p. 235. 



