86 DAIRY BACTERIOLOGY 



advantage of in the Swiss cheese making industry for many 

 centuries. Pure cultures are obtained in the acid vats by filling 

 up with warm " Schotte " (boiled clarified whey) so that the 

 temperature rises to about 60 C. The thermobacteria are 

 of no great interest in connection with pasteurised milk, as 

 they develop very slowly in it unless kept at temperatures 

 over 35 C. 



According to W eigmann's l investigations on low temperature 

 pasteurisation, practically the whole of the bacterial reduction 

 takes place during the first ten minutes, the remaining 1 or J per 

 cent, is halved during the second ten minutes, while practically 

 nothing happens during the last ten minutes. It is, however, 

 necessary to continue for the last ten minutes if the milk is 

 pasteurised in bottles, as it is considerably colder near the 

 bottom than at the top. Ayers and Johnson have even continued 

 heating for six hours without having observed any further bacterial 

 reduction. It follows that the few bacteria which are not killed 

 during the first ten to twenty minutes cannot be got rid of without 

 raising the temperature, so that there is nothing to be gained by 

 lengthening the time beyond the prescribed thirty minutes. 

 According to the author's investigations, the advantages of pro- 

 longed low temperature pasteurisation would be illusory if only 

 for the reason that prolonged heating (e.g., for five hours), even to as 

 low a temperature as 60 C., causes considerable chemical changes 

 in the milk, and, what is still worse, the development of the thermo- 

 philic putrefactive bacteria will be favoured by keeping the milk 

 for so long a time between 60 and 70 C. 



The concentration of milk consists either in condensing in vacuo 

 or complete evaporation to dried milk. 



Distinction is made between sweetened and unsweetened con- 

 densed milk, the latter also being called evaporated, milk. Both 

 are as a rule sold in hermetically-sealed tins, but the sweetened 

 milk may also be shipped in cans or barrels. As the condensation 

 is carried out at 50 to 60 C., the multiplication of bacteria is by 

 no means excluded during this process, and the milk should 

 therefore previously be freed from as many germs as possible by 

 heating to the neighbourhood of 100 C. 



Fourteen to sixteen per cent, of cane sugar is added to the milk 

 which is to be sweetened, and the product is evaporated to a third 

 of its bulk, cooled quickly and stirred meanwhile so as to prevent 



1 Weigmann, Wolff, Trensch and Steffen have confirmed Ayers' and 

 Johnson's results, and also shown that the proportion of lactic acid to other 

 bacteria is higher in stable milk than in pasture milk after low tempera- 

 ture pasteurisation, because the former is richer in lactic acid bacteria 

 than the latter (" Centralblatt f. Bakteriologie," 2 Abt., 1916, Bd. XLV., 

 p. 63). 



