xx PRESERVATION OF MILK 391 



If hydrogen peroxide is left in appreciable amount in the 

 milk, it imparts an unpleasant bitter taste, although it is 

 probably harmless in the small quantity present. Another 

 objection is that commercial solutions of hydrogen peroxide 

 are not always free from arsenic or other impurities, and these 

 substances may in this way be added to milk. 



The process does not appear to be extensively used in this 

 country, and if the contention of Bosenau is well founded that 

 heating to 60 C. for 20 minutes is sufficient to kill all the 

 pathogenic bacteria likely to be present in milk, it is decidedly 

 preferable to heat to 60 C. for 20 minutes than to heat to 

 50 C. for several hours and also add hydrogen peroxide. A 

 temperature of 60 C. is not more detrimental to the digesti- 

 bility of milk or the contained ferments than is one of 50 C. 



V. CONDENSATION AND DRYING 



Under this head may be mentioned two special methods, 

 designed for the preservation of milk, the resulting substances 

 being known respectively as condensed milk and dried milk. 



Condensed Milk. Condensed milk is prepared by concen- 

 trating either whole or separated milk, with or without the 

 addition of sugar. Theoretically, therefore, there are four 

 kinds of condensed milk, but the writer is not aware of any 

 unsweetened separated brands. The four kinds are : 

 (a) Sweetened condensed whole milk. 

 (&) Unsweetened condensed whole milk. 



(c) Sweetened condensed separated milk. 



(d) Unsweetened condensed separated milk. 



The condensation of the milk is usually carried out by 

 evaporating the milk in vacuum jars under reduced pressure 

 until sufficiently concentrated. In the sweetened varieties 

 cane sugar is added before concentration. The separated milk 

 variety usually contains more sugar (containing often as much 

 as 40 to 45 per cent) than the whole milk. The milk is sold 

 in hermetically sealed tins. In the unsweetened brands these, 

 after filling but before sealing, are heated to a temperature of 

 over 100 C. for some time. 



It has been shown in Chapter XII. that while bacteria are 

 present in condensed milk, the number present is relatively 



