322 THE CHEMISTRY OF DAIRYING 



contaminated with bacteria to the same degree; and as the 

 temperature is kept so much lower throughout the whole time of 

 standing, the bacteria do not multiply so rapidly. The amount 

 of lactic acid formed is, therefore, much less, 'and the motion 

 of the fat globules is so much faster that it more than com- 

 pensates for the greater depth through which they have to 

 rise. 



In the method of shallow setting, as a rule, not more than 

 about 80 per cent, of the original fat of the milk is recovered 

 as cream. In the methods of deep setting, over 90 per cent, 

 is recovered. Both of these methods have now been to a 

 large extent superseded by the centrifugal separator, by means 

 of which over 98 per cent, of the total fat in the milk may be 

 recovered. 



The composition of cream is extremely variable, and it is 

 impossible to strike any general average. Some samples 

 contain less than 10 per cent, of fat, i.e. less than has been 

 found in natural milk, and others contain nearly 70 per cent. 

 The ordinary limits of variation are from about 15 to 50 per 

 cent, of fat. Samples which contain less than about 20 per 

 cent, are generally described as thin cream, and those which 

 contain over 30 per cent, are described as thick cream. When 

 cream is produced by the mechanical separator, the percentage 

 of fat can be regulated almost at will. When it is obtained 

 by skimming, the percentage of fat depends largely upon the 

 temperature, method of setting, etc., and the conditions are 

 not so easily controlled. Separated cream is, of course, 

 perfectly sweet. So also, as a rule, is that obtained by deep 

 setting ; but the cream produced by shallow setting often has 

 an appreciable degree of acidity. 



The relative proportions of protein, milk sugar, and ash 

 in cream are practically the same as in milk. It is often 

 asserted that the ratio of total non-fatty solids to water is 

 higher in cream than in the milk from which it was produced ; 

 but when such is found to be the case it can generally be 

 accounted for by evaporation. 



Skim Milk. Inasmuch as the mechanical separator is 

 more effective than any method of setting and skimming, it 



