Tlte Viscosity of Cream. 313 



From these figuies it appears that, fur fresh cream, soured hy 

 additions of pure hictic acid, the value of the critical acidity 

 (expressed in the number of c.c. of XaOH IS/ 10 required to 

 neutralise the acid contained in the cream) is between 5G and 

 57 cc. N/10 per 100 cc. of cream serum; but if the souring bo 

 eflfected by means of a pure lactic culture, the critical acidity is 

 considerably greater, approaching 65 cc. N/10 per 100 cc. of 

 serum, or possibly even higher. 



It is, however, difficult to ol)tain the exact value for the critical 

 acidity in the latter case, owing, firstly, to lack of evidence as to 

 the probable effect of such factors as the age of the culture employed, 

 and the rate at which it produces lactic acid in the cream, and 

 secondly, to inability to calculate the interval of time required 

 for the culture to bring the acidity up to the critical j^oint, as it 

 is found that souring produced by this means is at first slow, but 

 at a certain stage the rate of acidification is greatly accelerated. 



Effect of Previous Heating on Critical Acidity of Cream. — It is 

 worthy of note also that previous heating of the cream may modify 

 the results of the experiments, and give a different value for the 

 critical acidity. Cream, wliich has been pasteurised after separation 

 from fresh milk, if acidified with pure lactic acid, has a critical 

 acidity slightly higher than fresh cream so treated. 



But if the pasteurised cream is soured by introduction of a pure 

 lactic culture, the viscosity rises gradually with increase of acidity, 

 so that instead of a sudden raj^id rise at the critical point, there is 

 a steady increase of viscosity with increased acidity over a certain 

 range, a pronounced rise of viscosity being attained at the degree 

 of acidity which corresponds to the critical acidity in the cases 

 previously described. 



A few experiments made with cream from milk kept at a high ■ 

 temperature for some little time are interesting in that they show 

 a difference in the behaviour of the cream so obtainc(\ on the addi. 

 tion of acid. 



The fresh milk was gradually heated under pressure in an auto- 

 clave until the indicator registered 105 deg. C. The milk wa."? 

 kept at this temperature for al)Out 20 minutes, then removed from 

 the autoclave and allowed to cool. The skin which had formed 

 on the top of the milk was skimmed off, and the milk separated. 

 The critical acidity of the cream thus obtained was found to be 

 considerably lower than that of fresh cream or pasteurised cream. 



When the milk, after heating, was cooled rapidly by being made 

 to pass through a condenser, around which circulated a stream 



17 



