XIV.] CHURNING 283 



in order to aerate it. Unless the cream is kept aerated 

 the bacteria making lactic acid may give place to others 

 producing butyric acid, to the great detriment of the 

 flavour of the resulting butter. When the cream is 

 properly ripe it should be brought to an appropriate 

 temperature before churning, this temperature being 

 about 60° in the v^^inter and 55° in the summer. The 

 length of time occupied by churning will depend upon 

 getting a proper proportion of cream and water and a 

 correct adjustment of temperature. When the churning 

 begins, it is necessary with sour milk in a closed churn 

 to open the ventilator in the churn from time to time 

 during the first five minutes of churning. The agitation 

 of the cream liberates a certain amount of carbon 

 dioxide which had been formed by the lactic acid 

 bacteria and was dissolved in a supersaturated condition 

 in the serum of the cream, and unless this carbon 

 dioxide is liberated from the churn the whole of the 

 cream will pass into a frothy or whipped condition. As 

 soon as the granules of butter have reached the size of 

 small shot the process is nowadays stopped, and the 

 butter is washed to free it from all adhering milk serum 

 before it is worked up into pats. Unless this washing 

 is thorough the butter will not keep, because the small 

 amount of casein and milk sugar left in when the 

 washing is imperfect provides a very favourable nutrient 

 medium for the development of bacteria, which will split 

 up the butter fat and set free some of the disagreeably 

 flavoured fatty acids. The resulting butter is not pure 

 butter fat but contains a proportion of water depending 

 on the velocity and the temperature of the churning, 

 and also upon the method of working the butter after- 

 wards. Rapid churning at high or at very low tempera- 

 tures will give rise to a number of very finely divided 

 globules of water inside the butter fat, and these cannot 



