200 BACTERIA IN BUTTER AND OLEOMARGARINE 
furnishing it. But though he cannot control this factor, he can, 
more or less satisfactorily, regulate the growth of the bacteria. 
Temperature of Ripening—At a temperature of from 65° to 
“o- the favorable lactic acid bacteria get the upper hand of other 
species more readily than at either a higher or a lower temperature. 
At temperatures above or below this, different species, mostly 
unfavorable, are more likely to gain the upper hand. Hence, by 
keeping the temperature at about 65°, the undue development of 
mischievious bacteria is more likely to be prevented. 
Duration of Ripening.—The butter-maker can stop the ripen- 
ing at any point, for, after the cream is churned into butter, the 
bacterial growth ceases. The necessary duration of the ripen- 
ing will vary, however, with the conditions. Sometimes cream, 
when brought to a creamery, is already sour and has, therefore, 
become ripened even before the butter-maker receivesit. In other 
cases, especially in winter, it will not only be sweet, but will con- 
tain small numbers of bacteria and require a much longer ripening. 
Moreover, milk produced under good dairy conditions, clean and 
fairly free from bacteria, will ordinarily require longer ripening 
than milk produced under less favorable conditions and containing 
already great numbers of bacteria. The length of time will vary 
also with the temperature, being, of course, longer at lower tem- 
peratures. To determine when the cream is sufficiently ripened, 
the butter-maker has two methods. One is the general appear- 
ance to his eye and taste, and the other is the degree of acidity. 
The latter factor is determined by the methods described on page 
333, and the ripening is generally continued until the acidity is 
0.5 to 0.65 per cent. 
THE USE OF STARTERS 
By far the most important change in the methods of cream- 
ripening is in the wide and almost universal introduction of 
starters. ‘Twenty-five years ago it was sometimes customary to 
add a starter to cream in cold weather simply for the purpose of 
