DAIRYING 19 



and feeble. The same thing- is true of bacteria; the healthy, 

 vigorous germs develop quickly, and bring about the changes 

 in milk which are peculiar to them much more rapidly than is 

 the case with wilted and enfeebled bacteria. The garden plants 

 may be sorted over and the vigorous specimens selected for 

 transplanting, but there is no similar means of detecting the 

 hardy bacteria. The buttermaker must form his opinion of them 

 from the acidity and age of the starter, as well as the tempera- 

 ture at which it has been kept and the rapidity with which it 

 has soured. 



549. Acidity of the Starter. It is a well known fact that the 

 vigor of bacteria is diminished by an accumulation of their own 

 products, one of which in the case of milk and cream is lactic 

 acid. When the acidity of milk has reached 0.8 per cent, the 

 bacteria cease to multiply rapidly, and those present in such a 

 strongly acid liquid are greatly reduced in strength and vigor. 

 The starter is therefore supposed to be in its best condition for 

 use before the acidity has reached 0.8 per cent; about 0.6 per 

 cent acid is probably a more favorable stage of acidity than 0.8 

 per cent. This amount of acid (0.6 per cent) is often developed 

 before milk coagulates ; better results will therefore be obtained 

 by using the starter just before rather than after it has curdled 

 or wheyed off. 



Thus, as we see, the two things most needed in a starter 

 are, first, a large excess of some desirable kind of bacteria, and, 

 second, a vigorous and healthy growth of the bacteria which 

 have been selected. 



550. Amount of Starter to Use. No fixed rule can be given 

 for determining the quantity of starter that ought to be used in 

 each lot of cream. The condition of the milk and cream and of 

 the starter must be considered in estimating the amount needed. 

 A quick ripening of the cream is aided by a large starter and a 

 slow ripening is obtained by using a small quantity ; a thin cream 

 needs less starter than a thick cream, and the sourness of the 

 starter itself is of importance. In a general way it may be said 

 that the amount of starter to be recommended in the summer, 



