Fermentations of Milk. 63 



The butter or cheese maker, receiving milk from a num- 

 ber of farms, often finds it necessary to trace to its source 

 on the farm the abnormal fermentation causing him trou- 

 ble and loss. The test most often used for this purpose 

 is designed especially to detect the most frequent of the 

 troublesome fermentations of milk, and is known as the 

 Wisconsin curd test. The sample to be tested must be 

 carefully collected in fruit jars which have been ster- 

 ilized by boiling, in water. The various samples must 

 not be taken by the use of a dipper unless it is sterilized 

 before each sample is taken. If this is not done, the bac- 

 teria remaining on the dipper may be sufficient to change 

 the result in any sample from what it would have been, 

 in case the sample had been taken in such a way as to 

 avoid such contamination. The best way to take the 

 sample is to fill the jar by pouring directly from the 

 can of milk to be tested. 



The milk is warmed to 98 F., ten drops of rennet 

 added, and as soon as a firm curd is formed, it is cut into 

 small pieces by means of a sterile table knife. The cut- 

 ting of the curd allows the whey to be expressed. As it 

 collects it is turned off until the small pat of curd is quite 

 dry. The jars are kept at 98-105 F. for ten to twelve 

 hours. When the curd forms most of the bacteria in the 

 milk are caught in the curd, the shrinking of the same 

 concentrating the bacteria into about one-tenth of the vol- 

 ume of the milk. They are held in place as in the plate 

 cultures described in Chapter II. They multiply rapidly, 

 forming colonies in the curd. The jars are kept at high 

 temperatures in order to favor the growth of the unde- 

 sirable bacteria. After ten to twelve hours the curds are 

 examined as to their texture, flavor, and odor. A curd 

 which presents an agreeable acid odor, a close texture, 



