604 MILK 



laboratory cultures of Str. lacticus, and these cultures are pur- 

 chasable for a small price. Butter makers have fresh cultures 

 mailed to them at regular intervals, since it is diflScult to preserve 

 a pure culture indefinitely under conamercial conditions. Most 

 commercial starters are really pure cultures, while some contain 

 contaminating organisms. They are sold either in hquid or 

 tablet form. While liquid starters are more active when fresh 

 than dry starters, the dry ones preserve their viabihty for a longer 

 period than do the liquid starters. 



To prepare a starter from a commercial culture requires care 

 and judgment. Other bacteria must be kept out, otherwise the 

 result will be a failure. The first step is the preparation of the 

 "mother starter." A quart milk bottle is cleaned and filled. about 

 two-thirds full with a good quahty of milk or skimmed milk. A 

 cotton swab attached to a piece of heavy copper wire (Bushnell 

 and Wright) or a silver-plated teaspoon with a piece of wire 

 soldered to the handle (Hastings) is inserted and the bottle then 

 closed with a cotton plug or an inverted tumbler, or both. The 

 milk is then steriUzed in steam for thirty or forty minutes. One 

 sterilization does not destroy all bacteria, of course, so some pre- 

 fer to apply steam for three successive days, although this is not 

 necessary. After the milk is cooled the culture is added, mixed 

 with the milk, and then incubated at 65° to 85° F. After eigh- 

 teen to twenty-four hours the mother starter is ready. The curd 

 should be smooth, without gas holes, and practically free from 

 whey. The starter should be examined for abnormal appearance, 

 taste, and odor. The acid present should be calculated by the 

 use of Farrington's alkah tablets. In order to observe the phys- 

 ical condition it is advisable to use a glass vessel. 



The incubation temperature can be maintained, according to 

 Hastings, by placing the bottle or bottles in an insulated box, 

 such as a fireless cooker, and keeping the temperature constant 

 by means of a pail filled with water at 80° to 90° F. 



The largest number of lactic acid bacteria are present when 

 the milk curdles at 70° F. and has an acidity of 0.6 to 0.7 per cent. 

 If the amount of acid is less, there are fewer bacteria, and if it is 

 greater, the number also decreases, owing to the destructive effect 

 of the acid. 



Fresh cultures frequently do not produce the desired effect, 

 and a fresh bottle of milk should then be inoculated from the 

 first one. This may have to be repeated several times before the 

 starter has gained proper vigor. 



If the mother starter is not intended for immediate use, it 

 should be placed on ice to prevent the souring process from going 

 too far. 



