THE KINDS OF MICRO-ORGANISMS IN MILK 361 



And furthermore, with the increase in acid the growth of Bacillus 

 coli and B. aerogenes is restrained and gas formation decreases. 

 The amount of gas in a curd is, therefore, not always an indica- 

 tion of the number of the coli-aerogenes group present in the milk. 



When, as Hammer has shown. Streptococcus lacticus and 

 either Bacillus coH or B. aerogenes are inoculated into milk at the 

 same time, a curd is formed rapidly by the acid produced by 

 Streptococcus lacticus, and consequently a large amount of gas is 

 retained. This phenomenon is more prominent when growth takes 

 place at 37° C. than at room temperature. 



Different strains of Bacillus coli and B. aerogenes vary con- 

 siderably in their power to produce gas, and also in other proper- 

 ties. This has been shown by Harrison, who studied 66 varieties 

 isolated from milk, and found many modifications which seemed 

 to link the extremes together. Hammer has made the interesting 

 observation that members of the coli-aerogenes group when freshly 

 isolated from milk produce more gas than when kept for some time 

 (eight months) on laboratory media. After this time they pro- 

 duced about as much gas as strains from sources other than 

 milk. It seems, therefore, that when gassy curds are formed the 

 cause may not always be looked for in large numbers of the coli- 

 aerogenes group, but rather in the presence of strains that have 

 acquired the property of producing much gas and have become 

 highly acid resistant because of their growing together with 

 Streptococcus lacticus. 



The desirable curd for making cheese is the one produced by 

 Streptococcus lacticus, and when this organism is present it re- 

 strains the coli-aerogenes group, as a rule, so that a suitable curd 

 is formed. This is especially true at temperatures below body 

 temperature. However, Doane and Eldredge have stated that 

 cultures of Bacillus bulgaricus are more suitable for suppressing 

 gas formation in Emmenthaler cheese than Streptococcus lacticus. 



The Wisconsin curd test, worked out by Babcock, Russell, 

 and Decker, gives the cheese maker a simple method of testing 

 the quality of the milk for this particular purpose, and is per- 

 formed as follows: The milk to be tested is placed in sterile pint 

 jars and warmed to 90° F. Then 10 drops of rennet extract are 

 added and the curd cut into pieces with a knife. After the curd 

 has settled the whey is poured off. Then the jars are kept at 

 100° to 105° F. After twelve hours the curd is examined. If it 

 is of soUd, firm texture and the odor agreeable, the milk is suitable 

 for cheese making. If the curd is full of gas-bubbles, is of spongy 

 consistency, and has an unpleasant odor, it indicates milk of poor 

 quahty for cheese making. By this simple and useful test the 

 cheese maker can easily trace any trouble to the proper source 



