BACTERIA IN THE INDUSTRIES 191 



typhoid bacillus. It sometimes withstands heating at 60 C. for five 

 minutes. All are killed at 60 C. for ten minutes. 



So far as can be judged from the meager evidence at hand, 60 C. for 

 twenty minutes is more than sufficient to destroy the infective principle of 

 Malta fever in milk. M. melitensis is not killed at 55 C. for a short time; 

 the great majority die at 58 C., and at 60 C. all are killed. 



Milk heated to 60 C. and maintained at that temperature for twenty 

 minutes may, therefore, be considered safe so far as conveying infection 

 with the microorganisms tested is concerned. 



Evaporated, condensed and dry milk are found upon the market and 

 are extensively used. Sugar is frequently added as a preservative. In 

 making condensed milk, it is evaporated in large pans until it assumes a 

 creamy consistency. Dry milk is prepared by spraying the milk on revolv- 

 ing hot cylinders. The thin film of milk is evaporated to dryness in a 

 moment, and in that state is scraped from the cylinders. Dry milk is a 

 common ingredient of baby foods and invalid foods, and is also very 

 extensively used in the manufacture of chocolate creams. The condensed 

 and dry milks do not keep long in spite of the greatest care in manufacture. 

 The containers and milk must be thoroughly sterilized or pasteurized, 

 and the cans must not be opened until ready for use. Such preservatives 

 as salicylic and boric acid are sometimes added to condensed milk. 



It is known that sweet cream yields a very insipid, flavorless butter, 

 whereas cream which has "soured" for a few days yields a pleasant tasting 

 and pleasingly flavored butter, provided the desirable species or variety of 

 bacteria are present. If the souring is continued too long the flavor may 

 be hopelessly vitiated. In the past it was customary to add a smaU amount 

 of old cream, having a desirable flavor, to a new lot of cream. This mother 

 cream was designated the ''starter." It contained the desirable cream- 

 ripening bacteria, mostly of the lactic acid variety. These old-time natural 

 starters are now largely replaced by starters, prepared in the laboratory 

 consisting of pure cultures of certain strains or varieties of cream flavor, 

 producing germs of the lactic acid group. A proper regulation of the 

 temperature is very important in the ripening of cream (60 to 75 F.). 

 It is also necessary to pasteurize the cream before adding the bacterial 

 starter in order to prevent the development of microbes which might 

 interfere with the proper development of the starter microbes. Naturally 

 the use of clean, sterilized utensils and uniformity of methods are all- 

 important, in order that uniform results may be obtained. 



Cheese flavors are also due to bacterial action, but not wholly so, 

 as many of the higher fungi, as species of Penicittium (Camembert Penicil- 

 lium) and of Oidium (0. lactis) also play a very important part as flavor 

 producers. The Roquefort cheese owes its characteristic flavor, in 



