CHAPTER XXXI ', ,j l I 9 R A R Y " ao] 



BACTERIA IN DAIRY PRODUCTS -^ 



L. A. ROGERS 

 Research Laboratories, Bureau of Dairy Industry- 

 United States Department of Agriculture, Washington, D.C. 



FERMENTED MILKS 



Primitive people in different parts of the world have developed various types of 

 fermented milks which have become an important part of their diet. The simplest 

 fermented milk is a result of the mild lactic fermentation produced by Streptococcus 

 lactis either spontaneously or by inoculation of pure cultures into pasteurized milk. 

 Nearly all of the ordinary commercial buttermilk is made by the latter method. 

 In selecting a culture for making buttermilk consideration should be given to the 

 fact that there are decided variations in the flavor and consistency given to milk as 

 well as in the rapidity of coagulation by different strains of St. lactis. An extreme case 

 of variation is found in the "taette milk," a stringy or ropy milk of the Scandina- 

 vian countries, produced by a variety of St. lactis which has been described under the 

 name St. taette. 



In Bulgaria, Turkey, and other near-eastern countries a fermented milk known 

 as yoghonrt is used very generally. This milk undergoes a lactic fermentation due, 

 primarily, to the acid-forming bacilli of which Lactobacillus bulgaricus is the type. 

 Bacteria of this genus in the fermentation of lactose produce lactic acid almost ex- 

 clusively while the streptococci form a number of acids in addition to lactic acid. The 

 lactobacilli ferment lactose rapidly, especially at temperatures of 35°-45° C, and 

 carry the fermentation to a pH of 3.8. Some strains produce a titrable acidity as high 

 as 2.5 per cent lactic acid. The tendency of the lactobacillus group to grow in chains 

 gives the milk a slightly slimy texture which disappears upon agitation. Pure cul- 

 tures of Lact. bulgaricus are used to prepare a sour skimmed milk which is sold as 

 "Bulgarian buttermilk" and under various trade names. The bulgaricus culture is 

 frequently used in combination with St. lactis to give a product with a milder acid 

 and a less viscous texture. 



The sour milk known as matzoon in Armenia and as leben in Egypt are evidently 

 similar to the yoghourt of Bulgaria and Turkey, although leben may contain a lactose- 

 fermenting yeast which gives a mild alcoholic fermentation. 



In Russia two fermented milks are made by using a combination of a lactobacil- 

 lus, a streptococcus, and a lactose-fermenting yeast. Kumiss is usually made from 

 mare's milk and is fermented in open vessels or skin bottles. Fresh milk is inoculated 

 from kumiss left in the bottles or by adding kumiss to fresh milk. In addition to the 

 acid fermentation, a small amount of alcohol is produced together with sufficient 

 carbon dioxide to make the milk slightly effervescent. Kefir differs from kumiss in 



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