396 BACTERIA IN DAIRY PRODUCTS 



that the milk is inoculated with a curious growth known as "kefir grains." These 

 grains resemble miniature cauliflowers and when broken up and put into milk under 

 favorable conditions increase in size, at the same time inoculating the milk with strep- 

 tococci, lactobacilli, and yeasts. The grains are masses of tangled filaments of these 

 organisms. The fermentation is similar to that of kumiss but is usually produced in 

 closed bottles so that the gas is held and the finished product is more effervescent. 

 A similar product can be made by adding about i per cent of cane sugar to sour milk, 

 inoculating with a bread yeast and fermenting several days in sealed bottles at 

 20-25 c. 



Great stress has been laid on the therapeutic value of fermented milks, particu- 

 larly those soured by Lact. bulgaricus, to which the name "bacillus of long life" has 

 been applied. More recent work has shown that such therapeutic efficacy has been 

 over-estimated, due to a misconception of the ability of bulgaricus to establish and 

 maintain itself in the intestines. 



Lact. acidophilus is now used extensively in preparing soured milk for therapeutic 

 purposes. Because this organism is much less active than bulgaricus, special precau- 

 tions are necessary to prepare a milk free from contamination. 



CONDENSED MILK 



The commercial term "condensed milk" is applied to milk preserved by the ad- 

 dition of cane sugar with subsequent evaporation to a little less than one-half of its 

 original volume. After condensing, the cane-sugar concentration is between 40 and 

 45 per cent, giving sufficient osmotic pressure to inhibit the growth of most ferment- 

 ative bacteria. Owing to the fact that in 100 gm. of condensed milk the total 

 amount of water in which 45 gm. of saccharose must be dissolved is less than 30 gm., 

 of which a part is rendered unavailable by combination with other constituents, the 

 sugar concentration is much higher than is indicated by the percentage composition. 



Not all bacterial growth is inhibited at this concentration. Some multipHcation 

 of bacteria usually occurs and may continue for a few weeks. If the saccharose con- 

 tent is relatively low, a coccus appears which ferments the sugar, thickening the milk 

 and giving it an unpleasant flavor.' Such thickening should not be confused with 

 the thickening which frequently takes place spontaneously and is not accompanied 

 by undesirable flavor changes. 



The concentration of cane sugar is not high enough to inhibit entirely the growth 

 of yeasts. It is not unusual to find growing yeasts in normal milk, but it is only oc- 

 casionally that the multiplication is sufficient to produce an appreciable amount of 

 gas. In normal milk the conditions become sufficiently anaerobic in a few days to 

 prevent further multiplication, and it is only when the infection is unusually heavy 

 that enough cells are present to continue an anaerobic fermentation with gas produc- 

 tion and spoilage of the product.^ 



EVAPORATED MILK 



"Evaporated milk" is the commercial term applied to milk concentrated under 

 vacuum to a little less than one-half of its original volume and preserved by heat 



' Rice, F. E., and Downs, P. A.: /. Dairy Sc, 6, 532. 1923. 



» Savage, W. G., and Hunwicke, R. F.: {Great Britain) Dcpt. Sc. b" Indust. Research, Food 

 Invest. Bd., Spec. Rep. ij. 1923. 



