120 BACTERIA AND THE CARBON DIOXIDE CYCLE 



alcoholic effervescent beverage that has been used for centuries in the 

 Caucasus, where it is usually made from mare's milk. With us cow's milk 

 is generally employed. The ' kephir grains ' with which it is prepared con- 

 tain both the bacterium and the yeast. The yeast cells secrete a peculiar 

 enzyme (lactase) that converts the milk sugar into grape sugar, which is then 

 fermented to alcohol and CO 2 . The lactic bacteria, besides giving the 

 pleasant acid taste to the drink, cause the precipitation of the casein in 

 a very finely flocculent, easily digestible form. Acetic and succinic acids 

 arise as by-products (91). 



In distillery operations (92) the development of butyric bacteria was 

 formerly much dreaded. In the preparation of sweet mash from green malt 

 the heating for two hours at 70 is naturally insufficient to kill the spores. 

 Experience showed, however, that a certain degree of acidity in the mash 

 prevented the development of butyric bacteria without injuring the yeast 

 cells, and careful investigation showed that the acidity was caused by the 

 lactic ferment. In practice the end is attained by inoculating some of 

 the sweet mash containing the yeast to be used with pure cultures of a lactic 

 bacterium, and keeping it at 50 C. (the optimum for these organisms) for 

 some time before adding it to the main body of the mash. At 50 C. the 

 lactic bacteria flourish and produce as much as i per cent, of lactic acid, and 

 the growth of the butyric organisms, whose optimum is 40 C., is inhibited 

 both by this temperature and by the lactic acid. 



This important application of lactic fermentation is now largely super- 

 seded by Efifront's hydrofluoric acid method. The yeasts are far less sen- 

 sitive to acids generally than the bacteria, and by continued culture with 

 increasing quantities of acid they may be accustomed to such large amounts 

 that the bacteria are reduced to a minimum, or even quite suppressed. 

 In a few months the alcohol yeasts may be accustomed to as much as 

 thirty milligrams of hydrofluoric acid per hectolitre mash, ten mg. sufficing 

 to kill the bacteria. Other poisons also have been tried ; formaldehyde, 

 for instance, which seems more efficacious even than hydrofluoric acid. 

 For electrical sterilization see p. 72. 



The spoiling and ' turning ' of beverages and articles of food through 

 lactic fermentation is a common occurrence. Beer must contain more than 

 7 per cent, alcohol if it is to be safe from the attacks of the lactic bacteria, 

 which, if they once get a foothold, soon cause it to ' turn,' making it turbid 

 and giving it a bad taste. Wine, too, is often spoiled by the same organisms, 

 and may contain as much as 2 per cent, lactic acid, derived from the fructose. 

 The acidity of wine is, however, more often caused by the acetic bacteria 

 than by the lactic. Boiled vegetables are not infrequently soured by both 

 lactic and butyric fermentation. 



The various methods of fodder preparation (93), brown hay, sour fodder, 

 and sweet ensilage, are fundamentally lactic fermentations by which the hay 



