32 MYCOLOGY 



P'ermentation is well exemplified in an old and well-known process, 

 the conversion of alcohol into acetic acid by a number of organisms 

 morphologically very similar. Hansen considers that there are three 

 different species concerned in the acetic fermentation, namely, Bacterium 

 aceticum, B. Pasteurianus and B. KiUzingianus, which are non-motile, 

 medium-sized rods often in chains and forming pellicles which appear 

 on the surface of the liquid, afterward sinking to form in the liquid a 

 deposit known as mother of vinegar. The changes which take place 

 in the conversion of alcohol to acetic acid may be expressed as follows: 



CH3.CH2.OH + O = CH3.CHO -f H2O 



Alcohol Aldehyde 



CH3.CHO + O = CH3.COOH 



[Aldehyde Acetic Acid 



This is conducted in barrels with wood shavings, where the alcoholic 

 fluid trickling over the shavings coated with the bacteria, and in contact 

 with the air, is changed to acetic acid. 



Lactic acid fermentation is important to man, because upon the 

 changes in milk by the lactic acid organisms depends the manufacture 

 of a considerable number of valuable products of the dairy, such as 

 buttermilk and cheese. This fermentation is an aerobic process whose 

 optimum is found between 30° and 3S°C. There is a considerable 

 number of bacteria capable of converting milk sugar into lactic acid, 

 such as Vibrio cholera, Bacillus prodigiosus and others, but the true lactic 

 acid bacteria are those which are the cause of the souring of milk. 

 Formerly, they were all classed as Bacterium acidi lactici, but recent 

 investigations have shown that not one species but a considerable 

 number are at work, sometimes one form; sometimes another being 

 active. A common kind is a short non-motile rod, o.^^xX. i to 2/^, 

 facultatively anaerobic, known by such names as Bacterium acidi lactici, 

 B. aerogenes, and probably comprising several races of one species. 

 The true lactic acid fermentation is the change of lactose, or milk 

 sugar, into lactic acid. As lactose is not directly fermentable it must 

 "be converted into such simple sugars as glucose and galactose. The 

 following equation approximately represents the chemic change 

 involved. 



C12H02O11 + H2O = CeHi.Ofi -f CeHisOe 



Lactose Water Glucose Galactose 



C6Hi20r. = 2C3Hfi03 



Lactic Acid 



