VARIOUS FERMENTATIONS 151 



bacteria, especially the " vinegar plant," Bacterium 

 aceticum, in the presence of oxygen, into acetic acid 

 and water : — 



ethyl alcohol acetic acid 



Their activity is the cause of the souring of wine or 

 beer if left standing in the air, and is used in the making 

 of vinegar. The " ripening " of cheeses, the " curing " 

 of tobacco, the " retting " of flax (the isolation of the 

 fibres of the flax plant when the stems are placed in 

 water), and many similar processes useful to man 

 depend on fermentations carried out by different 

 bacteria ; and so do many others which are not so 

 welcome, such as the turning rancid of butter and the 

 turning bitter of sweet fruit such as a cut melon, as well 

 as the putrefying of meat, etc. 



Putrefaction and Subsequent Changes in Nitrogenous 

 Organic Substances. — Putrefaction is the general name 

 given to various fermentations of the highly complex 

 proteins which form the basis of protoplasm. These 

 fermentations result in the formation of a great number 

 of compounds of varying degrees of complexity contain- 

 ing nitrogen and sulphur, among which are the evil- 

 smeUing gases so characteristic of the putrefactive 

 processes. Putrefaction is carried out by various 

 bacteria which are very widely distributed and many of 

 which form spores, so that practically no dead body 

 on the earth's surface can escape them. The process 

 takes place in all dead substances rich in proteins which 

 remain moist and not too hot or too cold to allow 

 of the activity of the putrefactive bacteria. Freezing 

 arrests their action, and so do the antiseptic substances 

 formed in peat bogs. That is why meat is kept in cold 



