78 THE MECHANISM OF LIFE 



Other micro-organisms have, on the whole, beneficial effects. 

 Yeasts are the causes of the fermentations in brewing and analo- 

 gous processes. Moulds and bacteria cause the ripening of 

 cheese. Fermentative and putrefactive bacteria act upon and 

 break down organic matter, and moulds and infusoria behave in 

 similar ways. 



Many substances that we meet with every day "go bad": 

 milk curdles; broths, soups, etc., "go sour"; and meat of all 

 kinds becomes tainted and then putrefies. Beers and wines may 

 sour. In the case of meat and fish, the " ripening " (as in the 

 case of game) and the tainting are the result of the activity of 

 bacteria and of enzymes that are naturally present in the flesh 

 itself. When the flesh is alive, the activity of these enzymes are 

 inhibited in some way, but after general death of the tissues they 

 begin to digest the flesh in which they occur. This process is 

 called autolysis, or self -digestion. At first it produces savoury, 

 and then noxious substances. 



By-and-by the meat putrefies, with the production of offensive 

 odours, and this is the result of the action of bacteria which 

 infect the decaying substance. If this activity is allowed to go 

 on unchecked, the offensively smelling meat begins to disappear, 

 and by-and-by the smell itself passes away. 



Any organic substances exposed to the air or contaminated 

 with dust, " dirt," or impure water, will " go bad," ferment, or 

 putrefy, and this is because fermentative and putrefactive 

 bacteria gain admittance, multiply, and break down the organic 

 matter, using the latter as a source of food. If rigorous and 

 successful means be taken to exclude all micro-organisms from 

 the organic matter, the latter will not go bad. Thus if milk be 

 " pasteurised " (that is, heated for several days in succession in 

 closed bottles to a temperature of about 80° to 90° C), it will 

 remain fresh and sweet for an indefinite period. If meat be 

 sterilised by heating to over the boiling temperature of water in 

 sealed tins for a sufficient period, it will be preserved in good 

 condition for years. If meat be frozen to well below zero centi- 

 grade it will remain good. If salt, boracic acid, formaldehyde, 

 chlorinated water, etc., be added to the organic substance, 

 putrefaction will not occur. Heating to a sufficiently high 

 temperature kills all micro-organisms, as also does antiseptic 

 substances like salt and boraeic acid. Freezing arrests the 

 multiplication and vital activities of bacteria, but does not kill 



