256 NUTRITION AND METABOLISM 



ensilage, salt-rising bread). Sometimes the acid-gas fermentation 

 is followed by a butyric fermentation. The true lactic fermentation 

 is not common, and is limited almost entirely to milk. This is ex- 

 plained by the circumstance that the organisms causing this decom- 

 position are parasitic in their habits, causing disease or living in the 

 intestine of animals. In the absence of acid and sugars, putrefaction 

 is the most common type of decomposition. 



Many factors aside from the chemical composition of the medium 

 are essential. Oxygen has already been mentioned as preventing buty- 

 ric fermentation. It will also prevent the acid-gas fermentation if too 

 abundant. Ensilage is trampled and pressed down to avoid air spaces 

 as much as possible, for molds will outgrow the acid-forming bacteria 

 if air has free access. Absence of oxygen will prevent mold growth, 

 and for this reason, jelly is paraffined, and butter wrapped tightly into 

 impermeable paper. The influence of oxygen upon the type of protein 

 and of cellulose decomposition has been pointed out previously. 



The moisture content is of great importance. As will be shown 

 later, not all organisms have an equal need of moisture; some molds 

 will grow on foods too dry for bacteria and yeasts. Molds are es- 

 pecially adapted for growing on dry media, as only part of their cell 

 substance is immersed in the medium. Their thread formation enables 

 them to search a dry medium, such as flour, for moisture, the extreme 

 of adaptation being Rhizopus, and the construction of the fruiting 

 bodies shows that they are destined by nature to be spread by air and 

 wind. It is no wonder that damp organic matter, if it can be de- 

 composed at all, will show molds, and nothing else, regardless of the 

 chemical composition, for there is no competition. Flour, moist seeds, 

 incompletely dried fruit, damp milk powder will always become 

 moldy. The same holds true with very concentrated sugar solutions 

 such as syrups, jellies and jams, while in concentrated salt solutions, 

 molds cannot thrive, and the torula yeasts are best adapted to such 

 conditions. 



A very important part is also the structure of the material. Micro- 

 organisms act mainly upon organic matter, and since this comes 

 from living organisms, it has usually definite structure, exceptions 

 being milk and blood. The structure of all living organisms is such 

 as to prevent the intruding of microorganisms. The body of plants 

 and animals is surrounded on the outside by tough and dry layers of 



