Microorganisms in Relation to Man 581 



although it may not affect the spores. Some spores have been 

 boiled in water for ten or more hours without injury ; indeed 

 few spores are destroyed by the ordinary boiling temperature 

 unless continued for more than an hour. This is a practical 

 point in preserving food materials, as in canning fruits and 

 vegetables. Where canning is done in factories, temperatures 

 higher than that of the ordinary boiling point are secured under 

 pressure, in order to sterilize the food completely. Fruits are 

 generally free from spores and do not therefore require more than 

 a few minutes of boiling. Vegetables, such as corn, beans, and 

 peas, contain heat-resisting spores which cannot be killed under 

 two or more hours of boiling under pressure. Where heating 

 under pressure is impracticable, the method of repeating the 

 boiling on at least three successive days is employed. Sub- 

 stances are boiled for a time to destroy bacteria, and as soon as 

 sufficiently cooled the spores begin to develop. Boiling ten or 

 fifteen minutes destroys germinated spores, and a third boiling 

 does away with possible survivors. Such fruit juices as cider, 

 or such vegetables as tomatoes, are easily sterilized with one or 

 two short periods of heating. Milk is exceedingly difficult to 

 sterilize, as boiling changes its character, and it often contains 

 spores that are very resistant to high temperatures. 



Of the remaining physical conditions, light is important. 

 Molds, yeasts, and bacteria, in their natural habitats, are 

 not wholly exposed to direct sunlight for any considerable 

 period. Practically all kinds are impaired, if not destroyed, by 

 bright sunlight in the course of a few hours. Other conditions 

 figure in this, such as moisture in the atmosphere. This suggests 

 the possibility that light is not so directly influential in killing 

 bacteria as in changing the conditions of the nutritive substances 

 on which they exist. It evaporates the moisture rapidly and 

 may produce other changes which make nutrition impossible 

 and death or a resting state inevitable. 



Certain kinds of bacteria are light-loving, as the purple 

 bacteria, which live largely on hydrogen sulfid and absorb 



