BACTERIA AND PRESERVATION OF FOOD-PRODUCTS 163 



and under ordinary circumstances would soon suffer the changes that 

 we have described above. These changes we call by such names as 

 decay, putrefaction, rotting, etc. They are always initiated by micro- 

 organisms (bacteria, moulds, yeasts, etc.), which are ever present in the 

 atmosphere and in the soil, and only wait for favourable conditions to 

 pounce upon their natural food. Our ability to prevent food-stuff from 

 suffering these changes is due to our knowledge of the laws which 

 regulate the growth and multiplication of these microorganisms. If, 

 therefore, we can place the food-material under conditions so unfavour 

 able that microorganisms cannot grow and multiply, we shall have 

 achieved our object. 



Now before putrefaction can take place the following conditions must 

 hold, the absence of any one of them being sufficient to prevent this 

 process setting in : 



1. The microorganism must be present. 



2. There must be an adequate supply of water. 



3. The temperature must be suitable. 



4. The food-stuff must not contain substances injurious to the 



growth of these small organisms. 



With regard to the presence of microorganisms, as explained in a 

 former chapter, there are very few places where they are absent, and it 

 may safely be stated that there are no localities in which the preserva- 

 tion of food is carried on as an industrial occupation which are free 

 from them. With regard to the second condition, no matter how suit- 

 able the food presented to them, bacteria cannot multiply when there 

 is less than 30 per cent, of water, and, approximately, the same per- 

 centage holds for moulds, yeasts, etc. However, there are very few 

 foods which in a normal condition hold such a small percentage of 

 water, so that very few can escape on this score. Next, there are no 

 places on the earth, even in the Persian Gulf, where the normal 

 temperature is too high for bacterial and other growth, though for 

 some distance from both the North and the South Poles, and on the 

 tops of the highest mountains, the temperature is too low to permit 

 of growth of any kind. Finally, there are no food-stuffs which 

 naturally contain substances that are imperishable, and which, at the 

 same time, prevent the growth of microorganisms. We therefore see 

 that, though all four conditions must hold before putrefaction can set 

 in, there is no hope of preservation except by artificial means. In 

 adopting artificial means it is necessary to secure the absence of one 

 condition only. 



