168 OUTLINES OF BACTERIOLOGY 



which it can be attacked, though only mildly, by a few species of 

 bacteria, to which the low temperature is only a partial hindrance. 

 This chiefly accounts for the superiority of fresh over frozen meat. 



With regard to fruits, although freezing ruins most of them, it is 

 possible, by keeping the temperature a little above the freezing point, 

 to preserve them for a long time without injury. 



5. FOUETH METHOD OF FOOD-PRESERVATION- 

 CHEMICALS. 



Great care must be exercised in the choice of the chemical that is to 

 be used as a preservative, for it must possess the unusual property of 

 being injurious to bacteria and similar organisms without being harm- 

 ful to the human system. There are, rightly, stringent laws with 

 regard to the use of these preservatives, for it would never do to 

 allow the food-preserver to be the sole judge as to the proper preserv- 

 ative to make use of, for whilst he could ascertain what chemicals 

 would prevent his food from becoming bad, he is not usually the 

 right person to estimate their effects on the human system. 



Fortunately there exist certain substances which, when present in 

 small quantities, are beneficial or even necessary to a plant or an 

 animal, but when present in larger amounts are highly prejudicial. 

 We see this exemplified in the case of the scurvy attacks which, in the 

 olden days, often afflicted sailors during long voyages. Their food was 

 almost all salted, and the disease arose through having too much salt 

 in their bodies. At the same time the human system cannot exist with- 

 out a little salt. The same principle holds with regard to bacteria, so 

 far as salt is concerned. In small quantities salt is generally added 

 when nutrient media are being prepared for the cultivation of bacteria, 

 but if more than a certain amount be used, it will have a prejudicial 

 effect on the growth of the bacteria. If still further increased growth 

 stops altogether. The value of salt as a preservative will therefore be 

 readily perceived. We are all familiar with salted corned-beef, salted 

 fish of various kinds, salt pork, and many other kinds of flesh that are 

 prevented from decaying by the use of this valuable preservative. 



Another preservative of the same nature is sugar, although there is 

 no substance which bacteria and moulds of all kinds find so suitable as 

 a food material. The same principle holds as in the case of salt, it is a 

 food only when present in small quantities. An excess of sugar acts- 

 like an antiseptic. In making jams, advantage is taken of this fact,. 



