260 THE PRESERVATION OF FOOD PRODUCTS 
develops also a large amount of lactic and butyric acids, the 
amount of lactic acid being as high as 7 per cent. and the butyric 
acid over 2 per cent. These acids are derived chiefly from the 
carbohydrates, as is shown by the great reduction in the amount of 
these bodies in the drying hay. A considerable part of the nitro- 
genous material is also lost, the total loss in the hay being about 
14 per cent. This loss of material is one of the objections to this 
method of curing hay. It is known as brown hay. 
Sometimes a slightly different method is used. The freshly 
mown grass is piled in heaps from ro to 13 feet high, the mass being 
trodden down as tightly as possible to prevent the admission of 
air. The temperature in these heaps rises rapidly, and is tested 
by a thermometer. When it rises to about 158°F., which occurs 
generally in from 48 to 60 hours, the heaps are opened and spread 
out in thin layers tothe air. The heat in the hay now rapidly dries 
the product and, with a single turning, itis ready for storing. Hay 
thus prepared is called burnt hay, and develops an aromatic odor 
which ordinary sun-dried hay does not possess. 
Lastly, it should be noticed that ordinary sun-dried hay will 
sometimes, especially if stored in too moist a condition, undergo 
a similar heating in the mow. The hay may be considerably 
injured by such heating, so that it will lose some of its nutriment. 
Sometimes the heat is sufficient to cause an actual ignition of the 
hay. 
Certain phenomena sometimes seen in cotton are clearly closely 
akin to the fermentation just described, for cotton may undergo a 
spontaneous heating sufficient to render it in danger of combustion, 
and this must be due to processes similar to those just mentioned. 
The same thing is true of hops which occasionally develop a like 
spontaneous heating during the curing. 
PRESERVATION BY COLD 
All the common species of bacteria grow more slowly as the 
temperature is lowered, and cease growing entirely when it reaches 
