258 THE PRESERVATION OF FOOD PRODUCTS 
Hay.—One of the most important applications of the drying 
process is in the preparation of hay. The fresh grass contains 
so much moisture that it could not be preserved in masses without 
undergoing extensive decomposition, and to obviate this the 
farmer resorts to the simple plan of drying out some of the water. 
But this phenomenon of drying is not always as simple as it looks, 
and sometimes a fermentation is certainly involved. Where the 
climate is moderately dry and the sun hot, the simple method of 
exposing the grass to the sun for a few hours is most widely 
adopted. But such a method is not possible in regions where 
there is likely to be a great deal of rain. 
Curing of Hay by Self-fermentation.—In countries where rains 
are frequent and sunshine rare, the sun’s rays cannot be de- 
pended upon for the curing of hay, so a different principle is used. 
If the moist grass is heaped in piles and allowed to stand for a 
few days, there appears a marked rise in temperature. This 
continues rapidly, the rate and the temperature reached depend- 
ing upon the conditions: the denser the packing, the higher the 
temperature. Commonly the rise is not above 160°, although 
under some conditions it goes above this, even to the point of 
spontaneous combustion. This latter phenomenon is of rare 
occurrence, however, although probably it is the cause of the 
spontaneous combustion that occurs occasionally in a barn when 
the hay is packed away in too moist a condition. 
The cause of such self-heating has not been definitely settled. 
It is evident that the phenomenon has a decided resemblance 
to the fermentation of tobacco and also to that of silage. Three 
possible causes may be concerned: (1) The respiratory changes 
in the still living cells of the grass. (2) The action of enzymes 
from the grass. (3) The growth of microérganisms. Experi- 
mental tests have not yet settled positively the relation between 
these possibilities. Sterilized hay will not undergo this heating, 
while the same sterilized hay, if inoculated with certain species 
of microérganisms (Oidium), will show a rise in temperature 
