12 BULLETIN 769, U. S. DEPARTMENT OF AORICULTURE. 
from the hulls are ground through a series of three or more heavy 
steel rolls, and finally carried into storage bins over the pressroom. 
In expressing most of the edible oils abroad several grades are 
frequently made by a repressing of the same batch of raw material. 
Cotton seed, however, in the United States, at least, is pressed only 
once, and when hydraulic presses are used it is always heated or 
cooked before pressing. The cooking is done in a shallow, steam- 
jacketed pan equipped with a mechemical stirrer, which, as it revolves, 
mixes the meats thoroughly and prevents uneven cooking. In many 
mills a second pan, called a^subheater, similar to the cooker, and in- 
stalled just below it, serves to hold the cooked batch until the presses 
are ready for it. 
Hydraulic press. — The type of press most commonly used in this 
country in the production of cottonseed oil is the steel box-frame hy- 
draulic. It consists of a series of horizontal steel plates set one above 
the other, and provided with closely fitting steel sides so that the 
whole machine is really a series of steel boxes without ends, piled one 
upon the other, the lowest box resting upon a hydraulic piston. One 
after another all the boxes are charged with cooked meats wrapped 
in heavy press cloths until the press is filled. The compressed air is 
then turned on, and the oil as it is squeezed out flows down over the 
sides of the press and through troughs to the settling cistern. As it 
comes from the press the dark-red crude oil contains some fine meal. 
Before being pumped or shipped to the refinery, therefore, it is held in 
settling tanks or cisterns until most of the finer particles have settled 
out. 
Expeller press. — An increasing amount of crude cottonseed oil is 
made in mills equipped with a type of continuous-working press 
known as the expeller. The expeller is built somewhat on the 
principle of the ordinary meat grinder, and is simply an interrupted 
screw revolving inside a slotted steel barrel. The ground seed enters 
through a hopper at one end of the barrel, is pressed along toward the 
opposite end, and finally discharged around a cone, which can be set 
in or out of the outlet orifice, to give any desired pressure. Squeezed 
from the seeds by the pressure of the screw, the oil runs out through 
the small slits in the barrel, and after settling or, better, filtering 
through a filter press, is ready for shipment to the refinery. 
As the yield of oil by either process is only about 45 gallons per 
ton, or less than 17 per cent of the weight of seed handled, and as 
a large part of the ground cake and hulls can be used as feed or 
fertilizer by the local farmers, the crude-oil mills often are located in 
the smaller towns throughout the cotton-growing sections. From 
these the oil is shipped in steel tank cars to more centrally situ- 
ated refineries or to the packing houses and cooking compound 
