PRODUCTION AND CONSERVATION OF FATS AND OILS. 9 
with a high melting point, tallow is subjected to a similar process, 
which in this case is called graining. The hard oleostearin thus 
obtained is employed in making lard substitutes, while the liquid or 
semisolid oleo oil goes into oleomargarine. When a fat or oil decom- 
poses, some of the glycerids break np, thus liberating the free fatty 
acids which are partially responsible for the characteristic burning 
taste of rancid oils. Aldehydes and probably other compounds occur 
in rancid oils, and to these more than to the fatty acids are due their 
disagreeable odor and flavor. 
To obtain pure animal fats, the tissues in which they exist are 
usually minced or chopped fine and then rendered, that is, heated 
until the melted fat separates out. After the free, separated fat has 
been drawn off, the cracklings, or cooked tissues, usually are pressed 
to recover as much as possible of the fat. Instead of heating them 
dry in steam-jacketed kettles, many lard makers cook the hog fats 
under pressure with live steam blown directly into the covered tanks. 
This gives what is known in the trade as steam lard, in contradistinc- 
tion to kettle-rendered lard. 
Vegetable oils are commonly pressed from the seeds or fruit con- 
taining them by means of heavy hydraulic presses or continuous 
expellers. They may also be extracted by volatile solvents, such as 
gasoline or carbon bisulphid. Some fruits, like the olive, which 
give up their oil readily, can be pressed cold after they have been 
properly ground. Most of the oil seeds, however, yield so little oil 
when cold pressed that they must be cooked and pressed hot. As a 
rule, oils pressed cold do not require any subsequent treatment to 
make them edible. Such oils are known to the trade as virgin oils. 
On the other hand, hot pressed and extracted oils contain coloring 
and flavoring matter which must be removed by refining before the 
oil can be used for food, or even, in many cases, for technical purposes. 
Refining is the chemical treatment of the oil by which undesirable 
flavors or coloring substances are removed. Various acids and 
alkalies, as well as numerous salts, like borax, water glass, potassium 
bichromate, and potassium permanganate, have been used for this 
purpose. The present commercial method, however, consists simply 
in treating the warm oil with a solution of caustic soda, thus neutraliz- 
ing the free fatty acids present, and destroying, or precipitating, 
much of the coloring matter. The alkali combines with the free 
acids and some of the oil to form a soap which gradually settles tc 
the bottom of the refining kettle. In the United States this semi- 
solid mass of soap is termed soap stock, or foots, and is used by soap 
manufacturers. 
After they have been refined, many oils are also bleached. This 
is accomplished by mixing the hot refined oil with a small quantity 
96503°— 19— Bull. 769 2 
