There are various methods for separating substances, such as handpicking,
threshing, winnowing and sieving.
Handpicking allows the good grains to be separated from the waste and impurities.
Handpicking is the basic method for separation of substances. It involves simply picking
out substances by hand and separating them from others. The substances being separated may
be impurities that have to be thrown away. It could also be that both the substances being separated are useful – such as separating green grapes from black ones.
The method of separating the grain from the stalk is called threshing. It is basically the
beating of dry stalks to shake off the dried grains. It can be done by hand, by cattle or by
machines. Traditionally, threshing was done by hand, but cattle help do the job quickly.
Nowadays, threshing machines are also used to separate large quantities of grain quickly.
Even though the large stalks can be separated from the grains by threshing, the grains still have dried husk and chaff, which have to be separated and thrown away before the wheat can be used. This method is called winnowing.
The husk is blown away as it is much lighter than the grain. So, when the grains are gently
dropped to the ground, only the wheat grains collect there, while the husk blows away.
Sieving is a method of separating substances that are of different sizes. For example,
wheat flour has some fine powdered wheat as well as some bigger impurities. When it is put this through a sieve, the fine powder falls through the small holes in the sieve, while the thicker
impurities remain as they are too big to pass through these holes. The substances are thus separated.