There are four melakhot that deal with separating okhel (food) from psolet (waste): Dash (threshing), Zoreh (winnowing), Borer (separating), and Meraked (sifting).
The melakha of Dash involves detaching food matter from its husk. The melakha is named for the act of detaching kernels of grain from their stalks and chaff. After the grain was harvested and gathered, its stalks were beaten with flails in order to separate the kernels of grain from them. When there was an abundance of grain, threshing was done by having an animal walk on the stalks over a hard surface. To make the threshing more efficient, a wide board with blades or stones attached to it would be attached to the animal. The animal would pull this board over the stalks, and this would separate the kernels from the stalks. (The details of this melakha will be explained below in sections 17-18.)
After threshing, some straw and chaff would remain mixed in with the kernels. In order to remove this, they would winnow the grain in the wind, that is, they would toss the mixture in the air with a dedicated instrument, and the wind would blow away the lighter chaff and straw while the heavier kernels would fall back to the ground. This is the melakha of Zoreh.
The remaining pile would still contain stones and clumps of earth, which would be removed by hand. This is the melakha of Borer.
After this, the wheat would be ground into flour. But because the outer layer of a wheat kernel, called the bran, is coarse, the grinding process would result in a mixture of flour and coarse particles of bran. In order to separate the flour from the bran, the flour would be sifted with a sieve. The finer flour would filter through the sieve while the bran remains on its surface. This is the melakha of Meraked.
Borer is done by hand, whereas Meraked is done with an implement. In Borer, the psolet is removed from the okhel, while in Meraked the psolet is left in the sieve as the flour passes through. Thus we see that there are multiple ways to separate okhel from psolet. All of these activities, when performed in their normal manner, are prohibited by Torah law. If they are done with a shinui they are rabbinically prohibited, and if they are performed as part of the normal eating process (ke-derekh akhila), they are permitted. All this will be explained below.
The proliferation of melakhot that deal with separating psolet from okhel shows us how central acts of selection and differentiation are in our lives. The world in general is confused and mixed up, and the ability to separate the good parts from the bad allows man to develop and improve the world. These melakhot also allude to man’s spiritual work, because the world is confused and mixed up morally as well, and our job is to distinguish between good and evil. If it were totally clear that good was on one side and evil on the other, it would be easy to always choose the good. The problem is that things are not so clear; even within the good there is evil, and even within the evil there is good. Evil things can sometimes be good in a different place and context. The great challenge that God presented to mankind is to choose the good from the evil, to put everything in its proper place, and thus repair the world.
All this applies throughout the week, when we must engage in the complicated work of separating the bad from the good, which demands that we engage directly with the world’s waste and refuse. But on Shabbat we must focus on the inner goodness of existence, enjoy it, and connect with the foundations of faith. When we draw on the sanctity and faith that we absorb on Shabbat, we gain the ability to distinguish between good and evil all week, and to engage in the work of refinement that is required to repair the world.