646. Separating on Shabbos

Shabbos 8:11

If a person winnows or separates the volume of a dried fig, he is liable. Curdling milk is a subordinate labor of separating. Similarly, one who separates the dregs from a liquid is liable for performing a subordinate labor of separating or of sifting. The exact labor is not clearly stated because winnowing, separating and sifting are all conceptually similar. The reason these three activities are counted as separate labors is because every labor that was performed in building the Mishkan (Tabernacle) is considered a different category of labor.

Shabbos 8:12

If a person separates food from undesirable material, or if he separates one type of food from another type of food using a sifter or a strainer, he is liable. If a person separates using a tray or a pot divided into sections, he is not liable. One is allowed to separate food by hand for immediate consumption.