Straining liquids can also involve a Torah prohibition, depending on the state of the liquid. If a liquid has psolet in it that makes it undrinkable, and straining makes it drinkable, then straining it is prohibited by Torah law. If the liquid is drinkable even without straining, one may strain it with a strainer even if this improves the drink slightly. The improvement does not fundamentally change it, and thus this is not prohibited.
In an intermediate case, where the drink is cloudy and most people would not drink it without straining it unless they had no choice, one may not strain the drink using a strainer. There is a disagreement about whether one may strain it with a piece of cloth, which is an unusual way of straining. Most Rishonim allow this, while Rambam prohibits it. Aĥaronim write that one should be stringent and follow Rambam (SA 319:10; MB 42).
Therefore, lees that are mixed with wine may not be strained to separate out the wine, since it is otherwise undrinkable. Doing so is prohibited by Torah law (SA 319:9; MB 32). If the wine is drinkable but so cloudy that most people generally would not drink it, one may not strain it with a strainer, and it is preferable not to do so with a cloth either. However, wine that is drinkable may be strained to improve its clarity. Since one could drink it even beforehand, straining it is not considered a melakha.
Accordingly, since most people are used to drinking orange juice with pulp and do not strain it, one may remove the pulp with a strainer on Shabbat.[12]
Similarly, one may turn on a filtered tap or pour water from a vessel with a built-in filter (such as a Brita pitcher). Since the water is drinkable even before it is strained, straining it is not considered a melakha.
[11]. Kaf Ha-ĥayim 319:47 and Yalkut Yosef 319:63 are lenient, following Ben Ish Ĥai. Igrot Moshe OĤ 4:74, Borer 7 is lenient when one does not have another option and for feeding a baby. SSK 3:17 cites both opinions and adds that one who is lenient and removes the seeds before eating has an opinion on which to rely. In n. 34, SSK states that R. Shlomo Zalman Auerbach is lenient because this is the way in which the watermelon is eaten. He points out that this case is less problematic than removing bones from fish. This is because it is possible to remove a bone together with some fish, put the combination into one’s mouth, eat the fish, and throw out the bone. In contrast, this cannot be done with watermelon seeds. The many poskim who permit removing fish bones before eating the fish would certainly be lenient regarding removing watermelon seeds before eating the watermelon.[12]. Nevertheless, one may not strain the liquid from cooked vegetables because they are considered two different types of food, and if one wishes to eat only one of them, the other is considered psolet for him. In contrast, orange juice with bits of pulp is just one type of food, and therefore separating the pulp from the juice is not considered Borer (see R. Shlomo Zalman Auerbach cited in SSK 3:53-54, 57 and n. 173; this approach is also found in Igrot Moshe OĤ 4:74, Borer 3).
