The question was raised concerning the status of saltshakers on Shabbat. Many people have the practice of putting some grains of rice in the saltshaker to absorb moisture; when one shakes the saltshaker, the particles of salt fall from the saltshaker while the grains of rice remain behind. At first glance, this would constitute a violation of the prohibition of Meraked. Meraked, one of the thirty-nine Melachot (categories of forbidden activity on Shabbat), means sifting removing undesirable elements from flour by having the flour fall through the holes of the sifter while the undesirable particles remain on top, in the sifter. This is precisely what occurs in the case of a saltshaker, which allows the salt to fall while keeping the rice on top, inside the shaker.
Nevertheless, Rabbi Moshe Halevi, in his work "Tefila Le'Moshe," permits using a saltshaker on Shabbat, even if it contains rice. He arrives at this ruling on the basis of several factors. For one thing, a saltshaker, unlike a flour sifter, is not an instrument specifically designated for this purpose, of removing undesirable elements from food. Secondly, a person shaking the saltshaker does not intend for the shaker to function in the capacity of a sifter, and has in mind only that salt should fall onto his food. Finally, one uses a saltshaker during his meal, as opposed to sifting flour, which is done as part of the process of food preparation. This type of "sifting" is thus considered "Derech Achila" a normal part of eating, which Halacha permits.
Thus, one may use a saltshaker on Shabbat, even if it contains grains of rice that are kept behind as the salt falls onto one's food.
