by R. Daniel Mann
Question: Is it permitted to use a fat separator on Shabbat?
Answer: (A fat separator is a regular container with a spout, except that the spout comes out from near its bottom. When one pours stock or gravy into it, forces of nature (lighter parts of a liquid mixture rise) cause the fat to rise to the top. By pouring the stock out from the bottom, the defatted part comes out first, and one stops pouring before getting to the fatty layer, which he later discards. Some models have a spout stopper, which traps air so that, until ready to pour out, the gravy with its fat stays out of the spout, so that all the fat remains in the container. (An on-line picture or demonstration may be helpful.) )
The baraita (cited in Shabbat 74a; see Tosafot ad loc.) mentions cryptically that selecting (borer) food from other types of food is sometimes forbidden and sometimes permitted. The following conditions for permissibility it brings to explain are accepted by the Shulchan Aruch (Orach Chayim 319:1-2). 1) The selection is done by hand, not by a utensil whose purpose is selection. 2) The food which one wants to eat is removed from that which he does not want now. 3) The food that is removed will be used in the short term. Only if all three are satisfied is the selecting not a violation of borer.
Let us analyze the fat separator. Most selecting takes place in one stage, either directly, by removing things from the mixture, or by putting something into a utensil that separates (e.g., a strainer) and removes immediately or one where the selection and removal occur when one shakes the mixture (e.g., a sifter). A fat separator, in contrast, employs two separate stages. The separation takes place over a few minutes after the gravy is poured in, and the removal of the defatted part occurs when one decides to pour it out. Let us first analyze each stage.
The first-stage separation happens naturally and is not significantly changed by the person’s action of pouring the gravy into the separator; the forces of gravity would perform the task in a pot or pan as well. Therefore, this cannot be prohibited.
The question is when one pours out the defatted part of the gravy and leaves the fat. There is enough intermingling between the parts of the stock to make it a question of borer. However, here you take the desired from the undesired (assuming one is interested in the defatted part, not the fat), which is good. (If there is some fat on the top of the spout in the beginning, do not throw it out alone.) Indeed, one should do this only if he plans to use the desired part shortly after removal.
The question is whether this is considered selecting that uses a (special) utensil. If we look at the whole process as one, then you are indeed using a utensil for it, and it would be forbidden borer. However, I am convinced one should look at it as two separate events: 1) natural separation; 2) removal of the good with a simple spout that does no separating. Besides an intuitive halachic conviction, a factor that indicates there are two separate stages is the fact that the second stage, which can come much later, occurs only if and when one decides to do it.
Therefore, pouring out only the part you want is not worse than pouring out some broth without vegetables from a soup pot, which is permitted if one does not use a pot cover etc. to hold back the vegetables. Indeed, one may pour from a utensil that which he wants and stop before getting to the unwanted material (see Shemirat Shabbat K’hilchata 3:47, who permits more severe cases). One could argue that a fat separator is worse because the pouring is effective only because this special utensil enables it. However, when discussing borer with utensils, the utensil is always involved in the separation, not in simple removal of that which was separated.
One could argue that the spout stopper is involved in separation, as it keeps fat out of the spout, and using it should be forbidden. However, this is wrong, as the air pressure does not hold back specifically fat but the entire mixture of gravy.