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Cooking Liquids on Shabbat

Rav Elyakim Krumbein
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Translated and adapted by Rabbi Eliezer Kwass
QUESTION:  Under what circumstances can one reheat liquids on Shabbat?  How and why does the halakha distinguish between liquids and solids?
Background Information - Cooking vs. Reheating
            Though the Torah prohibits cooking on Shabbat, in some situations reheating is permissible.  The well-known halakhic dictum states, "Ein bishul achar bishul," i.e., there is no prohibition of cooking a previously cooked food.  (Similarly, we find the principle "ein tochein achar tochein," e.g., regrinding a piece of matza into flour is permitted, while grinding grains of wheat is prohibited.)
Solids vs. Liquids - Several Approaches
            Authorities agree that reheating solid food is permissible once it has been one-third cooked (called by the Talmud "ma'akhal Ben Drosai" after a highway bandit who, in his haste, would eat partially cooked meat).  Reheating liquids, however, is the subject of a debate among the Rishonim:
1.  Some say it is prohibited once the liquid has cooled down beneath the level of "yad soledet bo" (literally, "when the hand recoils from its heat" - about 45 degrees Celsius).
2.  According to Rabbeinu Yona (quoted in Rabbenu Yerucham, Section 3), reheating a cooled down liquid is prohibited only if the application of heat improves the food's quality ("mitztamek ve-yafeh lo").
3.  The Rema (OC 318:15) maintains an alternate approach which prohibits reheating only if the liquid has completely cooled down.  As long as it remains somewhat warm, recooking is permissible.
Solids vs. Liquids - Why Distinguish?
            Why do the rules of recooking differ for liquids?  Why consider a solid "cooked" once the food has reached a certain stage of its preparation and a liquid "cooked" only while it is still hot?
            Based on the gemara (Shabbat 74b), one would infer that cooking means "softening" the food, not just making it edible.  For solids this definition works, but for liquids we need some other criterion.  Apparently, for liquids cooking means "heating."  This does not explain, though, why we do not also prohibit reheating solids which have fallen below "yad soledet bo."  If this temperature is a relevant criterion for liquids, why does it not also apply to solids?
Ritva's Cryptic Comment
            Surprisingly, except for the Ritva (Shabbat 39a), the Rishonim do not offer any explanation for the puzzling distinction between liquids and solids - and the Ritva leaves us only with a short, enigmatic statement.  Quoting the Tosafot, he says:  "'Basheil mevushal' (surely cooked) can apply to liquids but 'basheil mevushal' cannot apply to solids like meat and fish."  What does he mean by "basheil mevushal"?
            The expression appears in the Torah in conjunction with the Korban Pesach (Shemot 12:9).  The Torah requires it to be eaten roasted, not raw and not "basheil mevushal bamayim".  The word "bamayim" is open to two translations:
            1. [cooked] in water; and
            2. [cooked] with and by means of water.
Water is both a place or medium within which something gets cooked, and a cooking agent.  The Yerushalmi (Shabbat 7:2), basing the opinion that one cannot cook the Pesach sacrifice with the hot spring waters of Tiberias on the words "basheil mevushal bamayim," apparently translates the expression the second way - "with water," for clearly it meant to teach us something about the means of cooking and not merely the location.
            Liquids are not only cooked by fire, but they also become a partner with fire to act as a cooking agent.  A solid is passive in the cooking process; it can only be cooked.  A liquid, though, is an active participant in cooking the food which is in the pot with it.  It is heated by the fire below it, but  then it becomes that which cooks the meat, fish, or vegetables in its midst in a way which the fire itself would not have been able to. 
Cooking a liquid = Making it into a Cooking Agent
            Herein lies the distinction between liquids and solids.  To cook a liquid, say the Tosafot, is to turn it into a cooking agent.  Once a liquid has cooled down, it ceases to function as such, but it will resume its role if reheated.  Once a solid has been softened by heat, the change is irreversible.  Reheating it is thererfore permissible on Shabbat (at least on a Biblical level, though the Rabbis prohibit replacing it directly on a fire).  But every time a liquid drops below 45 degrees, its power to cook other foods is lost and, in effect, it becomes "uncooked."  Reheating it above that temperature is prohibited because this once again transforms it into a cooking agent. 
            With this approach we can answer a question that troubled the Acharonim.  Some Rishonim claim that the prohibition of cooking on Shabbat does not apply to fruit which is edible when raw.  Yet, the same Rishonim agree that it does apply to water (which of course is edible when raw)!  But now, the distinction is clear.  Water becomes a cooking agent when heated; fruit just become hot.  Since the heating does not essentially change the fruit - it was edible before - cooking does not apply to it.  Water, though, is qualitatively changed by the heating; it can now cook other things.
            The Ran (Shabbat Ch. 4) quotes Rabbenu Yona in explicit support of this idea (i.e. the prohibition of reheating liquids is based on returning to the state of a cooking agent).  Based on this assumption, he arrives at an exceptional conclusion; one may not pour boiling hot water from one "keli rishon" (= a vessel that was on a heat source) into another that has food in it, since there are views in the Yerushalmi that once poured from a "keli rishon," food no longer maintains its cooking ability (even if still boiling hot).  So "when water is poured, it immediately upon leaving the vessel, even if boiling, loses its ability to cook" ("passak ko'ach retichatah mi-levashel"), and then once again is re-cooked by the receiving "keli rishon."
Three Approaches to Cooking Liquids
            We can now understand the different opinions regarding when it is prohibited to reheat liquids:
1.  According to the first approach, once it is below 45 degrees ("yad soledet bo"), reheating it above that is prohibited because it becomes a cooking agent again.  This is how the Shulchan Arukh OC 318:4 rules.
2.  Rabbenu Yona only prohibits reheating above "yad soledet bo" if in so doing the food is actually improved, i.e. the liquid manifests its potential as a cooking agent.
3.  The Rema, who permits reheating as long as the liquid has not completely cooled down, is difficult.  Why ignore the relevant cut-off temperature of "yad soledet bo?"  His opinion can be understood if one views water that has once been heated as part of a "fire+water cooking system," to which it remains connected until the last remnant of heat is gone from it.  Only then would one be prohibited to reheat the water.  This is reminiscent of the opinion of the Drisha (OC 253), who permits returning even a fully cooled liquid to the fire as long as one is still holding it and has not set it down on the ground.  This retention in the hand is enough to maintain the water's identity as an integral part of the "fire + water cooking system" and, as such, it may be reheated.
(This article originally appeared in Daf Kesher No. 63, vol. 1 pp. 249-251.)
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