A fundamental issue in the laws of Shabbat is establishing the temperature of yad soledet bo (the temperature at which the hand recoils). The Sages state that this is the minimum temperature that is still capable of cooking food. However, it is uncertain precisely what this term refers to. Does it refer to a level of heat that the hand can only stand to touch for a few seconds? If so, this would be approximately 71° Celsius. Alternatively, does it refer to a level of heat that becomes unpleasant for one’s hand after a few minutes? If so, this would be approximately 45°C.
According to many poskim, since the matter remains in doubt, one must be stringent. Heat of 45°C must be considered capable of cooking. Thus, one may not put raw food into a pot that has reached a temperature of 45°C or higher. Similarly, one may not place raw food near enough to a fire that it could reach 45°C or higher (SSK 1:1).
This definition is also important for how the laws of Bishul apply to water. One may not raise the temperature of water from below yad soledet bo to yad soledet bo or higher. However, if the water has already reached yad soledet bo, it is considered cooked, and one may continue to heat it further, because of ein bishul aĥar bishul. Cold water may be left near the plata as long as the water is in a place where it would not reach yad soledet bo even if it remained there for an entire day.
Thus, in practice, one must be careful not to heat water to between 45°C and 71°C; since we do not know the precise temperature of yad soledet bo, any heating in that range may be considered Bishul. But one may heat outside of this range. For example, if water has been heated to 71°C, one may then put the water in a place where it will get hotter, because of ein bishul aĥar bishul. Similarly, one may put cold water in a place where it will heat up, as long as the water does not exceed 44°C, because this kind of heating does not qualify as Bishul.
There is another important position that defines yad soledet bo. This position maintains that food or drink that most people would be able to consume all at once cannot be considered yad soledet bo. However, if most people would be unable to eat or drink it all at once, that is a sign that it is yad soledet bo. Thus, it would be considered cooked, and one may heat it further (Ben Ish Ĥai, Year 2, Bo 5; Maharsham 1:187; Yabi’a Omer 3:24, 4-6). In a case of necessity, one may rely on this position.
To create a safeguard, the Sages prohibited leaving water in a place where it could potentially reach yad soledet bo, even if one is standing there watching and waiting to remove the water before it reaches that point. They were concerned that the person watching might get distracted and forget to remove the water before it reaches yad soledet bo, thus making him guilty of cooking on Shabbat. However, if there is no possibility that the water will reach yad soledet bo even if it remains there for an extended period of time, one may place the water there (SA 318:14).
. Igrot Moshe, OĤ 4:74:3 states that the temperature of yad soledet bo is somewhere between 43°C and 71°C. According to Or Le-Tziyon 2:30:12, it is between 40°C and 80°C, based on the notion that at 40°C a baby’s belly is scalded. This measurement is mentioned in SA 318:14. At the other extreme, some people drink tea that is as hot as 80°C. SSK states, based on a proof suggested by R. Shlomo Zalman Auerbach, that the lowest possible temperature for yad soledet bo is 45°C. Even though most poskim feel that one should be stringent and take into account both versions of yad soledet bo (that the hand cannot bear to remain on the heat source, or that it is merely unpleasant for the hand to remain), it is not necessary to follow the extremes within these positions. This is why I wrote that between 45°C and 71°C constitutes the doubtful range. Furthermore, if necessary one may rely on the position that uses the criterion of whether the food can be eaten or drunk all at once. There are some who are stringent and refrain from heating water even if it is already yad soledet bo. See SSK ch. 1 nn. 17 and 110. In contrast, Ashkenazic custom is that if the hot water started out at yad soledet bo and later cooled down, as long as it is still a little warm one may bring it back to a boil. (See the continuation of this discussion in sections 5-6.). According to Rashi and Rambam, one may place water in a place where it could become yad soledet bo on condition that someone is watching it to ensure that this will not happen. However, Tosafot, Rosh, Rashba, and most poskim maintain that this is forbidden, and this is the ruling of SA 318:14. In contrast, if one wishes to heat a cold liquid that had previously been cooked, like soup, one may be lenient and place it where it could reach yad soledet bo as long as someone is watching it and waiting to remove it before it reaches that temperature. This is permitted because (as we will explain in the next paragraph) according to Rambam, Rashba, and Ran, the principle of ein bishul aĥar bishul holds even for a liquid. This situation is considered a twofold doubt: first, perhaps the law really follows the lenient position of Rashi and Rambam; second, perhaps ein bishul aĥar bishul applies to liquids as well. R. Ovadia Yosef writes in Livyat Ĥen §51 that one may be lenient as a result of this twofold doubt. It is true that some are stringent in this case (SSK 1:13; Menuĥat Ahava 2:10:5; and Orĥot Shabbat 1:18) and are willing to endorse leniency only in a case of great need. Nevertheless, the primary position is to be lenient. This is because even according to those who are stringent and maintain that ein bishul aĥar bishul does not apply to liquids, if one forgot to remove the liquid before it reached yad soledet bo he will only have transgressed a rabbinic prohibition, since he did not intend to boil the soup but only to heat it a bit.