Bishul does not apply to a liquid until it is hot to the touch (Yad Soledet Bo, the hand recoils from it; Shabbat 40b). Twentieth-century Poskim debate the precise temperature equivalent to Yad Soledet Bo. Let us address this question at some length and note its major practical considerations.
Twentieth Century Poskim
Rav Ben Zion Abba Shaul (Ohr LeTzion 2:30:12) and Rav David Yosef (Halachah Berurah, Bishul, page 213) assert that Yad Soledet Bo is 40°C (104°F). Rav Moshe Feinstein (Teshuvot Igrot Moshe Orach Chaim 4:74:Bishul:3) writes that the minimum temperature of Yad Soledet Bo is 110°F. Rav Shlomo Zalman Auerbach (Teshuvot Minchat Shlomo 1:91:8) creatively demonstrates that it cannot be less than 113°. Rav Aharon Kotler (cited by Rav Shimon Eider, Halachos of Shabbos 2:243 note 19) asserts that Yad Soledet Bo is not less than 120°. Interestingly, the Darkei Teshuva (105:51) cites that the traditional practice of Halachic authorities to determine if something is Yad Soledet Bo is to put a finger in it and see if your hand recoils.
Zomet Institute – 120 Degrees
The Zomet Institute’s Rav Yisrael Rosen (Techumin 43:63) writes that he has heard from several people that Rav Zvi Pesach Frank believed Yad Soledet Bo is 50°C (122°F). Rav Rosen records that Rav Shlomo Min-Hahar reports that when he told Rav Frank that this is a high number, Rav Frank responded, “Consider it 48°C (118.4°F), and that satisfies all opinions.” The Kaf HaChaim (Orach Chayim 318:143) and Rav Ovadia Yosef (Chazon Ovadia, Shabbat 4:370 and Halichot Olam 4:42) cite as normative the Ben Ish Chai’s (Bo 2:5) assertion that Yad Soledet Bo is determined by the temperature at which the average person would refrain from drinking the water because it is too hot. Zomet’s Rav Yisrael Rosen (Techumin 43:63) conducted tests and concluded that most people can comfortably drink water at 50°C.
Rav Asher Bush reports that North American kashrut professionals (such as from the CRC, OU , and Star-K) regard the lower edge of Yad Soledet Bo to range from 115 to 120°F. This assumption has numerous practical applications, including the production of whey, chocolate, and many other products.
Rav Auerbach proves his point that the Gemara Chulin (8b) concludes (and is codified by the Shulchan Aruch Yoreh Deah 10:3) that the place of Shechitah is “cold,” i.e., is not Yad Soldet Bo. Rav Shlomo Zalman further notes that a sick bird’s temperature is even higher, and the blood rushing out of the shechita point rises at least 1 degree Celsius. Thus, Rav Shlomo Zalman raises the possibility that this additional information might even prove that 48 Celsius (=118.4 F) is not yet Yad Soledet Bo.
In addition, the United States Consumer Product Safety Commission recommends 120°F as a safe temperature to avoid tap water scalds. Similarly, the American Journal of Public Health prefers a maximum of 120 degrees for hot water. The minimum temperature according to the New York City code is 120 degrees Fahrenheit.
Rav Schachter and Rav Willig
Rav Schachter ("Cooking and Warming food on Shabbat," p. 177) concludes that 120 degrees Fahrenheit is the accepted number for Yad Soledet Bo. In contrast, Rav Willig ("Cooking and Warming food on Shabbat," p. 14) insists that it is 113 degrees Fahrenheit. It seems compelling to follow Rav Schachter, as we rely on major Kashrut organizations that use 120 degrees Fahrenheit as the minimum Yad Soledet Bo temperature. This is particularly the case when there are other lenient considerations.
Nafka Minot
There are numerous Nafka Minot (ramifications) of this assertion. Examples include making hot tea on a Kli Shelishi, dipping Challah in hot chicken soup, and reheating food from the refrigerator in a way that will not reach Yad Soledet Bo. One can avoid these questions by ensuring that hot water, chicken soup, and reheated food remain below 120 degrees Fahrenheit. The drinks and food will still taste very good, since temperatures below 120 degrees are still quite warm.
Safek Yad Soledet Bo
Even if one insists that 113 degrees Fahrenheit is the lower threshold of Yad Soledet Bo, it is only a Safek if the item is Yad Soledet Bo at that point. Rav Moshe Feinstein (Teshuvot Igrot Moshe 4:74 Bishul 16) writes that regarding Shabbat, we are uncertain up to 160 degrees Fahrenheit if it is Yad Soledet Bo. Thus, one may, at least, use the 120 degrees standard regarding tea, chicken soup, and reheating food as a compelling lenient consideration.