How much is Yad Soledes?
Throughout the laws of cooking on Shabbos, we find the term of Yad Soledes used in regard to the temperature of a food, and understanding its proper definition is vital for implementing the Halacha’s in which this matter is relevant.
The Halachic ramifications: Knowing whether a food is the heat of Yad Soledes plays a role regarding various cooking related laws on Shabbos, as well as in the laws of Kashrus, such as in the following cases:
- Heating a food on Shabbos: The general rule is that less than Yad Soledes heat does not have the ability to cook a food. It is thus permitted to heat a food on Shabbos to less than Yad Soledes even if the food generally contains a cooking prohibition, so long as the food will not reach Yad Soledes even if it remains there for the entire Shabbos. For example: One may pour warm non-Yad Soledes water directly onto a bag of tea without needing to have a Keli Shelishi, and one may add salt to ones non-Yad Soledes pot of soup even if it is a Keli Rishon. One may add cold water to a hot Mikveh, if the Mikveh water is not Yad Soldes. One may reheat a moist food in an area near the Blech if the food will not reach Yad Soledes even if it remains there the entire Shabbos.
- Reheating a food that is below Yad Soledes: According to all Poskim, there is no cooking after cooking by a liquid if it maintains its Yad Soeldes heat. Thus, one may pour replace a pot of soup back onto an upside pot on the Blech if the soup is still Yad Soeldes. However, if the food is no longer Yad Soledes, then accoridng to the Sephardim it is forbidden to do so.
- Taaruvos of non-Kosher foods: The taste of a non-Kosher food cannot penetrate a Kosher food unless the food is Yad Soledes. Thus, for example, if one accidently spilled hot milk onto a meat vessel, the meat vessel remains Kosher if the milk was below the heat of Yad Soledes.
In feeling of warmth: Yad Soledes is the amount of heat that would burn the stomach of a baby. Some Poskim rule that this is defined as the amount of heat that one’s finger does not feel comfortable to remain inside of for a long period of time [even if one would not instinctively remove his finger right away upon touching it], and hence if one’s finger can comfortably remain inside the heat, the heat is defined as less than Yad Soledes. Other Poskim rule it is the amount of heat in a food that one would avoid entering his mouth, and hence if one can comfortably enter the food into his mouth [without needing to blow on it or swish it from cheek to cheek] then it is defined as less than Yad Soledes.
In degrees: Some Poskim rule Yad Soledes is 40 degrees Celsius [104 Fahrenheit]. Others rule it is 43 degrees Celsius [109 Fahrenheit]. Others rule it is 45 degrees Celsius [113 Fahrenheit]. Practically the custom of many is to measure Yad Soledes by 45 degrees Celsius. However, Sephardim are to be stringent regarding reheating foods that have cooled down, to consider Yad Soledes 71 degrees Celsius [159 Fahrenheit].
Practical application: It is forbidden to measure a temperature on Shabbos even with a non-digital thermostat. Accordingly, one is to use one of the above two mentioned methods for measuring Yad Soledes on Shabbos, either as the amount of heat one feels comfortable leaving his finger inside of even for a while, or the amount of heat one feels comfortable entering into his mouth.
 See Maharsham 1:197; Pischeiy Teshuvah 105:7; Minchas Yitzchak 29; Minchas Shlomo 1:91; SSH”K 1 footnote 3; Igros Moshe 4:74; Yoreh Deah 2:52; Middos Vishureiy Torah; Oar Letziyon 2:30; Maor Hashabbos 1:2; Piskeiy Teshuvos 253:1
 See Admur 253:5; 21, 22; 26; 27; 28; 254:4; 257:9; 318:9; 11; 14; 23; 24; 29; Michaber 318:14; Rama 253:1; Shabbos 40b
 See Admur 318:9; 11; 24; 29; Michaber 318:14; Shabbos 40b
 Admur 318: 24; 29
 See 318:9; Michaber 318:4
 Michaber ibid
 Michaber 105:2
 Michaber 318:14; Shabbos 40b “What is Yad Soledes Bo? Rechava says so long as a baby’s stomach would burn from it.”
 Vayeishiv Moshe 178; Hilchasa Rabasa Leshabbaso in name of Chasam Sofer that he would check with his finger; Yeshuos Chochmah 80; Minchas Yitzchak 5:127; Birur Halacha [Zilber] 318; Piskeiy Teshuvos ibid; See Rashi Shabbos ibid “His hand goes backwards due to fear that he may get burnt”
 Kitzur SHU”A 80:3; Ben Ish Chaiy Bo 5 “How does a person know to measure Yad Soeldes? Use this rule, so long as its cool enough to be fit for eating or drinking then its not considered Yad Soeldes, while if it is too hot to enter the mouth, then it is Yad Soeldes”; Kaf Hachaim 253:30; 318:143; Maharsham 1:197; Birs Olam Ofeh 93; See Poskim in Piskeiy Teshuvos ibid footnote 5
 Chazon Ish, brought in Piskeiy Teshuvos ibid; Bris Olam Ofeh 92; Chut Shani 29:7; Or Letziyon 2:30-12; Kovetz Mibeis Levi 6:35
 Igros Moshe 4:74
 Minchas Yitzchak Likkutei Teshuvos 29; Rav SZ”A in Minchas Shlomo 1:91; SSH”K 1 footnote 3
 Igros Moshe O.C. ibid; Y. D. 1:60 [170 degrees]; Y. D. 2:52 [175 degrees]; Y. D. 3:31 [165 degrees]; Or Letziyon 2:32 footnote 12; Maor Hashabbos 1:2 in name of Rav SZ”A
 See Admur 306:18; Michaber 396:24; Piskeiy Teshuvos 306:26
 See Poskim ibid regarding these two methods who explicitly write that one may use these methods of the finger or one’s mouth to practically measure Yad Soeldes on Shabbos. See Ben Ish Chaiy ibid,; Kaf Hachaim ibid; Piskeiy Teshuvos ibid footnote 5