In the realm of kashrut, yad soledet bo is significant because it is at this temperature that a liquid, a food item or a utensil used for food preparation begins to transfer its “flavor.” Since no transference takes place in cold mixtures, a cold dairy spoon placed in cold leftover chicken soup will not be affected in the same way as a cold dairy spoon placed in a hot chicken soup or a hot spoon placed in a cold chicken soup (or hot into hot). – Please note that there are other ways that “flavor” can be transferred (be careful with pungent or spicy foods and sauces!).
On Shabbat, recognizing the temperature of yad soledet bo is important because of the prohibition of cooking on Shabbat. While one may place a liquidy food near a source of heat to take off the chill or warm it to room temperature, causing it to reach yad soledet bo would be considered cooking. – Again, please note that this is a simplified summary.
Because of the significance of yad soledet bo, rabbis have continually attempted to define it, especially as the need to withdraw one’s hand is extremely subjective. The Talmud (Shabbat 40b) defined it as being hot enough to scald the skin of a baby’s abdomen. Later sages, having ways to actually measure temperature, have expressed a range of opinions from 110 degrees (43 Celsius) to 160 degrees (71 C). The wide range of definition allows one to be stringent in both how hot a cold item becomes or how low the temperature on an item that was yad soledet bo must be to be considered to have cooled off.
NOTE: As with all Treats dealing with halacha (points of Jewish law), one should consult one's local rabbi for practical application.
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