How Hot is Too Halachically Hot
Yad SoLeDes is the point at which Halacha deems we have sufficient heat to initiate cooking. This is relevant to the laws of cooking during Shabbos and also Kashrus. It is not defined by the boiling point of water or any metric we are familiar with today in our scientific empiric world of precision. It is an approximation, as are all the measurements Halacha works with - the size of an olive, the time taken to walk a certain distance, the time taken to eat a certain amount of a particular type of food, etc. Yad Soledes refers to heat which is uncomfortably hot.
Today, Hlacha is seeking to be as accurate as the world and society in which we live; otherwise we appear to be unsophisticated.
Today, Kashrus must accurately identify temperatures in food production facilities that are manufacturing and/or processing both Kosher and non-Kosher on the same machinery. If the temperature is below Yad SoLeDes, a surface clean will suffice to ensure there is no contamination from the non-Kosher to the Kosher. However, if the food is processed at Yad SoLeDes or higher, then we must deep clean to remove the non-Kosher food flavours that have been absorbed in the machinery. This is known as Kashering.
Typically, a snack food may be processed with a variety of flavours, some being Kosher whilst others are not Kosher. The base product, potato, rice or pop-corn etc. is Kosher, as are most of the flavours. However, cheese, chicken, beef and balsamic vinegar flavours are not Kosher. The trouble is that the base product is cooked plain and coated afterwards, often in a tumbler in which the flavours are sprayed on. Invariably this can only be effectively accomplished when the product and the flavourings are warm or hot.
Not only is this machine itself Kashrus compromised but the conveyors which carry the hot non-Kosher product from the tumbler also become Kashrus compromised and can Halachically infuse non-Kosher flavour to all the other products that come into contact with that machinery.
The CRC declare that the temperature of Yad SOLeDes is as high as 120°F or 49°C, however, they have problems measuring this temperature in the situation described above. Infra-red thermometers are unreliable, and these type of snack products mostly do not lend themselves to be measured with a regular probe.
The CRC, in a surprising move, instructed the Mashgiach to prepare a couple of hot water samples between 100°F and 140°F and exercise himself to become familiar with the varying temperatures and then use his hand felt evaluations to determine if the foods in the factories are more or less than Yad SoLeDes; see article. Presumably, this harks back to the origins of all measurements in Halacha being approximations.
The CRC however, will not employ this approximation universally. It argues that although our Sages, Chazal, certainly determined Yad SoLeDes in such a manner, it is nevertheless appropriate to use more accurate methods wherever possible. [In truth this makes little sense because the temperature they have chosen as Yad Soledes is itself initially determined by such approximation. What they really mean is, they choose a temperature which they are confident is not Yad Soledes and in so doing are confident that even when employing the approximations of the Mashgiach, they are confident they are nowhere near the actual temperature of Yad Soledes]
So they still maintain that in this situation measuring by hand is "safe" because
a) other options are not possible since the product cools so rapidly [this really is a little confusing, as the CRC appears to be suggesting that the product is possibly more than Yad SoLeDes yet it cools so rapidly that a mechanical or electrical temperature device will not detect what can be detected by the hand of the Mashgiach - it is a great mystery why the hand can detect the momentary hotter temperature whilst the device cannot.]
b) they were making a determination that would be as strict as possible. They would need to be absolutely certain that temperatures were less than Yad SoLeDes. Had there been any doubt by the Mashgiach, the CRC would not have Kosher certified the product. [This is also quite strange; they might just as easily have used the infra-red and included a safety margin to account for any suspected unreliability. In fact these devices usually are provided with guidelines that identify the margin of error, which most likely would be a far more efficient and more accurate method of determining the temperature]
See Igros Moshe Orach Chaim 4:74, that 110°F (43°C) is to be used as Yad SoLeDes although in fact it may be as high as 160°F (71°C) This variation is relevant in cases that are BeDiAvad, in cases of Safek and in cases of DeRabbanan.
Rabbi Sh. Z. Auerbach proposes a compelling method to accurately determine Yad Soledes - he notes that body temperature of a healthy duck is almost 45C whereas people are generally 37-38. A sick duck will be hotter. Furthermore, the Gemara asserts that Shechita prompts the temperature to rise; Rabbi Auerbach assumes this means at least one degree more.
Rabbi Auerbach accordingly calculates that 48C is less than Yad Soledes.
[Chullin 8] Rav rules that a beast slaughtered with a Nochri's knife, will become tainted by the non-Kosher knife. The reason for this is that a living beast is warm and the incision is additionally heated by the consequential flow of blood. The remedy is to shave off the layer that was in contact with the knife.
However, Rabah bar bar Chanah disagrees, ruling that it suffices to rinse the incision.
The Gemara proposes that they argue about whether the place of Shechitah is RoSeAch, very hot, or not.
However, the Gemara counters that perhaps there is agreement that it is RoSeAch. Rabah bar bar Chanah rules that rinsing is adequate since the heavy bleeding prevents flavour moving from the non-Kosher knife into the flesh.
Alternatively, there is agreement that it is not RoSeAch. Rav however, requires cutting off a layer since the pressure of the knife forces non-Kosher flavour into the meat. See Minchas Shlomo below.
Similarly, when a beast is Shechted and found to be a Tereifa, there is concern that the knife has absorbed some non-Kosher flavour from the Tereifa. Rav Acha bar Yakov and Ravina argue about the remedy. Does the knife require HagALah (Kashering in boiling water) or is a cold water wash adequate.
However, this is an issue even when Shechting a Kosher beast since the knife will absorb Eiver Min hHaChai (a limb of a living animal, before the Shechita is completed) this Issur only being removed at the conclusion of the Shechita.
The Gemara explains this is not a problem since the incision only gets hot at the conclusion of Shechita, at which time the beast is Kosher.
During Shabbos one may not cook oil but we may warm it. The Gemara [Shabbos 40b; R. Shimon ben Gamliel] describes and permits the practice of applying oil to ones hands and then warming them and the oil on them, before a fire. The warmed oil was applied to the bodies of infants. So, whatever temperature an adult can in normal circumstances tolerate when heating them before a fire, is less than Yad Soledes. We can safely assume that this would be significantly higher than what would be comfortable for, or even cause a burn to, a baby or infant.
However, the Gemara seems to take a conservative approach, measuring Yad Soledes by what is safe to apply to a baby. Furthermore, it proposes by what is safe to apply to a baby's stomach, a more sensitive part of the baby.
Shulchan Aruch (OC 318:14) defines Yad SoLeDes as heat that will scald an infant's stomach.
The Gemara Chullin [see above] documents a Machlokes if the Beis HaShechitah is cold or very hot, RoSeAch.
The Rema rules that Iruy suffices to Kasher a Shechitah knife because the heat of the Beis HaShechitah is not really equivalent to a Kli Rishon that is Yad SoLeDes. When the Gemara describes it as RoSeAch it means it is only slightly RoSeAch.
There are at least 15 opinions see,