Yoreh De’ah Tidbits

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Times change and practices evolve but texts last forever. What follows are some random tidbits from the commentaries surrounding the sections of Shulchan Arukh about Meat & Milk and Mixtures (of permitted and forbidden foods). These are items that I find interesting for a variety of reasons.

  1. Three second rule — When food falls on the floor, many people observe a variation of the three second rule: if you pick it up quickly, we assume you may eat it because it did not get dirty. This is only convincing if you want it to be. More plausible, though, is the rule that it takes a few seconds for a hot forbidden food to affect a hot permitted food on which it falls. If you pick it up quickly, the bottom item remains permitted (Pischei Teshuvah 105:8).
  2. Sharing By Default — Personally, I object to other people, even friends, grabbing food off my plate. Evidently, this was not a universal sentiment. The Taz (88:3) states that sharing food on friends’ plates is common practice.
  3. Plates and Placemats — The Shulchan Arukh (89:4) forbids eating meat on a placemat on which one ate dairy. The Radbaz (Responsa 2:721), cited in Pischei Teshuvah (89:8), points out that eating patterns have since changed. In the time during which the authorities established this ruling, people ate food directly off placemats. Nowadays, the Radbaz wrote in sixteenth century Egypt, people eat on plates so we need only clean off the crumbs and turn the placemate over (as a stringency).
  4. Explain Extremes — In extenuating circumstances, the Shulchan Arukh (92:7) permits serving a meat dish cooked in a pot on which a drop of milk has fallen, provided there is more meat than sixty times the drop of milk (in normal circumstances, the rule is more complicated). The Taz (92:22) quotes Mahari Mintz (Responsa 15) as stating that a rabbi must explain to the congregant why he is ruling unusually leniently, so that people don’t get confused over his apparently conflicting rulings.
  5. Math Made Easy — Can a pot be constructed so that its contents are sixty times the volume of the pot’s walls? The Shakh (93:1) says that it can and the Pischei Teshuvah (93:1) quotes a Mirkeves Ha-Mishneh which claims to prove that such a pot exists through logic and something called Algebra.
  6. Rational Judaism — Must all customs make sense? According to the Rema, not necessarily. In describing the status of the lid of a pot in which a forbidden food has been cooked, the Rema quotes a particularly stringent opinion. In a memorable statement he declares that he follows this view because it his custom even though it is a stringency with no reason: “וכן אני נוהג מפני המנהג והוא חומרא בלא טעם.” Sometimes custom trumps logic.
  7. Where Do You Throw It? — And finally, what do you do with food that you not only may not eat but also from which you may not derive any benefit at all? The Taz (94:4) quotes earlier authorities who say that you must throw it in the outhouse. I find this particularly relevant regarding discussion of how to dispose of chametz on Passover eve when burning is not an option.

About Gil Student

Rabbi Gil Student is the Publisher and Editor-in-Chief of TorahMusings.com, a leading website on Orthodox Jewish scholarly subjects, and the Book Editor of the Orthodox Union’s Jewish Action magazine. He writes a popular column on issues of Jewish law and thought featured in newspapers and magazines, including The Jewish Link, The Jewish Echo and The Vues. In the past, he has served as the President of the small Jewish publisher Yashar Books and as the Managing Editor of OU Press. Rabbi Student serves on the Executive Committee of the Rabbinical Council of America. He also serves on the Editorial Boards of Jewish Action magazine, the Journal of Halacha and Contemporary Society and the Achieve Journal of Behavioral Health, Religion & Community, as well as the Board of OU Press. He has published five English books, the most recent titled Search Engine volume 2: Finding Meaning in Jewish Texts -- Jewish Leadership, and served as the American editor for Morasha Kehillat Yaakov: Essays in Honour of Chief Rabbi Lord Jonathan Sacks.


  1. I tried to work on that Mirkeves Hamishne a few years ago- about the walls of the pot/contents shishim ratio. I thought it should just depend on the thickness of the walls. Any of the actuaries out there…

  2. There is a great appendix in the “Kitzur Shulchan Aruch al Hilchos Basar be-Chalav” picture book that proves that this is essentially impossible… Granted, the geometry makes some rounding assumptions but the logic is pretty sound. I remember going through it with my chavrusa and then going home to ask my wife, the mathematician…

  3. When I learned Basar B’chalav 10 years ago, I was told that tanker trucks are the only keilim big enough that the contents are more than 60 times the walls.

  4. “There are two types of people: those who share food and those who don’t.”

    I guess that’s one for Yitznewton’s marriage compatibility list 🙂

  5. This is crazy. Walls don’t have volumes! If you wan to determine the mass of stuff (stuck to the walls) that leaches from the surface of the walls, it depends on factors such as the nature of the food and the “nature” and cleanliness of the of the walls. Regardless, it is easy to design experiments to calculate the amount (mass) that leaches out of walls, and determine if that is 1/60 of the mass or volume of the contents. But it doesn’t matter, bc minhag trumps halacha and math every time.

  6. assuming one takes into account the volume of the wall (which is how i was taught the 60 times rule) it’s pretty obvious that a theoretical 1 molecule thick wall etc…..
    btw r’hs frequently says the rabbis should get together and change the rule by stainless steel due to its lack of absorbtion.

  7. Here’s the algebra (assuming I got this right):

    r=radius of pot

    1) volume of pot >= 60 * volume inside pot

    2) pi*r^2*h >= 60 * t* (pi*r^2 + 2*pi*r*h)

    3) pi*r^2*h >= 60*t*pi*r^2 + 120*pi*r*h

    4) pi*r^2*h – 60*t*pi*r^2 >= 120*t*pi*r*h

    5) pi*r^2*(h-60*t) >= 120*t*pi*r*h

    6) r*(h-60*t) >= 120*t*h

    7) r >= 120*t*h / (h-60*t) where h > 60*t

  8. Gil, the thickness of the wall is irrelevant. Food only adheres to the exposed metal. In the old days, when iron pots were used, I suppose that rust/corrosion could make the wall surface a lot more adsorbent (not absorbent). You also need a “coefficient of adsorbence” (Cad) for each food/pot pair.

    Psa = surf area inside pot
    Fd = density of adsorbed food
    Pv = pot inside volume
    Cadpf = coefficient of adsorbence for a particular pot and food
    Fv= volume of adsorbed food (I assume we need an food vol (adsorbed) to pot vol ratio of 1:60)
    Fv >= 60 * Pv
    Fv = (Psa*Cadpf)/Fd
    Psa = 2*pi*r*h
    Pv = pi*r^2*h

    Now all you need is a published table of Cadpf, or derive your own experimentally. That is left as an exercise for the reader.

  9. Not sure if the starting assumption of your math is right. On the RHS second term, you are assuming that the volume of the cylinder is the outside circumpherence x height x thickness (2*t*pi*r*h). And to that you add the volume of the bottom of the pot (t*pi*r^2), which is correct.

    I say that the volume of the cylinder is the difference between the area of two concentric circles, one with radius smaller by t, times height or h*(pi*r^2 – pi*(r-t)^2). I haven’t worked it through the rest yet, but I don’t think it will come out as neatly in the end.

  10. Forgot the bottom of the pot:
    Psa = 2*pi*r*h + pi*r^2

  11. Gil, the thickness of the wall is irrelevant.
    This is not what I was taught-the concern is what is absorbed into the walls over time and the maximum possible absorption is the issue -thus you’d calculate the volume of the walls and need 60 times that. The interesting (yes-to me) question was always what if that was say the volume of 20 zeitim and you know you had only used the pot to cook 5 zeitim so far in its life.

  12. Joel,
    I think that you and many others were taught wrong. “Absorption” is a misnomer. Metals do not absorb, they adsorb. If you don’t understand the difference, then you should look up these terms. You can easily measure the amount of adsorption by weighing the empty and dry pot before and after cooking. It will be a trivial amount (<<1/60) if the pot is new and clean.

  13. OK, so this raises the question of what is the halachic concern with regard to the pot that raises the whole 60th issue.

    Seems to presume that the entire volume of the pot’s metal becomes milchig (or flaishig) based on a recent previous use and you accidentally used it for the Flaishig (or milchig)??? And so we want to compare volumes?

    I thought that it’s assumed that the food on the inside won’t halachically affect the outside (eg. take a brand new pot, pour in hot meaty fluid, if carefully poured wouldn’t the outside remain parve at that moment?)

    Or is it as AL is saying and you need to calculate the amount adsorbed to the inside and thickness is irrelevant?

  14. AL,

    The metal doesn’t absorb itself, but the metal is porous. Thus fat or oil is “absorbed” into the pores in the metal. Treating cast-iron pans with oil would be pointless unless the oil were taken in by the pores. I’m not sure why you are mentioning adsorption here — that doesn’t seem to be the issue at hand. (Presumably, you could measure the porosity of a given metal type and then know the maximum volume of material that could be “absorbed” by a given volume of that metal)

  15. I don’t think the 1/60 thing has anything to do with kashrut of pots nowadays. A misunderstanding of absorption may have applied back in history, but now we have this minhag. In the old days, I bet a lot of stuff really did stick to the pot surfaces, so there was some backing for the 1/60 rule. And not everyone cleans their pots so well, so it may still be a good idea.

    But to claim that metals aBsorb any significant amount of food? That’s ludicrous. Show me the data.

  16. I have heard it said — maybe even by Rav Schachter, but I’m not sure — that these laws are no longer scientifically accurate and “we have a problem”. I don’t think it is sufficiently urgent to change them, if we can.

  17. Skeptic,
    Metals are definitely not porous. Even a pot made of thin aluminum foil would not get wet on the outer surface. Oil is adsorbed to the surface, probably in a layer no more than a few molecules thick after a good washing. If the metal is rough or scratched, the surface area is greatly increased, and thus more can adsorb to the surface. But it’s still a tiny amount in a clean pot.

    In the absence of data, there is no reason whatsoever to deviate from observable facts, which you can easily verify for yourself.

  18. So are we saying that what I said at 3:24 pm about the outside vs the inside of a pot is scientifically true but not halachically?

  19. “Metals are definitely not porous”

    Since when? Metals have defects in them, which occur during the smelting process. If you look at a metal surface under magnification, you see pores. Am I missing something?

  20. So are we saying that what I said at 3:24 pm about the outside vs the inside of a pot is scientifically true but not halachically?

    AIUI in a word – Yes.

  21. ” If you look at a metal surface under magnification, you see pores. Am I missing something?”

    I think the point is that what you call “pores” are just surface irregularities. They may increase the surface area, but they do not imply that the internal structure of metal is “porous,” i.e., contains voids that are or can be filled with a non-metal substance (e.g., chicken soup). A sponge has “pores” and one could reasonably, per current scientific knowledge, be concerned that chicken soup seeped into its entire volume. A metal pot, not so much.

  22. If you really want to have fun, look into the subject of porcelain. Many (all) of us were taught that the reason it can never be kashered is that it is porous, unlike glass. Well, ask a material scientist…

    And whole Pyrex vs. glass kashering mythology is also an interesting one.

    And, finally, the difference in how “formica” surfaces are treated in the US vs. Israel.

  23. I agree with R’ Gil’s derivation of the volume of a cylindrical pot relative to the volume of its walls. It would, however, take a very large pot with thin walls to achieve the 60:1 ratio. The formula derived by Gil also holds for a square pan, and a similar formula can be derived for a hemispherical vessel. The cooking vessels in use at the time of the Shulchan Aruch and its commentaries were, I believe, thick-walled. In which case, a ratio of 60 was unabtainable. The argument of Toronto Yid on calculating the volume of the walls by subtracting the inner pot volume from the outer one is correct, in principle. However, the thin-walled case reduces to the simpler formula for wall volume given by Gil, i.e., approximated by the area of the walls x the thickness.

    Al may be correct about the reality of taste absorption by the walls of a metal pot – particularly with stainless-steel cookware. Nonetheless, the Ashkenazi practice still follows the Rema who maintains that metals absorb throughout their volume. I presume that this, rather radical, assumption was based on the cast iron or bronze cookware of his time which may have been characterized by surface roughness and microcracks. It certainly doesn’t characterize stainless-steel cookware which are shiny and remain so after much use. Aluminum ware, in contrast, will become pitted, scratched, and stained with use. It would be a boon to caterers and homemakers to reflect reality and to treat stainless-steel as a non-absorbing material, but that would require acquiescence of major poskim. A similar situation would exist with glassware where the Mechaber declares that it is non-absorbing, while the Rema states that it absorbs like porous ceramics. Perhaps the glassware seen by the Rema in Cracow was molded and had a rough, blistery finish, it is certainly not true of more modern glassware. I suspect that the practice is to treat glassware as per the Mechaber rather than the Rema.

  24. Fortunately for Jews, we live in an age where the cost of utensils is so inexpensive that we can ignore the bad science that led to the minhagim we keep. But, we should be honest in how we teach the subject and admit that we our practice is now based on years of minhag rather than based in scientific fact.

    That said, poskim should also be apprised of the scientific facts as we know them today (e.g. minimally to enable a kula in for those in need).

  25. IH, I have some background in glass chemistry and physics, and agree with your suggestion about the reality of porcelain. A true porcelain is made of a white clay mineral that is fired at a sufficiently high temperature to sinter the clay particles and to provide a glassy surface. That surface has the non-absorbing properties of clear glass. The same is true of our stoneware dishes. They are made of clay fired at a lower temperature than porcelain. Consequently their body is porous. However a glaze is applied prior to firing. The glaze melts to form a glassy non-absorbing surface. Halachicly, however, no distinction has been made between such non-porous ceramics and the older porous ones.

    The distinction drawn by some between Pyrex cookware and glassware used at ambient temperature has no basis in reality – as you alluded. The basic difference in compositon between ordinary glass and Pyrex is the substitution of sodium borate (borax) for the sodium carbonate(soda) and lime of ordinary glass. The dominant ingredient in both materials is quartz sand. All such glasses are sand products and not clay products. They are therefore quite different materials both in composition and properties.

  26. Perhaps this is a business development idea for the growing Kashrut business: hechsherim on dining/kitchen-wares that can be used like glass from a kashrut perspective.

    You heard it hear first! 🙂

  27. As I mentioned – R’HS really does agree in principle that the halacha should change based on our “insert proper adjective that won’t insult chazal or anyone else” understanding of the facts. Unfortunately (I say with a real tear) we don’t have the mechanism to do this (R’HS seems to say that it would take “all the Rabbis getting together” to agree and change this – of course this brings to mind that such an event will certainly herald the coming of Mahiach.

    Keep in mind that this would take away a lot of the “fun” of learning hilchot kashrut 🙂


  28. As I mentioned – R’HS really does agree in principle that the halacha should change based on our “insert proper adjective that won’t insult chazal or anyone else” understanding of the facts. Unfortunately (I say with a real tear) we don’t have the mechanism to do this (R’HS seems to say that it would take “all the Rabbis getting together” to agree and change this – of course this brings to mind that such an event will certainly herald the coming of Mahiach.

    Keep in mind that this would take away a lot of the “fun” of learning hilchot kashrut 🙂


  29. Joel, that strikes me as being too broad an assertion. The Pyrex rules, for example, are an entirely modern affair.

    And with greater knowledge about the science, as new materials are introduced into the market, they can be sanctioned in their own right (instead of falsely corresponding them to specified older materials where it is harder to change the halacha despite updated knowledge).

    But, this comes back to the general hashkafic issue of whether one thinks about halacha as a museum, or as a living/breathing/adapting system.

  30. There is also the issue of kulot for those in need. Thank God many of us are able to be machmir because: a) the dining/kitchen-ware is relatively inexpensive; and, b) we can afford to be machmir.

    But, let’s not forget there are Yidden who are not as fortunate and for whom a decision as to whether something is accidentally treif’ed is not insignificant.

  31. R’ Joel,
    regarding your comment The interesting (yes-to me) question was always what if that was say the volume of 20 zeitim and you know you had only used the pot to cook 5 zeitim so far in its life.

    See the Shulchan Aruch 98:5 (and Rama 92:5), we only need 60x the 5 zeitim.

  32. I fail to understand the great reluctance to modify halachot when the physical reality should dictate a change. While I appreciate the fact that a stable legal system must perforce be conservative, that doesn’t mean that it can and should be static. A static legal system must become increasingly unrealistic and irrelevant.

    Why, for example, should anyone treat Pyrex cookware the same as pottery? The fact that a leading decisor of the past generation mistakenly treated it as such, should be irrelevant. Why, also, should the pesak of the Rema about the metal vessels in use in Cracow 500 years ago govern new, non-absorptive metallic cookware such as stainless steel (ss)? While there is no inclination to argue for the use of such cookware for both milchig and fleishig, occasional inadvertant mixed usage is another matter. Instead of their being virtually no chance of finding shishim k’negdo if the entire metal volume is assumed to consist of the problematic species, we should always find that there is more than enough of the contents of the ss pot to nullify whatever miniscule amount is left on the walls of a pot. Why are major poskim reluctant to rule that way? It seems to me that making life easier for observant Jews is more important that avoiding controversy.

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