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Throughout all of Yoreh Deah Chelek Alef and Hilchos Pesach (and perhaps other places in Shulchan Aruch as well) we see the idea of "blias" ("absorptions") by celim. That is to say there is a "taste" that is left in a pot or pan after cooking in it. Therefore if someone would cook meat in a pot, then within the same day cook milk in it (even though there is no meat in the pot now), still there is a "taste" of meat that was left inside of the pot walls when it was first cooked. That taste will now come out of the pot when cooking milk in it and will prohibit the milk.

This is all true of a metal pot. However, a glass pot, according to the Machaber, doesn't take in any taste at all. On the other side of things, if one has a pot made of "cheres" (earthenware), he would never be able to expunge taste from it once it was cooked in (not like a metal pot which one could kasher by making ha'galeh on it).

I'm having a difficult time understanding how this din (rule) is still relevant to today's pots. Is there really a taste left inside of a metal pot? Once upon a time a metal pot that was cooked in over time would slowly become dirty, grimy, filthy, covered in black, clearly used. One could look at this and understand that there is a "taste" left in the walls of the pot. However, I don't see how this is true of a normal metal pot in today's market, which is smooth, finished off, and more or less retains it's original color and look and stays clean over time. Where is the "taste" in the walls of this pot? If someone would cook milk or meat in it and then cook water in it, would the water have any taste to it (assuming the pot was cleaned in between)? While I don't see any posek (halachic decisor) using this argument even as a "tziruf" (argument in combination with other leniencies) to be lenient in a question of issur v'heter (forbidden combinations), still, I'm looking for any poskim that perhaps spoke about this or entertained the possibility. Maybe today's "metal" pots are more like a glass pot, which the poskim say doesn't retain any taste of what's cooked in it.

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Thank you to @msh210, who addressed my first concern. On my second concern, I still think pans might be different from pots (assuming you are correct). Frying definitely leaves behind something you can see over time, at least with metal pans; Teflon® might be different. –  Seth J Feb 14 '13 at 15:04
    
Incidentally, Yehoshua, the Rema (and therefore, Ashkenazim) hold that glass is like חרס. Glass would certainly show a visible absorption, if what was visible was the concern. At least for the Rema, it would seem that visibility has nothing to do with it.. –  HodofHod Feb 14 '13 at 15:59
    
@HodofHod Some Achronim only apply that Rama for Pesach. Also, it seems from the wording of the Mordechai that he quotes that the issue is a gezera one, not real blios. –  Double AA Feb 14 '13 at 17:16
    
@DoubleAA Where is the Mordechai? Where is this quoted? (doesn't seem to be the Rema in SA. Maybe the Darchei Moshe in Tur/Beis Yosef or is this somewhere else? –  Yehoshua Feb 16 '13 at 22:18
    
@DoubleAA ^^^^^ –  Yehoshua Feb 17 '13 at 13:02
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2 Answers

From a Kosher Spirit interview with Rabbi Chaim Cohn:

KS: Can you share a unique experience that you had while working at the OK?

RCC: I once had an argument with a plant engineer concerning whether or not stainless steel can absorb or not. He maintained and brought extensive documentation to prove that stainless steel can’t absorb anything and therefore should not need kosherization. I told him, your proofs are impressive but the Rabbis decreed 2000 years ago that metal needs kosherization and I do not have the authority to go against them. We agreed to differ.

Approximately a year later, I was in the process of supervising the kosherization of a 1000 gallon reactor and after the workers had meticulously cleaned it we brought it to a boil. There was about a quarter of an inch of scum on the top of the pot. The engineer was passing by and I called him over to look at it. I said, “This was a completely clean pot.” I turned to the workers and asked, “Right?” The workers nodded their heads vigorously up and down. The engineer looked at it and said, “ I can’t argue with empirical evidence,” and walked away.

Six months later I got a call from the engineer. He said, “Rabbi Cohn, I now understand.” “Great,” I thought, and asked, “You now understand what?” He answered, “I understand why a stainless steel vessel will absorb.” I asked, “Why?” He answered, “You have to understand, when it leaves the factory it can’t absorb anything, however, when it is heated it expands and contracts and it will produce micro fissures. Your Rabbis were right, you can’t measure this and therefore have to assume the worst-case scenario that the entire volume has absorptions in it.”

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But Maasim BeChol Yom that I clean a pot with soap and water and cook other food in it without the slightest fear it will affect the taste. If anything is absorbed it's next to nil. –  Double AA Feb 14 '13 at 17:41
    
That's the way gezeiros work -- sometimes there is a concern, sometimes not, but Chazal couldn't leave it up to each individual to decide, so they made a universally-applicable law. –  Dave Feb 14 '13 at 17:59
    
I'm fine conceptually with saying it's an arbitrary gezera that sometimes we do even when the reason doesn't apply (I'm not sure that's accurate in this case, but I'm fine with it conceptually). But you seem to be arguing in the story that the reason does apply to my stainless steel chicken soup pot. –  Double AA Feb 14 '13 at 18:09
    
Blias by a metal pot is only a gezera? –  Yehoshua Feb 14 '13 at 18:18
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@DoubleAA - according to the story, the reason could indeed theoretically apply to your stainless steel chicken pot. Not that it will happen every time with every person and every pot, but the possibility does exist. –  Dave Feb 14 '13 at 18:26
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A kashrus inspector told me the following "Very little, if any, perceptible ta'am exists in modern equipment. For most production, even a minimal washing (not heat sensitive) is considered adequate. Equipment used for flavor and flavor chemical production will sometimes be steam cleaned for days to ensure no residue of the prior flavor. Also, only glass is considered to be 100% impenetrable, metal are considered to allow some penetration through them."

I heard in the name of an unnamed great rabbi that when Moshiach comes, he is going to make some big changes in halachos like this. The halacha assumes the entire volume of the pot's walls and base are full of absorbed taste and therefore the volume of food inside the pot can never have 60 times the volume of absorbed taste in the walls. That was never possible even with the oldest pots and even more unrealistic today but I have never heard of a posek using any of your observations to be lenient.

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Which Hashgacha, so I can avoid it :) –  Shmuel Brin Dec 11 '13 at 18:00
    
@ShmuelBrin, what is the problem with the statement? (If it means "considered adequate" from a production perspective - a company won't care to do more if not for Kosher requirements). –  Yishai Dec 11 '13 at 19:52
    
@Yishai I understood it to mean that the Hashgacha won't kasher equipment (as it doesn't absorb) –  Shmuel Brin Dec 11 '13 at 20:01
    
ShmuelBrin, I understood it like @Yishai. –  David Wave Dec 12 '13 at 20:59
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