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I'm making veal stock that's later going to be reduced to a glace viande.

In the past, I was always close enough to observe the process and strain the stock when done. I also realize that an hour or two and even longer isn't an issue when making stock.

But today I will be away for most of the day and my stock pot will remain unattended on the lowest possible heat on the stove. I don't expect any problems from this procedure. But it prompted a question:

How long could a pot of bone stock be simmered?
Is there an upper threshold when unwanted substances are extracted from the bones? When the flavour profile does not get "better" but starts to deteriorate?


Clarification:

  • I am not talking about cooking a day or two, I know that this is typically no problem. And yes, if necessary I could add water to counter evaporation. For this question, you may assume "no time limit".
  • I am familiar with the concept of perpetual stew. But for a perpetual stew, parts of the stock are removed and new ingredients added. I'm asking explicitly about one unchanged stock: a pot of bones in water, optionally some spices and mirepoix.
  • I'm fairly certain that I've seen evidence that there is a maximum time, and that the gelatin you want actually starts breaking down at some point. I will try to dig that up, assuming I'm remembering correctly. – Josh Caswell Dec 25 '17 at 21:35
  • It might be helpful if you could describe the actual goal you have in mind. Are you trying to maximize flavor extraction from the bones? Are you trying to get as much gelatin as possible? Are you just trying an experiment? What's the reason for wanting to run a stock pot for a very long time without replenishing the flavoring ingredients? – barbecue Dec 28 '17 at 22:04
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You could implement a perpetual stock, where you keep it simmering indefinitely and renew it by adding fresh ingredients over time. The tradition of keeping a perpetual stew boiling for weeks or even months dates back centuries, and was a way to keep the ingredients from going bad in days before refrigeration.

https://alehorn.com/2016/01/07/is-medieval-perpetual-stew-for-you/

If you just want to simmer a single pot of stock indefinitely, you can do that safely by adding water on a regular basis in small quantities (small enough that the stock temperature does not drop below 140 degrees) but if you're not using some and replacing the ingredients, over time (several days) the stock will eventually acquire a sort of acrid taste (speaking from personal experience.)

I think the issue is that there's a limited amount of the aromatic compounds which produce the desired flavor, and over time, these tend to evaporate. Replenishing water doesn't replenish the flavor. The smell of stock simmering means those aromatic compounds are now in the air and no longer in the pot. Eventually, the amount of nice flavoring compounds leaving the broth to the air exceeds what you can extract from the remaining bones etc. Oxidation of the remnants leads to a less pleasant flavor. There is also the question of the pot itself adding metallic flavors if the stock is at all acidic.

  • The last sentence at least partly answers my question. Sorry if the initial question was somewhat unclear re. perpetual stock. – Stephie Dec 25 '17 at 18:21
  • Added a bit more info. I couldn't find any citations to support my claims, but I'm sure they are out there. Will try to add more . – barbecue Dec 25 '17 at 18:41
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A book I used to have (called simply soup) said the best stock they ever made in their restaurant was when they left a pot simmering for two days unexpectedly.

Making stock in a slow cooker is typically done for a minimum of 12 hours on high, and I often leave it longer so the cooling down takes place at convenient time. For dark stocks (as I usually make) the flavour doesn't deteriorate. Lighter stocks may well darken.

The limit is really evaporation - however well the pot is covered, if the stock is simmering or even soaking at a lower (but still safe) temperature this will be significant. To some extent it doesn't matter, but want the ingredients to be mostly covered, and you certainly don't want it boiling dry.

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I find when making a chicken stock w/ wings, a drumstick and spine/backbone/trim, there's really no point in going over 4 or 5 hours. Add mirepoix in the final hour, and it should have enough flavor. I find that simmering the bones and onion and various veggies for 6-8 hours seems to dull the flavors.

If you want to strengthen the flavors, you should just Reduce the stock.

But if you're going for some kind of Mythical Ancient Village style stock, you could keep it going for days and days and days, topping it off with water. I could see this working better if you actually replaced the boiled-off simmering ingredients with fresh ones to fortify the stock.

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I've made all my own stocks for forty years and finally found I got the best results from making them in a pressure cooker in an hour or less after first roasting the bones and vegetables in the oven.

  • Thanks for your contribution. I absolutely know how to use a pressure cooker and my question is not about making stock per se, but about cooking much longer than necessary. – Stephie Dec 26 '17 at 18:31
  • Sorry to have wasted your time, I refer to flavor, not cooking off all the aromatics which prolonged cooking does. – user23186 Jan 1 '18 at 22:39

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