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May 22, 2010 08:35 PM

airtight lid = sour stock?

Julia Child recommends to never cover the kettle airtight or else the stock will

Does this make sense to anyone? Because I have a crockpot with a rubber seal to
keep in heat.

5 million Frenchmen can't be wrong!

A cook told me that professional cooks always leave stockpots uncovered or only partially covered.

Is this true? If so I'd like to know why.

Here's my guess FWIW:

An airtight seal with added heat can induce anaerobic fermentation, a chemical process that releases CO2, ammonia, and organic acids. Perhaps this chemical reaction produces the "sour" Julia Child is referring to?

I got this idea from :
and here

thx for any help!

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  1. As I understand making stock, you leave the stockpot partially covered in order to allow some reduction of the liquid, which results in a more concentrated flavor.

    1. I don't know about the stock going sour with an airtight light, but I've made stock at home both with a lid and without a lid, and I prefer the flavor that results from slow cooking with no lid - as lawmann states, the stock reduces as it cooks without a lid, for a more concentrated flavor. With a lid, little to no reduction occurs.

      1. thx for the replies. Are there any professionally trained chefs that would like to throw in their 2 cents?

        1. I am no professional but I have heard people make stock in a pressure cooker (=tightly sealed lid), and when I make vegetable stock I do it that way. Works really well.

          1. There are a lot of 'old wives tales' in cooking and this may be one of them.

            Anaerobic bacteria cannot survive at the temperature at which stock is simmered so the 'souring' can't be from that.