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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.

            1. Could she possibly be referring to cooling or storage rather than cooking?

              I once bought a wide-mouth thermos with a plastic interior, so I could bring soup to work for my lunch. It didn't retain heat well enough, and on a couple of occasions when I took homemade chicken or turkey soup with vegetables, including cabbage, it was fizzy and souring by lunchtime. I blamed the cabbage at the time.

              1. Stock simmered uncovered or partially covered allows reduction of the liquid and concentration of flavor, as lawman mentioned. Partial covering allows for more even maintainance of cooking temperature. Chill uncovered and as rapidly as possible, to prevent spoilage. Stock covered completely while cooling can sour, and this is what I think Julia was referring to.

                There's no way you can be guaranteed an absolute air tight seal necessary for anaerobic fermentation with a stockpot or even a slow cooker, to create the necessary oxygen free environment, to say nothing of the temperature. I don't believe that's what Julia was talking about. Btw, I doubt Julia knew much about anaerobic digestion or biogas production; not her forte.

                I think what Julia may have been referring to as well, and I have seen this happen in a professional setting, is that the stock simmered, (and I hesitate to call it simmering as that indicates a temperature of at least 180-200°,) at too low a temperature, say, between 140-160°, contains bacteria, and left too long at that temperature, sours. This happens largely due to inattentiveness or inexperience on the part of the chef. Aerobic fermentation requires a sustained temperature of 140-160° to occur. Not chilling properly and in a timely manner will cause stock to sour as well.

                I have to say that somewhere in the back of my mind, I seem to remember someone in a professional setting telling me not to cover the stockpot completely while simmering or the stock will sour. But from what I've learned about stock making, it doesn't make sense.
                As John E. wrote upthread, this may be an old wives tale; as long as the stock simmering temperature is maintained above a safe level and is chilled properly, fermentation risk is very low.

                4 Replies
                1. re: bushwickgirl

                  ahhh ok. Thx for the great answers! greygarious and bushwickgirl are probably right on.

                  With an airtight lid anaerobic fermentation could happen during a prolonged cooling period, or while at a temperature below simmering. Careful simmering overnight if it gets cold!

                  Julia's words:

                  "Partially cover the kettle, leaving a space of about 1 inch for steam to escape."

                  "Never cover the kettle airtight unless its contents have cooled completely...".

                  ...and how completely droll to suggest "my" expertise with anaerobic digestion and biogas production! You are simply the most delightful character. My word. And such marvelous advice; absolute scientific workability. Warble blarble brurble. Bon apetit!

                  1. re: limesatpirate

                    Marion Cunningham, in The Fannie Farmer Cookbook, also warns against covering cooling stock and writes," If stock is covered tightly while cooling, it may turn sour." Marion and Julia were on the same page.

                    FYI, Ms. Cunningham received the Grand Dame award from Les Dames d'Escoffier and Julia Child received the French Légion d'honneur, both great lifetime acheivements.

                    1. re: bushwickgirl

                      If CH ever institutes a Légion d'Hound, I nominate vous!

                2. In an episode of the Jacques Pepin/Julia Child series Julia kvetches about people closing the lid of the kettle tight and says it makes the stock/soup sour. Jacques replies that he's never heard of such a thing. Take your pick.

                  3 Replies
                  1. re: morwen

                    I vote Jacques. After all, Julia also said female eggplants have more seeds. There's no such thing as eggplant gender. She also said frozen peas are inferior to fresh and apparently meant supermarket fresh, though because frozen peas are frozen right after picking, they retain more sugar than supermarket fresh, in which the sugars have been converting to starch since harvesting. Jacques explained that when peas are washed after shelling, the babies, which have the most sugar, float and are then frozen, so they are very good. He's right on that.

                    1. re: greygarious

                      Yes, I'm with Jacques as well. I can't understand where Julia got that stock souring idea. Five million Frenchmen? No.

                      It was the eggplant gender thing that decided it for me.

                      Not to hijack this thread, but here are some interesting comments by learned folk about eggplant sexes, or lack thereof:


                    2. re: morwen

                      Julia was talking about potatoes, not stock! The exchange you mention occurs in the "Potatoes" episode of the "Julia & Jacques: Cooking at Home" series.

                    3. McGee advocates (oven) braising without the lid, claiming that it is easier to keep the temperature in the simmer range this way. With the lid on, the liquid can get hotter (less heat loss to the air and via evaporation), and rise to a (slow) boil.