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Cool stock before covering ... why?

I understand that you're supposed to cool stock before covering and refrigerating or freezing. What I've read is that it will sour if you cover it while it's still hot; have always wanted to taste-test that, but haven't yet.

My question is - WHY does the stock sour? What reaction happens when you cover the stock before it's cool? And will that reaction also happen if you cover stock-based soups, gumbos, etc. if you cover them while still hot?

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  1. My mom told me to do this (allow it to cool) to allow a layer of fat to form on the top of the stock as it cooled--a natural flavor seal and preventative against bacteria (air can't get it as much).

    I know nothing about souring.

    1. There is a post on chow hound about this

      Anerobic fermantation

      I don't know if it is true

      1 Reply
      1. re: Alan408

        Sounds like myth to me. First of all, stock is usually too salty to support much reproduction of microorganisms, and it's going to be refrigerated or frozen relatively soon regardless of whether it's pre-cooled with the lid off or on. Despite the fact that the stock has been boiled and covered, I have to believe there is enough free oxygen remaining that covering the pot is not akin to what happens during true pressure canning or "retort sterilization" as I think the food processors call their food canning procedure. There of course is the possibility of botulism and that sort of thing in true canning procedures where the food item is completely deprived of oxygen. But stock, even if well-boiled, is still mostly water, and if it's not totally sealed as in a canning procedure, it's hard to imagine that no free oxygen/air remains.

      2. I was always told to leave the lid off while cooling because it allows the heat to escape so it cools faster, which makes sense.

        I've never heard anything about "souring."

        8 Replies
        1. re: weezieduzzit

          I'm curious if "souring" means literally just TASTING sour, or souring as in possibly going bad.

          My mother always told me to cool the stock (or soup) with no lid because it will cool faster. Also to portion out into smaller or more shallow dishes, and then refrigerate as soon as it's cooled near room temp. The idea being to chill the stock as quickly as possible. In a stockpot, the stock will cool quickest on the surface or outsides, but the center of your pot will remain hot for far longer than 2 hours (the "safe" zone for holding warm food).

          When I make soup, I ladle it out into shallow dishes and it's cooled to room temp within 30 minutes. Then I refrigerate or freeze in Ziplocs in smaller portions.

          Have you ever noticed how many people get food poisoning from purchased soups? I've read this is usually due to improper (slow) cooling of mass-produced soup. The soup in the center of the pot has plenty of time to grow bacteria while it's cooling.

          1. re: nothingswrong

            I've actually never heard of anyone getting sick from purchased soup. I set the stock pot in a sink of ice water and toss a frozen bottle of water in it. It cools very quickly and then I portion and freeze.

            1. re: weezieduzzit

              The was a famous (and tragic) case of people being fatally poisoned by botulism in Bon Vivant vichyssoise about 40 years ago. Which was the first time botulism entered the general public discourse.

              None sense that u can recall.

              1. re: rjbh20

                I remember this! My uncle's brother and his wife were near death from eating this soup their son found them and went through the trash to see what they had eaten and got them to the hospital in time. This became an urban legend at our family dinner table. My mother never served Vichyssoise again.

              2. re: weezieduzzit


                When I've had conversations with people about the #1 food they avoid due to having gotten sick from it, the answer is maybe 8 times out of 10 "soup." Both from restaurants or from grocery store cold-cases (not canned soup, but the kind made by the store or brought in from third party manufacturers and packaged in plastic containers).

                Just last week, my best friend got a horrendous case of food poisoning after eating chicken tortilla soup from a local restaurant. She was nonstop puking for 24 hours. She's had food poisoning 3 times in her life from soup.

                And last year my mother got food poisoning from soup at a restaurant. She will still order it on occasion but always prefaces her meal by saying "I hope this doesn't make me sick."

                1. re: nothingswrong

                  Really, nothingswrong, I've never known anyone to say they've been ill from soup or even heard anyone say that you're more likely to get ill from soup.

                  I hadn't heard of the Bon Vivant case, either, probably because it happened 6 months before I was born and the illnesses appear to have happened on the East Coast, far from where I grew up- so I looked it up:

                  On July 2, 1971, the U.S. Food and Drug Administration (FDA) released a public warning after learning that a Westchester County, New York man had died and his wife had become seriously ill from botulism after partly eating a can of Bon Vivant vichyssoise soup.[1][2] A family in Dobbs Ferry, NY, reported to the CDC that they had dined on Bon Vivant vichyssoise at home only hours before the news broke. They were especially concerned because they had spoon-fed a few sips to their son, Christopher Farrell Wilson, who had just turned two years old the month before. Fortunately, in spite of the terrifying batch number they later found stamped on the can they had consumed, they did not become ill.

                  The company began a recall of the 6,444 cans of vichyssoise soup made in the same batch as the can known to be contaminated. The FDA then discovered that the company’s processing practices made questionable all products packed by the company. The FDA extended the recall to include all Bon Vivant products. The FDA ordered the shutdown of the company’s Newark, New Jersey, plant on July 7, 1971. Five cans of soup out of 324 were found to be contaminated with botulinum toxin, all in the initial batch of vichyssoise recalled.

                  The recall destroyed public confidence in the Bon Vivant name. Since Bon Vivant also marketed some of its production as store brands, this led many people to be suspicious of any soup on grocery store shelves. The company filed for bankruptcy within a month of the start of the recall. It changed its business name to Moore & Co.[3]

                  The FDA resolved to destroy the company's stock of canned soup. Moore & Co. fought this proposed action in court until 1974.[4] (From Wikipedia.)

                2. re: nothingswrong

                  You can also freeze some water in a heat-proof bottle and dip the bottle into the pot and swirl it around as a sort of "cooling wand." There are commercial versions of this sort of thing, typically made out of polycarbonate.

              3. As noted elsewhere, it cools faster uncovered. And as to the well-traveled folklore that stock needs to be cooled to room temp before it goes in the refrigerator or bad things will happen -- total nonsense. It dates back to icebox days when putting hot stock in the icebox made the ice melt faster & wasted money. Same thing for soup, gumbo, etc.

                1. I too have read references to potential "souring" of stock due to covering (and slowing down the cooling process) and wondered if it was a literal flavor transition or a reference to undesirable microbes and accelerated contamination.

                  Having suffered once from a violent bout of food poisoning due to sloppy stock into soup handling I aggressively cool stocks, soups, stews in a deep sink ice bath dropping their temperature from 200 down to 45 degrees in less than 20 minutes. From that point, covered, refrigerated or freezer free from triggering a microbe laden soup.

                  1. That's never made sense to me either. It would seem that leaving something that has been boiled, thus killing any bugs, uncovered creates the risk of introducing new bugs or something else. I make stock almost every week and often in large quantities for the freezer or to keep in the refrigerator. I fill the quart jars while the stock is boiling hot and screw on lids immediately. The heat dissipates faster in smaller volume. Then I leave them on the counter until they've cooled down enough to handle, and they go right in the refrigerator while they're still pretty warm. The fat layer forms just fine on top. They last at refrigerator temperature for up to four weeks without going sour. They might last even longer, I just haven't kept any longer than that.

                    1. I let it cool with the cover off because - and here's my deep, dark secret - it cools faster with the lid off. that's the only reason why. if it keeps my stock from souring then i'm thrilled!

                      1. I don't know anything about stock souring. I cool stock by first setting the kettle in the kitchen sink in an ice water bath in front of a table top fan and stirring frequently. I used to also add frozen bottles of water, but I don't bother with that anymore.

                        Frankly, what I have done in the last several years is to make stock in the winter when the temperature in our attached garage is 20° and cool the stock that way.

                        1. <What I've read is that it will sour if you cover it while it's still hot; >

                          I heard of that before, but I don't think it is true. Yes, it is true that uncovering help to cool faster. However, covering the pot when it is hot and allowing to cool down without opening actually keep it more sanitary.

                          <What reaction happens when you cover the stock before it's cool? >


                          1. This discussion reached out to me and last night I woke up around 1 am and refrigerated the beef stock I left out. It was still hot when we turned in. Nanoo nanoo.

                            1. Some of this is just food safety common sense. If you cover a hot pot it will cool more slowly and the "souring" effect is that organisms are growing in the pot. I work in a commercial kitchen and everyone who handles food is required to get ServSafe training. I've written this before on other posts but here it is again...You need to be cooling things like meat stocks down as quickly as possible to avoid growth of micro-organisms. Food safety rules dictate that you cool items from 135 degrees to 70 degrees in 2 hours or less and from 70 degrees to 41 degrees in less than an additional 4 hours. Not doing this may not kill you but can cause you to become ill. Organisms produce toxins and it's the toxins that make you sick. You can kill the organisms by heating but you do not remove the toxins they leave behind.

                              In the vichyssoise example, potatoes are one of the foods particularly susceptible to growth of anaerobic bacteria.

                              1 Reply
                              1. re: Cittafarina

                                I use a cooling paddle to cool stocks down quickly. Home cooks can make one by freezing water in a plastic bottle.

                                As far as the vichyssoise example, potato and cream puree sealed in a can (and therefore deprived of oxygen) definitely requires proper retort canning--which is where I guess the Bon Vivant company had a glitch. But for home cooks who are going to refrigerate or freeze a relatively clear stock within a few hours, I can't imagine the likelihood of C. botulinum gaining a foothold is very high.

                              2. I generally lets stocks cool with the lid off until they reach a temperature where it won't heat up my refrigerator too much. This week it's been easy as my whole backyard is a freezer. I remember working in commercial French kitchens where we constantly were making large batches of stocks and sauces that were put into 5 gallon plastic buckets. The walk-in coolers would warm up significantly, especially in the summer, with all that still warm liquid brought in at the end of dinner service. But keep in mind you are starting with a sterile product (cooked stock) so you do have quite a bit of time before bacteria multiplies to dangerous levels.

                                1 Reply
                                1. re: zackly

                                  Really well equipped commercial kitchens back in my day had large cold (or hot) water bain maries, sinks where you could put buckets of things you wanted to cool and were designed to run off the water continuously before it got to the top of the container. The moving water would dissipate the heat. Here's a home version.

                                2. Here's a tip kids.

                                  Once you've got your stock to the right flavor, reduce by half again. Then put in sink and replace the water you just evaporated out with cold tap water, bottle water, Ice, or whatever you use. Stir and refrigerate immediately.

                                  3 Replies
                                  1. re: Zalbar

                                    I've long wondered whether this idea would have any adverse effect on the flavor. None, eh? I have not experimented.

                                    1. re: LorenzoGA

                                      Depends on what you're trying to achieve. Boiling down a delicate fish fumet is going to make it taste stewy/overcooked. But for long-boiled meaty stocks, it need not make a difference to over reduce then rehydrate.

                                    2. re: Zalbar

                                      I find it useful to over reduce stock just for storage space considerations in order to store a smaller volume. I don't add back water until I use the stock.

                                      I would have some concern that having created a close to sterile stock by long boiling that stirring in ice or cold water introduces contaminants. As I noted above, my stocks last a long time under just refrigeration packing them away hot. I would think that introducing non-sterile elements would reduce that shelf time.

                                    3. I have never covered a pot of stock or soup to cool it - why would I do something that makes the cooling process take longer? BUT, I unwittingly did something similar, with bad results. Before I learned that foam-insulated thermos bottles don't maintain temps well (glass or steel-lined ones do), I had a soup thermos. I heated low-salt homemade turkey vegetable soup, which included cabbage, to just a bit hotter than comfortable eating temp, then put it into the thermos to take to work. 5 hrs later, when I unscrewed the lid, there was a warm geyser of fizzing broth that smelled like sauerkraut.

                                      1. I've never done this. I always just put it directly in the fridge as soon as it is done.