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This question already has an answer here:

I am making vegetable soup for about eight people and would like to cool it in little time. Refrigeration seems to be a good choice, but I would like to know if there are better methods/techniques out there.

marked as duplicate by rumtscho Jun 12 '16 at 12:06

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    I'm not sure I agree with the classification as duplicate of here. That question specified an iron pot and a target goal of freezing, which both raise different concerns. I personally wouldn't tend to immerse a (bare) iron kettle in an ice bath, both for rusting concerns and because thermal shock could lead to warping or cracking. And if soup was to be frozen, I'd recommend other steps, like pouring into bags (on trays) or shallow containers to promote rapid freezing (and for convenience in using later), aspects that don't apply here as much. – Athanasius Jun 13 '16 at 1:13
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Ice bath.

Put a bunch of ice in your sink or in a container large enough to place your soup pot in. Add enough water to cover the ice. If the soup pot is large or wide, you can speed up the cooling by periodically stirring it: this is particularly important for thicker soups or stews, where the middle section of a pot can stay warm for a long time. Or, for maximum cooling, break up the soup into smaller -- preferably more shallow -- containers first and then place them in the ice bath (though this isn't generally necessary if you're using ice).

If you don't have that much ice, you can use a cold tap water bath to at least get it down to near room temperature before refrigerating. But you will need to replace the water frequently for fast cooling.

(Unless you're breaking up the soup into smaller containers first, I would strongly recommend an ice bath over simply putting a large pot in the refrigerator. It can take many hours for the contents of a large pot to cool completely in a refrigerator, which can be a food safety hazard. If you do use the refrigerator alone, put into smaller containers and don't stack them to allow air circulation, and obviously don't place them next to highly perishable foods like raw meat, etc.)

  • This was gonna be my answer. – Escoce Jun 11 '16 at 20:22
  • This technique can be improved by filling a number of food quality bags (that can deal with the temperature of soup) with ice cubes, ensuring they are sealed, and immersing them in the soup. – abligh Jun 12 '16 at 10:07
  • Stirring cools the soup a lot faster if it's thick because the natural flow would be slow, and convection weak. – user1306322 Jun 12 '16 at 11:19
  • @user1306322 - good point. I didn't think about mentioning the distinction for thicker soups, but will add it to the answer. – Athanasius Jun 12 '16 at 23:43
  • You can save quite a bit of ice by putting your pot in a sink of cold water for 10 minutes or so. Metal pots work best. Stir occasionally. Don't turn the faucet on while it's directly over the soup pot. – Wayfaring Stranger Oct 3 '18 at 0:07
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I usually let soup cool at room temperature for 15-20 minutes in a wide, shallow, very thermally-conductive pot or pan set on a cooling rack. A large saute pan works well. Moving it to a different vessel than the one the soup was cooked in leaves behind all the heat retained in the cooking vessel, especially if you're making soup in something heavy like enameled cast iron. With lots of exposed surface area and occasional stirring, it's cool enough to store pretty fast. I put it in containers and refrigerate it while it's still warm.

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An ice bath is about as good as you're going to do. And if you're really serious about the speed of cooling, then don't forget the trick of adding salt to the ice bath - a bath of water, lots of ice, and lots of salt (when well mixed/agitated) can reach a much lower temperature than just water and ice (all the way down to -4F (-20ºC) if you're good!)

As an alternative plan, you might leave some of the water out of the recipe (if possible) and put the equivalent amount of ice directly into the soup to cool it later. I think this will cool the soup even faster than the ice bath because the heat exchange situation will work much more quickly than the relatively limited conduction-heat-transfer area between pot and ice bath.

http://chemwiki.ucdavis.edu/Reference/Lab_Techniques/Cooling_baths

  • I don't see why going lower than 0°C with salt would be worthwhile in this application – the temperature gradient is already plenty big enough even with ice alone, or indeed with just cold tap water. Adding salt will mostly make the ice melt even faster – with a given amount of ice, you may well get the soup colder if you don't add salt! This really would only make sense if you need to freeze the soup. — If it should go really fast, then thermal conduction through the pot walls is the bottleneck either way, only bypassing that with ice cubes in the pot will indeed make it much faster. – leftaroundabout Jun 12 '16 at 10:49
  • @leftaroundabout: The physics answer to your comment is: cooling from ice does not increase the temperature of the ice-water mixture. Cooling from ice is caused by heat absorbed from the soup causing phase change of ice from solid to liquid water. Adding salt to ice causes the phase change to occur at lower temperatures and cools the soup more rapidly. Once all (most) of the ice is gone, the water will then heat up from the soup. So adding salt to ice does cool things faster. – Mark Ripley Jun 12 '16 at 11:12
  • @MarkRipley: I didn't contest that adding salt will speed up the initial cooling, I just said it's not efficient. If the soup is initially 90°C, then adding salt to the ice will increase the heat transfer rate through the pot by at most 20%, but it will increase the heat transfer through the ice bath container by ~100%! I.e. you mostly cool the environment faster, which is not what you want and it wastes ice. – leftaroundabout Jun 12 '16 at 11:43
  • I have to agree with leftaroundabout: unless you are trying to freeze the soup outside a freezer for some reason, adding salt is horribly inefficient. To think about this another way, the major cooling contribution of an ice bath is the "heat of fusion" that comes when the ice melts. Whether that ice melts at 0C or -10C or whatever, it's going to absorb roughly 334 J/g, whereas every degree the ice or water rises only absorbs around 4 J/g. Adding salt just means wasting energy cooling water below 0C, since the real action comes from the melting process, not a few extra degrees temp difference. – Athanasius Jun 13 '16 at 0:49
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If you want a literal answer to what's the fastest way, I say pour in some liquid nitrogen.

As to fastest practical way, I too would recommend not using any active cooling technique while the soup is above 50°C. In particular, refrigeration is a really bad choice, because you introduce a lot of heat into the refrigerator which it will have to work hard to get out again. It's a big waste of energy. While the soup is hot, the heat will happily go away itself if only you give it some path to escape. One good way, as suggested by Dan C, is to use a big surface area. In particular, that takes away a lot of heat through vaporisation.

Another useful technique is to put the soup in the sink in a closed pot and keep cold water running all over- and around it for a couple of minutes.

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