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I'm cooking soup as a starter for Christmas dinner. I've made this soup before and I used 1.5 litres of water for 4 servings. I'm making 12 servings for Christmas day so that should be 4.5 litres of water required if I just put in 3 times as much of everything.

I'm not sure if I've correctly scaled up the water needed, 4.5 litres sounds like a lot of water. Maybe when cooking 4 servings 1.5 litres of water is used but 0.5 litres boils off, then with 12 servings 3.5 litres is used and still 0.5 litres boils off. Is this a smarter way to scale up the water needed?

Edit: To make 4 servings I add 6 tomatoes, a leek and 3 potatoes; I simmer it for 30 minutes with the lid on.

  • Boils or just simmers? A third of your volume sounds like a lot. What's the final volume of soup? – Catija Dec 22 '16 at 17:45
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    We need more info to answer this. How long is the soup held at a boil or simmer? Do you cover the pot or not? What ingredients go in this soup? – John Feltz Dec 22 '16 at 17:48
  • @Catija I didn't mean to use 0.5 litres as an exact number I was trying to show another method of scaling the water needed – Hugh Dec 22 '16 at 17:54
  • @JohnFeltz I simmer the soup for 30 minutes with the lid on, would that make very little water lost? For 4 servings I add 6 tomatoes, 5 stalks of celery, 3 potatoes and a leek – Hugh Dec 22 '16 at 17:57
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    With a leek, maybe a lot of water lost – Lorel C. Dec 22 '16 at 19:45
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1.5 litres for 4 servings is 375ml per serving (plus some volume from the veg which I'll ignore) assuming no water boils off. That's a sensible portion. I reckon my soup bowls hold just a little less than that, but you'll leave some in the pan when serving . So I doubt you lose a lot of water when you normally cook it.

That said, I'd err on the side of caution and (i) measure the bowls (both the normal ones and the ones you plan to serve it in if they're not the same); and (ii) start with a little less water than you think you need - adding water is quick, boiling it off less so and you don't need the hassle of your soup taking too long.

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    +1, measuring is the best way to be sure. Seeking theories for explaining a gut feeling, and then guessing how strong their effect is will likely to be quite far off. – rumtscho Dec 22 '16 at 19:53
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Generally speaking, you are correct that less water will boil off as a % of the total when you increase water volume. Mainly because the surface area of the water will not increase enough to offset the extra volume. The amount of water evaporating will be directly related to the surface area of the top of the water.

Imagine a tall test tube of water with a flame directly underneath it. It would take a long time for all the water to boil. Now imagine the same volume of water in a frying pan. It's going to evaporate a lot quicker.

Based on all the variables at play, the best solution is to add the amount of water you think is close to what you actually need. If you need to cook it longer to boil more water off, that will be easy to do. The longer the soup cooks, the more the flavors will come together. If you need to add water/stock you can always do that as well.

  • Thanks for the explanation, I will measure out the extra 8 bowls needed and add a bit more to be safe – Hugh Dec 22 '16 at 19:01
  • The energy input during boiling is the most important factor on the rate of water loss. (Note: The answer has a second equation that uses Area and doesn't appear to use energy, but it says, "assume the water stays at that temperature" and prescribes "xs is the ... at the same temperature ..." which implies that energy will be added at the same rate it lost.) The reason that less water by percent boils off is because you are inputting the same amount of energy, but you are still boiling off the same quantity of water. – Zach Mierzejewski Dec 23 '16 at 14:35
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If this is a short cook soup, and I would consider 30 minutes lid on to be short cook for sure, then not a lot of water will be cooked off in your usual proportion, so maybe reduce from 4.5L to maybe 4.25L.

If this was a longer cook with more opportunity for evaporation, then what your gut is warning you about would be more likely. If you are concerned it is still the case, or if you wanted to do the same thing with a longer cook and open crock, the I would lean to multiplying the other ingredient by 3, but the water by only say 2 times, start the cooking, and then taste as it goes. If the broth starts tasting strong, then you will want to add water to dilute.

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    Thanks for the broad answer on short/long soups, this will help others with a similar problem – Hugh Dec 22 '16 at 19:03
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    Couldn't you quantify how much water evaporates by comparing the total volume of ingredients to the amount of soup you end up with? Then multiply that by the ratio of surface area of the larger and smaller pots. – Random832 Dec 22 '16 at 22:05

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