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All recipes call for simmering a dish if you wish to reduce. I know that if for example if I wish to reduce wine, it will take me double the time or even more if I simmer instead of just cranking the heat up to a boil. But there must be a reason for this - what is that? There is only a 6c difference between a simmer and a boil.

I also presume boiling is more permissive in some cases than others, so an ability to distinguish the importance of simmering is necessary to know when you can crank the heat up to save time.

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    Too much heat breaks down some aromatic compounds, boiling to reduce will change the flavor, often for the worse. – GdD Mar 10 '16 at 8:35
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There's another reason for not boiling liquids, besides the possibility of making a mess (boiling over) or ruining it (scorching, etc.).

You actually reduce the amount of flavor by boiling. As Kenji explains on Serious Eats :

But here's the deal: when simmering, water is not the only thing escaping. Ever notice how when you come home to a pot of sauce simmering on the stovetop or perhaps a beautiful pot roast braising in the oven, your entire home smells of it?

Guess what: if those flavorful aromatic compounds are reaching your nose, it means they are leaving the pot.

So ... if the only goal was to make there be less liquid, boiling's fine. But if you actually want to concentrate the flavors, you want a slow simmer.

The article also goes into more details about reducing alcohol, and some problems with boiling tomato sauce (which I believe applies to most pectin & other hydrocolloid thickened sauces).

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    I wondered about this, but I wasn't sure: you can smell things when simmering too, and it takes a lot longer, so it's hard to tell whether you come out ahead or not. Kenji's quote even mentions the loss of aromatics from simmering in that quote. But he does go on to say you can try it side-by-side and slower is better. – Cascabel Mar 10 '16 at 3:03
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    Boiling tends to force a lot of unvaporized liquid into the air, blown out of the surface of the liquid by violently escaping steam. – rackandboneman Dec 12 '16 at 8:22
  • @rackandboneman : and if anyone wants proof of this ... just boil some tomato sauce. (but be prepared for a mess to clean up) – Joe Dec 12 '16 at 18:36
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    I think tomato sauce is a pathological case since it is thickened and has oil drops in it in many cases :) But even plain water will do it, that is the smoke-like stuff you actually SEE as "steam". – rackandboneman Dec 13 '16 at 10:43
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As you pointed out, liquids reduce a lot faster when you crank up the heat compared to when you leave it at a gentle simmer. The reason is simply that you're introducing a lot more thermal energy into the liquid when you crank it up to the max. Once the liquid reaches the boiling point, any extra heat you provide will be canceled out by the cooling effect of evaporation. So adding more heat to a boiling liquid causes the evaporation rate to go up while the temperature stays the same.

As for the reason to use a simmer, that's going to depend on the liquid. Some liquids handle heat a lot better than others. A simmer is nice to avoid burning, curdling, boiling over, etc. But if you're reducing something that can take the heat, feel free to crank it up.

Liquids that shouldn't be boiled include: anything with a lot of milk (can scorch or boil over), anything with a lot of starch (can boil over), emulsified sauces (can break), chocolate (can scorch), oil (very hazardous when boiling), and custard (can curdle). Some people also prefer not to boil stock because it can become cloudy, but that doesn't necessarily ruin it.

  • Good answer, but I feel as if giving a rule of thumb to know on the spot what can take the heat and what cannot will produce a better answer for this type of question. – Bar Akiva Mar 9 '16 at 21:27
  • @BarAkiva What would such a rule look like? A list of all liquids and a suggestion if they can take the heat? That would be too long. – rumtscho Mar 9 '16 at 22:00
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    @rumtscho Sometimes there are categories of things with common properties. For example, things with lots of sugar are probably prone to nasty splattering and maybe burning if they boil hard as they reduce, and I'm guessing things with starch are prone to boiling over. I'm sure you can't cover every possibility, but you can definitely make some generalizations and give some examples. – Cascabel Mar 9 '16 at 22:05
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    @Jefromi I was trying to think along these lines and failing to come up with a coherent list which is not just a collection of over a dozen of random categories. But maybe somebody can do better. – rumtscho Mar 9 '16 at 22:08
  • @rumtscho More like the characteristics. I do not need a list of all the cooking oils in the world to know what I need to fry a chicken breast - I know fats transfer heat really well, so if it is a fat I can make an independent judgment. I am looking for the same sort of general rule. Like if I am going to end up with just the flavor of a liquid (reducing 90% of wine) then cranking the heat up is a good idea. – Bar Akiva Mar 10 '16 at 0:22

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