Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn’t, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you’re not sure why, send a tweet to The Food Geek to find out what’s happening.
megpasz asks via twitter,”what temperature does a sauce need to ‘reduce?'”
As with most of these sorts or questions, there’s a quick and thoroughly valid answer, and the more in-depth answer. With this one, you get a third answer that’s somewhere between the two.
You generally want to reduce at a simmer, which is around 200°F (93°C) for sauces that are close to water in consistency. The exact temperature varies based on what’s in it, but look for just a few bubbles rather than going for a full-on boil. Most recipes suggest jumping to a boil first, then going back down to a simmer after, but that’s generally just to get done with the sauce faster.
Strictly speaking, a sauce will reduce at any temperature above absolute zero (-273°C or about -460°F), it’s all a matter of timing. The higher the temperature at which you reduce, the more you change the nature of the flavors in the sauce. Essential oils will break down at higher temperatures, so when possible, you would add these after the simmering (citrus zest, herbs, and so on), but by and large, the lower the temperature, the less damage you’ll do to the flavors. However, the higher the temperature the faster the reduction, so if you’re in a hurry, you can turn up the temperature. Well, as long as the Michelin folk aren’t dining at your home that evening.
Let’s think about temperature for a moment. When we take a person’s temperature, we generally find a fairly insulated spot to measure from, such as under the tongue. If we haven’t been consuming anything hot or cold, that is a decent place to find out what the body’s temperature really is. If you take the temperature of someone’s fingertips, for example, they’ll generally be colder, having been exposed to the air with not a lot of blood going through them.
The same thing happens with food. If you take the temperature of a piece of meat, you generally want it to be as deep into the meat as possible, without getting too close to the bone. This way you’re away from the air, but not touching the bit that conducts heat so much better than meat does.
The interesting thing is that the same thing happens all the way down to the molecular level. Many people think of temperature as being the amount of heat something gives off, but it’s not. If you take some air that’s at 400° and some water that’s 400°, you’re much better off passing your hand through the air than the water. That’s because water holds so much more heat than air, and so will have that much more heat to transfer to your hand. So heat is the amount of energy that can be transferred from one thing to the next.
On the other hand, temperature is the average speed that a group of molecules have. Air is spread out and doesn’t have a lot stopping it, so its molecules can zip along without impediment; they can have a high temperature easily. Water doesn’t move as easily, because there are a bunch of other water molecules in the way, and they’re all jammed together. But when they do get going, it’s hard to stop them. Still, when the air and water are moving at the same average speed, they have the same temperature.
Another way to think of it is to imagine a room filled with kittens. These kittens are running everywhere, climbing on drapes, bouncing off the walls, and so on. Still, if you were in the middle of the room, and a kitten ran into you, the kitten bounces off easily and doesn’t really move you at all. Now imagine a room filled with elephants. Chances are, they’re just lounging around, possibly walking gently from here to there. Still, if you could get them to run around as quickly as the kittens do, and one ran into you, then you would be in trouble and/or squished.
Now that we understand temperature and how it differs from heat, let’s think about how temperature affects reducing the amount of liquid in a sauce. Let’s remember the kitten room, but now imagine there’s a big cat door along one wall. You know, the kind with the rubber flap that hangs down. The kittens are young, so they don’t really know how it works, but if one happens to be going fast enough in the right way, it’ll pass right through it. The faster the kittens go, the more likely they will be to go through the cat door. And, when a kitten that is moving faster than the average speed leaves the room, that means the average speed of the kittens is lower, so the room is effectively “cooler.”
The same thing happens with water. If it’s boiling, that means the water molecules are, on average, moving really fast, and if any given water molecule is going in the proper direction, it flies off the surface and goes into the air and turns into steam. The water cools a bit, but you don’t really notice because you’re generally adding more heat into the water.
Even at room temperature, water molecules fly off the surface of the water, it just doesn’t happen as quickly as when you’re boiling, because they aren’t moving as quickly. We generally call that evaporation. This is also why you cool off more quickly when you are wet and exposed to the air: the water molecules are evaporating, which lowers the total speed of the water on your skin since the fast ones tend to leave more quickly than the slow ones.
So, when you’re reducing a sauce, you could let it sit out and eventually it will reduce itself. On the one hand, this is very handy, as it saves on energy bills and won’t destroy the flavors with heat. On the down side, it will take a long, long time, and it will probably spoil before it’s done. This is why we generally go with a simmer: it’s hot enough that it will kill nasty food-bourn microbes, but slow enough that it will preserve most flavors.
If you wanted to optimize for flavor, you could bring the liquid up to 160°F, then drop it down to 140°F for as long as it takes. That will kill the bacteria and keep them from re-growing. Still, that’s a bit of a pain to do without specialized equipment, and I don’t know of anyone who does it that way. That doesn’t mean that someone doesn’t swear that it’s the only way to properly reduce a sauce, but people think all sorts of things. Simmering not only gives you a reasonable time frame to cook in, but it has handy visual cues, so you won’t even need a thermometer.
Understanding how heat and temperature work are vital to your ability to cook well. It will come up again when baking a cake or roasting a large cut of beef or just about anything in the kitchen. The important thing to remember is that temperature is your goal, and heat is how you get there. A jet is faster than a bicycle, but if you’re just going down to the corner grocery store, you’re probably better off using the bicycle. If you’re going to a country 3000 miles away, though, bicycling won’t get you there in time to do anything worthwhile.