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Why is cooking in oil or fat considered dry heating, while cooking in wine is considered moist heating?

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2 Answers 2

up vote 17 down vote accepted

Oil is dry heat because oil contains no water. Wine does.

The "moist" in moist heat really means water. In moist heat cooking, water acts as a solvent and actually dissolves much of the solid matter in the food - hence the reason why steaming and boiling tend to make food rather soft or even soggy.

Oil, on the other hand, is very rarely a solvent. There are certain food compounds that are fat-soluble, but most are water-soluble and oil, being hydrophobic, actually blocks any contact with water, thus preventing any dissolution, and generally making the food crispier as well as promoting the Maillard reaction (which can only occur with dry heat, due to the low boiling point of water).

It may seem counterintuitive, but oil isn't wet. It just looks that way.

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wine contains water and oil doesn't? i'll ask a chemist friend. I didn't think they added water to wine –  barlop Apr 26 '11 at 7:14
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@barlop: Yes, grapes are mostly water. –  Ray Apr 26 '11 at 13:11
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@barlop: They don't add water to wine, it already has water in it. Most wine is in fact 75-85% water. Most liquids in general (milk, juice) and many solids (meats, fish) are composed largely of water. We are mostly water. –  Aaronut Apr 26 '11 at 13:32
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@barlop: This is honestly what I would have considered common knowledge. If you really feel the need to test it, then mix it with water. If it dissolves, then it's likely water-based. If not, then it is (or contains) some other immiscible solvent, such as lipids (oil/fat). You can also heat it; if anything starts to boil off at or very close to 100° C, then it's probably got water in it. Oils and fats can be heated to several hundred °C before they start to smoke and eventually ignite (they never boil). –  Aaronut Apr 26 '11 at 18:34
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(There are several exceptions to the miscibility test - for example, alcohols are miscible in water, but it's pretty easy to tell when something is alcohol-based by the smell. Really the test is more effective the other way around; if it's immiscible in water then it clearly has no water to begin with.) –  Aaronut Apr 26 '11 at 18:37

To build on Aaronut's answer, one issue with cooking in a "moist" environment is that it limits the temperature.

In dry-heat cooking, such as roasting, searing, frying, grilling, etc., the flavor from the Maillard reaction (to which Aaronut alluded) is a very important result.

In moist cooking, such as braising, poaching, steaming, etc., the temperature cannot (with the exception of pressure cooking, but there are other issues there) rise high enough to allow your meat to become browned. You can also have flavor and nutrient loss, as well as texture issues.

Consider a steak, for example. Whether it is pan-seared or grilled, it will be browned on the outside. Imagine if, instead, it was boiled. Personally, I would not want to eat that gray lifeless mass of meat.

Or another example: french fries. You can put potatoes in hot oil, and you get a crispy delicious snack. Drop them in boiling water, and you get.. boiled potatoes?

This isn't to say there is no value in moist cooking; just to illustrate the difference between moist and dry.

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Just to clarify, unless the ambient pressure is increased, water will not generally heat above its boiling point (around 100C/212F) since all of the hotter water just escapes as steam. The Maillard reactions do not begin to occur until much higher temperatures. Steam can actually get much hotter---even hot enough for Maillard reactions---but that usually requires high pressure (e.g., steam engines can get up to 350C+). Air and oil can of course be heated to much higher temperatures at STP that are in the Maillard range. –  ESultanik Apr 26 '11 at 14:09
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Good explanation, although neither air nor oil can be heated to high temperatures at STP... or else it wouldn't be STP. –  Ray Apr 26 '11 at 14:29
    
Heh, good call. By "STP" I guess I really meant "SP" (i.e., average air pressure at sea level). –  ESultanik Apr 26 '11 at 15:09

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