Wet != Dry

originally written 5/12/2014

If you listen to the America's Test Kitchen podcast, this week, in the "Test Kitchen Tip" segment (around 1:30 in), they say, "If you asked 100 people if hot oil will cook food more quickly than hot water, everyone would choose the oil. In fact, the exact opposite is true ..."

This is a very unnatural comparison, because they're talking about oil and water at the same temperature, but under normal conditions, you can get oil much hotter than water, and the functions that they serve are totally different.

Wet heat (e.g. poaching, boiling, braising) is generally capped at 212F (100C). For things where you're going to consume the cooking liquid (i.e. not blanching in salty water), you're generally extracting flavor from the food, into the cooking water, because water's a good solvent, and osmosis is a thing.

Frying, in spite of the fact that you're submerging food in a liquid, is a dry heat method of cooking. That is, it's more similar to roasting and grilling. The difference is the fluid medium is oil, instead of air. Dry heat cooking does a few things:

1. It concentrates flavor by reducing moisure, so the relative concentration of flavor compounds is higher.
2. If the temperature is high enough (around 300F-350F [149C-177C]), it allows for the Maillard reaction and caramelization, both of which create new flavors.

If you like those flavors, you're not getting them with liquid water unless you have something like 100+ PSI (7+ bar) (your standard pressure cooker goes to 15 PSI), and I honestly have no idea what would happen to your food at that pressure. I'm imagining straight liquification, but I'd love to see some experiments.

Texturally, wet heat generally makes your food softer. Dry heat makes it harder. There are also considerations to be made for stuff like the general solubility of your food (e.g. Don't boil bread all day), protein reactions (e.g. collagen to gelatin conversion -- which can be done wet [braising] or dry [confit]), etc.

In any case, I don't think you're generally in a position where you have a piece of food that you want to cook, and your only consideration is time to doneness, where this tip tells you anything meaningful that's going to affect your decision to use one or the other. The one case I can think of where you're getting almost the same result for the thing you're cooking (braise vs. confit), you're largely doing them because you want a flavorful water, or a flavorful fat.

I guess my point is that cooking is complicated, and the fact that water is a conductor is kind of the least interesting thing about it.