of oil, and you’re looking at a drop closer to 50 degrees. So
at home, you have to heat your oil hotter in order to
compensate for this loss.
Q: OK, I think I’ve got it: New oil = good, old oil = bad,
right?
Not necessarily! You may think that using fresh new oil is
the best way to fry foods, and you would be forgiven for
thinking that. Forgiven, but wrong. Here’s why:
Completely fresh oil is highly hydrophobic: it doesn’t
want to get anywhere near water. Any food that you drop
into a deep fryer is bound to have a very large percentage of
water in it (after all, the whole point of frying is to drive off
water), which means that the oil is not going to like it. It
hates it so much, in fact, that it has trouble getting close to
its surface. Have you ever noticed that when you drop
battered food into fresh oil, there’s a shiny bubble that forms
around the food? That’s a layer of water vapor rapidly
escaping from its surface and preventing the fat from getting
too close. Because the fat can’t come in contact with the
food, heat transfer is inefficient with fresh oil. This means
longer cooking times, less crispness, and less “fried” flavor
(remember, fried flavor comes from a combination of
browning, dehydration, and fat absorption—see “What Is
Deep-Frying?,” here).
Slightly older oil, on the other hand, has got a few
surfactants in the mix—those molecules that allow fat and
water to come close to each other. Because of that, older oil
is better able to penetrate foods, cooking them far faster and
giving you crisper, better—flavored crusts.