The Food Lab: Better Home Cooking Through Science

(Nandana) #1

Unfortunately, when you fry, all three of these types of
exposure occur in abundance. On its own, exposure to
oxygen causes oxidation, a process that causes the large fat
molecules to break down, resulting in many smaller
molecules, among them ketones and short-chain fatty acids.
These are the foul-smelling molecules that are the true cause
of that fishy smell in fry shops, and this type of reaction
occurs even when you don’t add any food to the hot oil. In
extreme cases, it can even occur in poorly stored bottles of
oil (for this reason, you should never store your oil near the
stove—you’re simply inviting rancidification).
Once you actually start frying things, it gets worse.
Hydrolysis, a reaction that occurs when you combine water,
oil, and heat (i.e., fry something), compounds and speeds up
the effects of oxidation. That’s why oil used for frying
eventually breaks down and becomes stinky and unusable.
Depending on how hot you fry and how much food you fry
at a time, a container of oil will get anywhere from a half
dozen to a few score of uses before it becomes unusable.
Finally, the last way in which oil breaks down is a process
called saponification—literally, the conversion of oil to
soap, and when we say soap, we’re not talking Ivory or
Dove bars, we’re talking about the chemical definition: a
chemical salt of a fatty acid.‡ Soaps are a surfactant, which
means they have a hydrophobic (oil-loving/water-hating)
end and a hydrophilic (water-loving/oil-hating) end. They
are the peacemakers of the oil and water world, allowing the
two to coexist without separating, as they are wont to do.
In this case, though, peaceful coexistence is a bad thing:
the more surfactants your oil contains, the more water it can

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