I’ve blogged on this subject in the past, but will happily re-summarize in light of the recent posts on oil, fat and frying, reader Sally! As outlined below, frying fats that are solid at room temperature are desirable for a number of reasons. But if solid animal fats are either too expensive, too rare or off the table for dietary or health reasons (as they were twenty or so years ago) what is there to do? The answer of course: turn liquid oils into solid fats.
That’s what a German chemist by the name of Wilhelm Normann did in 1902. He found that if one were to bubble hydrogen through liquid oil in the presence of a catalyst metal, unsaturated fatty acid chains would pick up hydrogen atoms (remember those teachers and school children from the post below). The result would be a solid oil, also known as a “fat.”
Why is that? Well remember how I wrote that lipid (fat or oil) molecules are “E” shaped, three fatty acids attached to a “backbone” of glycerol. Each of the fatty acids on that “E” can be — indeed usually are — different. Thus while all lipid molecules more or less look like the letter “E” some of the fatty acids on them are longer than others, are curved, etc.. Those differences are a factor of their basic structure, but also of their degree of saturation. Less saturated fatty acids tend to be curvy, fully saturated fatty acids tend to be straighter.
That matters when it comes to crystallization. As I’ve written many, many times before, molecules that are alike tend to stack up on each other, thus forming firm crystals. You can imagine an oil as a jumble of languid, differently-shaped lipid molecules that can never quite click together, and so they flow.
Add hydrogen atoms, however, and the fatty acids begin to straighten out, the molecules become more uniform in their shape, and they begin forming crystals. The oil starts to become solid. The longer the reaction is allowed to continue the more the crystallization goes on and the firmer the oil gets. This is why most oils are only “partially” hydrogenated, because, while cooks (especially fry cooks) like their fats firm, they don’t want them rock-solid.
The rub there is that it’s partial hydrogenation that creates trans fats, which many people find objectionable, even though trans fats are a regular, natural feature of things like, oh, cow’s milk. But as usual, I digress.
So how then do producers of trans-free fry oils avoid getting trans fats in their products? Simple: they combine fully-hydrogenated oil with non-hydrogenated liquid oil, thus keeping the “fat” soft while avoiding partial hydrogenation. Neato, no?