So why do some people use solid fats to fry and others use liquid oil? An excellent question, reader Jay. As many people know, just about all commercially made, oh, doughnuts for example, are fried in solid shortening. They could in theory be fried in liquid oil, but solid fats offer several important advantages, among them a greater resistance to breakdown and rancidity.
But why exactly is that? To answer I’ll need to get into a little bit of chemistry. The world of fat, as you’re probably aware, is divided into categories based on whether the fatty acid molecules they contain are saturated or unsaturated. What does that mean? Well, if you remember, I recently talked about how fat molecules are “E” shaped: a glycerol molecule with three fatty acid molecules attached. Those fatty acid molecules are nothing more than long strings of carbon atoms – atoms which can each be bonded to up to two hydrogen atoms (making a so-called “hydrocarbon chain”).
If that sounds confusing, think of a line of pre-school teachers (carbon atoms) walking down the street, and each of them is holding either one or two toddlers (hydrogen atoms) by the hand. (Can you tell I have young kids at home?)
If every teacher has both hands occupied by a toddler, then that fatty acid is said to be saturated, full up with hydrogen atoms (as a side note, should you ever see something like this in real life, hurry up and enroll your kid in that pre-school, because the teacher-to-student ratio is unbeatable). If one of the teachers in the line has a free hand, then the fatty acid is said to be mono-unsaturated. If more than one teacher has a free hand, or even two free hands, then that fatty acid chain is said to be poly-unsaturated.
The general rule of fats is that the ones that are high in unsaturated fatty acids tend to be liquid and the ones high in saturated fatty acids tend to be solid. But that’s not the only difference. Unsaturated fats, since they have molecular bonding sites available (remember those free hands) can bond to other types of “free radical” atoms (imagine unruly teenagers joining the parade). This process occurs as the oil is heated and exposed to air. It causes the fatty acid chains to break into pieces, yielding all kinds of weird compounds including foul-smelling short-chain fatty acids, aldehydes and ketones. In the end, it can cause the fat to go rancid.
So you can see why solid fats are valued so highly by people who fry a lot. They’re durable. They also have the advantage of being solid at room temperature, which means that once the food that’s been fried in them cools down, the residual oil won’t “weep” onto plates, napkins or cardboard boxes. Firm fat also tends to feel less greasy on the tongue.
But doesn’t most shortening have trans fats in it, Joe? That was the rule up until very recently. Just about two years ago now, inexpensive trans-free solid shortening became available to the food industry. Nowadays more than a few doughnut shops fry in trans-free fat, though by no means everybody. As someone who’s never believed the hype surrounding trans fats, that’s not important to me. If it is to you, you’ll be comforted to know that you can once again enjoy doughnuts without the specter of trans fats hovering over you.
But of course there’s always lard, no?