My oh my I've received a lot of questions about doughnuts the last couple of days! Reader Seth asks:

Joe, I notice you use liquid oil for your frying medium. I've heard that professional doughnuts makers use mostly solid shortening. Is that true? And if so, why?

An excellent question, and one I've been meaning to get to the last couple of days. The answer is yes, just about all commercially made doughnuts are fried in solid shortening. There are several reasons for this. First, as I mentioned earlier in the week, a solid fat is much more resistant to breakdown and rancidity than a liquid fat.

To explain why, 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 chains attached. Those fatty acid molecules are composed of strings of carbon atoms, each of which can be bonded to up to two hydrogen atoms (a 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 one or two toddlers (hydrogen atoms) by the hand. Can you tell I have little kids?

If every teacher has both hands occupied by toddlers, then that fatty acid is said to be saturated, full up with hydrogen atoms. 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 (and should you see something like that, by all means hurry up and enroll your kid in that pre-school, since the teacher-to-student ratio can't be beat).

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 atoms, often undesirable (think in terms of unruly teenagers joining the line). Frequently those undesirables alter or breaking the hydrocarbon chain, which accelerates breakdown or causes 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" oil 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.