Does all hydrogenation produce trans fats?

No, only partial hydrogenation does it. You’ve heard that term before, I’m sure. Why are liquid oils only partially hydrogenated? Because the more you hydrogenate fatty acids, the more hydrogen atoms you add to those hydrocarbon chains, the firmer the fat gets. Fully hydrogenated cottonseed oil, for example, is so uniform and crystallizes so well, it’s rock hard at room temperature. Try spreading that on a piece of toast! So, the process is usually stopped before the reaction progresses that far. The problem of course is that any time you have partial hydrogenation you get those darn trans double bonds. It’s one of those all-or-nothing propositions.

The new “trans free” solid fry fats, so I understand, are a combination of fully hydrogenated vegetable oil and liquid oil, which gives the fat that in-between, butter-like consistency.