Hydrogenated oil 是不是一定有 trans fat?

如果係 Fully Hydrogenated Oil 就不會有 trans fat, Partially hydrogenated oil 就一定有.

Hydrogenation 令液體菜油變成固體, 但如果 Full Hydrogenation, 固體油就會好硬, 好難用, 所以只係Partial Hydrogenation. Partial hydrogenation 就會形成trans fat, full hydrogenation 雖然冇trans fat, 但就增加左飽和脂肪含量.

"Fully hydrogenated oil does not contain trans fatty acids. Instead, it contains more saturated fat"
FromTrans Fatty Acids 101
"When liquid vegetable oil is fully hydrogenated, however, almost no trans fats remain. The resulting fat is even more solid, taking on a hard, waxy consistency, even at room temperature. Full hydrogenation increases the amount of saturated fat, although much of it is in the form of stearic acid, which is converted by the body to oleic acid, a monounsaturated fat, which doesn’t raise levels of bad cholesterol. This makes fully hydrogenated fats less harmful than partially hydrogenated fats."
From Partially and Fully Hydrogenated Fats
雖然Fully Hydrogenated oil 可以用黎製成 trans-fat free 既固體菜油 (by mixing really hard fully-hydrogenated oil with liquid oil), 不過要注意: if a package simply lists "hydrogenated oil," without expressly stating whether it is partially or fully hydrogenated, it may not be trans-fat free. Sometimes the terms " hydrogenated" and "partially hydrogenated" are used interchangeably. If the package clearly states that it contains fully hydrogenated oil, then it will be trans-fat free.

即係話, 寫住"Hydrogenated oil " 就多數有 trans fat (因為都有機會係 Fully hydrogenated, but if that's the case 佢多數會寫明係fully 囉); 而 "Partially hydrogenated oil " 就一定有.
~:)

Hydrogenation is the process of heating an oil and passing hydrogen bubbles through it. The fatty acids in the oil then acquire some of the hydrogen, which makes it more dense. If you fully hydrogenate, you create a solid (a fat) out of the oil. But if you stop part way, you a semi-solid partially hydrogenated oil that has a consistency like butter, only it's a lot cheaper.

Because of that consistency, and because it is cheap, it is a big favorite as a butter-substitute among "food" producers. It gives their products a richer flavor and texture, but doesn't cost near as much as it would to add butter.

Partially Hydrogenated Oils Make You Fat!
Partially hydrogenated oils will not only kill you in the long term by producing diseases like multiple sclerosis and allergies that lead to arthritis, but in the meantime they will make you fat!

You Eat More
It's not like you have any choice in the matter. Remember that the essential fatty acids are vital to every metabolic function in your body. You will get the quantity of essential fatty acids that you need to sustain life, no matter what. You will not stop being hungry until you do.

If you are consuming lots of saturated fats, you really have no choice but to become fat, because saturated fats contain only small quantities of the polyunsaturated fats that contain the essential fatty acids you need. The key to being thin, then, is to consume foods containing large amounts of polyunsaturated oils. (Those foods include fish, olives, nuts, and egg yolks.) Over the long term, those foods remove your sense of hunger.

Note:
The difference between a "fat" and an "oil" is temperature. A "fat" is a lipid that is solid at room temperature. An "oil" is one that is liquid at room temperature. Both are lipids (or "oil/fat"). Change the temperature, and you can convert an oil into a fat, or vice versa.

Partially hydrogenated oils make you gain weight the same way that saturated fats do -- by making you consume even more fat to get the the essential fatty acids you need. But partially hydrogenated fats are even worse. Not only do they produce disease over they long term, but they interfere with the body's ability to ingest and utilize the good fats!

Picture it like this. The trans fats are now the guards along the watchtower. The essential fatty acids (the support troops) are waiting outside to get into the fort (the cell), so they can be distributed along the watchtower (the cell wall). But the guards won't let them in! So they have to find someplace to stay in town. Over time, more and more troops are finding lodging in town. So new houses (fat cells) have to built to keep them in. The town grows more and more swelled with troops (fat), and it gets bigger and bigger (fatter). It's not a pretty picture at all, when you realize that the town is your belly, buns, face, and neck.