Trans fats: help me out

I had understood that trans-fats were the underlying evil in hydrogenated oils and that this was why butter substitutes were now even worse for us that actual butter and everything on the grocery shelf was suspect because the hydrogenated oil has a long shelf life, and hence, is ubiquitous.

But today I noticed that my microwave popcorn is made with partially hydrogenated oil, yet also boasts a big call-out on the front that says, "0 Trans Fats!!!"

How is this possible?

How high up are the partially hydrogenated oils in the ingredients list? It could be that there isn't enough of it to register more than half a gram per serving.

The FDA allows the term "zero trans fats" if there is less than 0.5 grams of trans fats per serving.

http://www.cfsan.fda.gov/~dms/qatrans2.html

Best,
Michael

Ok... a little organic chemistry:
When you partially hydrogenate a hydrocarbon, where there is still a double bond present, it will be in cis or trans configuration. These two options are called isomers. ASCII art doesn't do it justice, so I recommend this Wikipedia article on cis vs trans, or this one on trans fatty acids.

Polyunsaturated can be "cis". I don't know if it's possible to partially hydrogenate oils and keep the "cis"-ness.

A little more organic chemistry.

Many biological processes produce only one form of an isomeric compound. Laboratory processes (on up to industrial scale) tend to produce a mixture of the isomers, so the result differs from the natural product. A good example is vitamin E. The natural for is d-alpha tocopherol while the synthetic form also has l-alpha tocopherol. Vitamin pills using the synthetic form will show dl-alpha tocopherol as the active ingredient.

Biochemical activity of two isomers can be quite different, which is at the root of problems with trans fats.

There is an additional issue with partial hydrogenation in that the degree of hydrogenation can vary a lot. Two batches of partially hydrogenated soybean oil could have substantially different trans fat counts, for example, since partial covers anything from very slight to almost complete. Cis fats in the original oil could remain. Of course, if hydrogenation is complete, there will be no double bonds in the resulting fully saturated fat and therefore no cis or trans fat. Degree of hydrogenation, the mixture of fatty acids in the source oils and amount of oil per serving all affect the amount of trans fat in a serving even if serving size hasn't been chosen in a tricky manner. There are lots of ways to manipulate the variables to have partially hydrogenated oil and still have less than half a gram of trans fat per serving.