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I apologize in advance for my lack of knowledge in how a fat comes to be saturated, and perhaps therein lies the answer to this question. However, due to my wife's gall stones I need to remove hard to digest fats from my cooking routine. In addition to buying fewer raw ingredients that have high saturated, hydrogenated, or trans- fats, I want to be sure that food preparation methods do not change the chemical structure of fats that I do happen to be working with.

I may just be over-thinking this, but in cases of suspending one fat in another, emulsification, etc I wanted to know if there are any methods I should avoid due to the chemical changes they entail. For instance, deep-frying foods (which typically use the fats I hope to avoid) is said to make them difficult to digest.

Please list any others that do; or let me know if I'm just being a worry wart.

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By the way, I hope you are operating on the advice of a medical professional here, because even with all of the criticism around saturated fats (a lot of which may be valid), one criticism I definitely haven't heard is that they are more "difficult to digest". Concern tends to be more over the long-term health effects. –  Aaronut Jan 20 '11 at 16:52
    
@Aar working with the surgeon who was originally going to remove her gallbladder, we were given recommendations to avoid foods that impair liver health and to lower fat consumption generally and improve fiber consumption. He also recommended taking standard steps to reduce cholesterol and we are eventually meeting with a dietician to get some more tangible guidelines. –  mfg Jan 20 '11 at 17:38
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Fair enough @mfg. Just wanted to make sure, since I'm so used to seeing other people acting as their own doctors and dieticians. –  Aaronut Jan 20 '11 at 18:00
    
There's nothing wrong with me being responsible for my own diet. Why would I ever yield up that responsibility into less capable (and no doubt profiteering) hands? –  zanlok Jan 21 '11 at 14:47
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4 Answers

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The simple answer is no, you cannot convert monounsaturated/polyunsaturated fats into saturated fats through cooking alone.

Before I can even begin to answer in detail, I have to start by pointing out that "saturated" and "unsaturated" fat is already an oversimplification. These are very rough classifications of fats and the chemical reality is far more nuanced. I'd encourage you read the Wikipedia entry on Fatty acid for a relatively basic explanation.

Fatty acids are actually classified, chemically, in three different ways:

  1. The existence of double bonds (CH=CH) somewhere in the molecular structure. Fatty acids with double bounds are unsaturated. Fatty acids with single bonds only are saturated.

  2. The chain length (number of Carbon-Hydrogen groups). Another distinguishing characteristic of saturated fatty acids is that they are generally (maybe always) long-chain. The reverse is not necessarily true, however; not all long-chain fatty acids are saturated.

  3. The configuration, either cis or trans. Trans fatty acids are not actually a different type of fat in the same sense as saturated vs. unsaturated; they are actually unsaturated fatty acids, just in a different configuration.

Although certain studies suggest that it is possible to create TFAs from edible oils with prolonged heat, it is also extremely difficult, to the extent that it's nearly impossible to do in meaningful quantities by accident. I will refer you to this study of heat-induced cis/trans isomerization which says that after 8 hours of heating at 180° C, the researchers found up to 6.5 mg of trans isomers per 1.0 g of oil, which comes out to a grand total of 0.65% by mass. This is practically nil as far as a home cook is concerned - these quantities only matter if you're doing commercial processing of vegetable oils, or maybe if you're using the oil for deep-frying and you reuse it dozens of times (far beyond what any experienced fry cook would recommend).

The study also says that edible oils (i.e. the ones you cook with) experienced less isomerization than other kinds. So really, the amount of isomerization you're going to get (conversion of the "good" cis isomers to "bad" trans isomers) is minuscule and simply insignificant as far as mainstream baking or frying applications are concerned. So forget about trans fats.

Can you create saturated fats? That would effectively mean breaking down the double bonds into single bonds. In order to do that, you need to add hydrogen (that's why saturated fatty acids are "saturated" - more hydrogen).

There's a name for this process, which you actually stumbled upon in your question. It's called Hydrogenation. It's adding hydrogen atoms to an unsaturated (double-bonded) fatty acid.

Hydrogenation requires a substrate (involving benzene or some other hydrocarbon), a hydrogen source (that's pure, dangerous, H2 gas), and a catalyst (heavy metal). My guess is that your kitchen has none of those things, unless you're cooking in a chemistry lab. So there is simply no chance for you to accidentally hydrogenate your oils.

What you really need to be more worried about with oils (unsaturated fats) is lipid peroxidation. That's the oxidative breakdown and, eventually, rancidity of fats, and polyunsaturated fats are particularly prone to this. Heat is a catalyst for peroxidation, so if you "burn" your oil (or other fats), you may end up creating the same sorts of free radicals normally associated with rancidity due to improper storage. The long-term effect of these free radicals is not firmly established but the consensus seems to be that they aren't good for you in the long term (cancer risk and so on).

So don't worry about converting your oils when you cook with them. It's practically impossible. You should be more concerned with overheating them or letting them go rancid in storage.

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I notice a downvote; if I'm over-simplifying or if my answer is wrong somewhere, please let me know where. I want this to be strictly factual and correct. –  Aaronut Jan 20 '11 at 18:02
    
I take exception to the strong claim that one can't convert cis-configuration bonds to trans- by heating. Check out Real-Time Monitoring of Thermally Induced Trans Fats in Corn Oil Using the FatIR™ Oil Analysis System. Admittedly I am not a professional in this field, but the concept of the "tail" of the molecule turning over so one of the two hydrogen molecules opposite the double bond moves to the opposite side is not unreasonable. –  ErikE Jan 20 '11 at 23:32
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P.S. I think there's a misconception in calling cooking "non-chemical." Cooking in many ways is all about chemistry. While most of it may be the rearrangement of whole molecules in relation to each other, I don't think it's accurate to say the molecules are themselves never changed. For a simple example, doesn't the yeast organism produce new molecules rather than just rearrange existing ones? That's cooking. –  ErikE Jan 21 '11 at 0:10
    
@Emtucifor: Yeast is alive, for one thing, so you're really talking about a biological process, not a chemical one. I take your point; however, chemical reactions require chemical reactions and simply heating or mixing oils isn't going to cause that (except for the heat breakdown, i.e. peroxidation mentioned earlier). I didn't mean to imply that cooking in general was non-chemical, just the specific actions being talked about in this question. –  Aaronut Jan 21 '11 at 1:12
    
First, do you mind replying to the more important point about trans fats being produced by heating? Second, hold on there: you said "any other culinary process" which you equated to "non-chemical". But such an exclusion seems artificial. I'm not convinced that "culinary processes" can't cause chemical reactions. Do you have a source for this? Something as simple as baking soda + vinegar can make new molecules. Restricting that statement now to "only the processes mentioned in this post" seems an afterthought. Please understand this isn't an attack, just an inquiry. –  ErikE Jan 21 '11 at 1:16
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Fats don't generally change structure as a result of cooking. As you heat up a fat, it's structure loosens, but doesn't fundamentally change. (Cocoa fat is an exception.) So no, how you cook your food will not create more unsaturated fat (unless you're cooking it in unsaturated fat).

I think deep frying makes foods harder to digest not because it makes chemical changes to the food, but because it 1) dries out the food and 2) introduces more fat from the cooking oil.

If you want more info on what saturated and unsaturated mean, and which fats are less saturated, On Food and Cooking has a short section with lots of good info.

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Counterintuitive though it may be, deep fat frying, if properly done introduces less fat than other fat based cooking methods. That is because it is cooked at a higher temperature, so it sits in the fat not as long as other methods. The outward pressure of moisture escaping the food works to limit the oil seeping in. The problem with deep fat frying is that it is rarely done with the healthier types of oil like olive oil (doubt that you could). Also items deep fat fried are almost always coated with a batter of some type. –  Bill Jan 20 '11 at 23:13
    
@Bill: I think it's really the batter that's the issue. There are plenty of oils with high smoke points that are low in saturated fats - sunflower oil, for instance. –  Aaronut Jan 21 '11 at 2:45
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I'm not sure if I understand your question, but if the question is "Can I turn a monounsaturated or polyunsaturated fat into a saturated fat through cooking?" then the answer is "No, you can not."

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The process of saturating or unsaturating a fat involves breaking double bonds and adding hydrogen to the molecule. The more double bonds, the more UN saturated the fat.

This is all determined by the initial production of the fat. Animal fats are more saturated and certain 'desirable' fats are LESS saturated when they are created. There is no way that I know of to change this DURING the cooking process.

Extreme heat methods will make some chemical changes, but saturating the fat would involve breaking the bonds and adding hydrogen, and when this is done commercially it requires catalysts and other specific conditions.

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