From S., 7 December 2010, Fear of frying … which oils should I use?
Q: I’m starting to hear a lot from my hippie dippie circles about not cooking at high temperatures with any oils except for coconut and walnut because even olive oil “turns rancid” at high temps (like when roasting veggies or sauteeing). I could just look it up on Google, but why would I do that when I could be lazy and ask you?
A: Hi and thanks for your question. Oh, yes. The hippie friends and their love of the coconut oil. Let’s discuss, shall we? With science.
Fats can be divided into two broad categories – saturated and unsaturated. Within the unsaturated category are monounsaturated and polyunsaturated fats. I’m going to oversimplify a lot to avoid getting into too much organic chemistry here. Fats are compounds of glycerols and fatty acids. Fatty acids are strings of carbon atoms, bonded with other atoms such as oxygen and hydrogen, that branch off into long chains. Saturated fats are so-called because the carbon atoms are “saturated” with hydrogen atoms. Saturated fats are very stable and solid at room temperature. Animal fats like lard, butter, and tallow are usually at least 40% saturated fats (with the remainder monounsaturated fat), although the proportion varies; coconut (palm) oil is over 80% saturated fat.
In the case of monounsaturated fats, the carbon atoms in component fatty acids are not completely saturated with hydrogen, and it is possible for one carbon atom to bond to another carbon atom within the chain. That one double bond in the fatty acid chain is the “mono,” and a single bond connects the remainder of the fatty acids. In the case of polyunsaturated fats, it is possible for one carbon atom to bond to another carbon atom in the chain more than once. Accordingly, at least two double bonds are present in the fatty acid chain (hence the “poly-”). Both mono- and polyunsaturated fats generally are liquid at room temperature, and in their liquid state are known as oils. Both can be made solid by hydrogenation – the process of mimicking saturation by adding hydrogen to the fatty acids in the otherwise unsaturated fat. Shortening, like Crisco, is an example of hydrogenated fat. Most fats of plant origin are a combination of mono- and polyunsaturated fat, to varying degrees. Olive, canola, and peanut oils are primarily monounsaturated fat. Soybean and sunflower oils are primarily polyunsaturated fat. According to some research (which is sort of inconclusive), monounsaturated fat is better for your health because high heat has particularly nasty effects on the double bonds in the polyunsaturated fats, such as during the frying process, especially if the fats burn.
Rancidification is the decomposition of fat, which in culinary terms usually takes place in the presence of heat or water (although it also takes place when fats are exposed to enzymes and bacteria). Some fats become rancid – or decompose – at lower temperatures than others; this temperature commonly is known as the fat’s or oil’s “smoke point.” The heat tolerance of the fat or oil per se should not be your only consideration, however. Certain fats and oils contain other substances, such as volatile oils that provide flavor and aroma. For example, walnut oil and olive oil both have characteristic aromas. The volatile oils responsible for those aromas are sensitive to heat and can become bitter or unpleasant when exposed to cooking temperatures. This reaction is distinct from rancidification and is one reason why many cooks choose to cook with relatively neutral-tasting refined oils rather than more flavorsome nut oils.
Interested in the smoke points of common cooking fats and oils? Enjoy this table.
You asked specifically about coconut oil and walnut oil. It is not true that coconut and walnut oils are less prone to become “rancid” at high temperatures than other oils. All oils – including coconut and walnut – will decompose at high temperatures, and refined oils fare better than unrefined oils. Unrefined coconut oil has a smoke point of 350F. At that temperature, it is unsuitable for deep-frying or for, say, roasting at hot oven temperatures. Similarly, unrefined walnut oil has a smoke point of 320F, making it unsuitable for most heat-based cooking applications other than poaching, and trust me on this: no one poaches anything in walnut oil. Also, unless your friends are rich, they probably can’t afford to cook with walnut oil anyway, because it costs about $10/500ml, last time I checked. It makes a great vinaigrette and finishing oil, though, so feel free to drizzle it all over your gem lettuce and local microgreens. Refined coconut oil has a smoke point of 450F, making it quite suitable for frying, but refining involves a bleaching process that I tend to think your hippie friends would reject. If you want to cook with the most stable fat – that is, the one that retains its integrity through the hottest heat, nearly all the refined oils are fine choices, although refinement may seem objectionable on various grounds.
In case you’re interested, I cook primarily with grapeseed oil, and, to a lesser degree, canola oil. I use olive oil and tree nut oils (hazelnut, pecan, walnut) for vinaigrettes and as finishing oils. Other oils of interest, such as avocado, are too expensive for day to day use in my opinion.
Oh, and one last thing. I read on someone’s Facebook page the other day about making “fudge” (it was a “clean eating fudge” that made me shudder, but I’ll just leave it at that for now) with a product called coconut butter. A question arose whether coconut butter and coconut oil were the same thing, because coconut oil appears to be solid at room temperature, and the fudge-maker huffed that they certainly were not, because butter is solid and oil is liquid. Well, not necessarily. This is a perfect example of the need to read labels. Coconut oil becomes solid at 25C/77F, or more or less room temperature in late spring/early summer. In winter, though, coconut oil will be solid in most parts of the United States. So what appears to be coconut butter may in fact be simply solid coconut oil. To complicate matters, though, some companies sell a product called “coconut butter” that contains not only coconut oil but also little ground up bits of coconut meat, for that extra-coconut taste. The moral of the story? Read the label!