Oxidative Rancidity in Cooking Oils

A few oils in my kitchen…

One of my best gal pals (let’s call her Laura) recently posted a great three-part question, listed below. Laura happens to be wrapping up Vet school and is certainly no stranger to all things science-related. (And she’s a great cook, and hilarious, and sweet, and very cute…and for the right guy she just might be single… Note: if you’re reading this and I’ve stopped posting, it’s because she’s killed me.)

Okay, the three questions.

Q: Is most cooking oil rancid by the time we actually get around to using it?

A: I usually rely on “best used by” dates as a consumption guide. I’d venture a guess that cooking oil won’t make it out of the factory if it’s already rancid. Oil producers test their products for something called a peroxide value, which indicates the degree of oxidation/rancidity that has occurred in the oil.  (Peroxide value is only one of the many tests performed to ensure oil quality.) Additionally, most manufacturers pack oil under a nitrogen blanket which eliminates oxygen from head space in shipping/packaging containers, thus mitigating risk for rancidity from residual O2.  That said, many of us subject our cooking oils to abuse in our home kitchen, exposing them to oxygen, heat, and light – the three culprits for catalyzing oxidative rancidity.

FYI – Unsaturated fats will go rancid faster than saturated fats, and polyunsaturated fats will degrade fastest.

Q: How would rancid oil affect the taste/quality of the end product?

A: Some describe rancidity to taste like cardboard, some liken the flavor to old french fries, and others creatively describe the flavors as “off.” Whichever way you describe it, you’ll know when rancidity rears its ugly head.  Chances are good that the off flavors will carry over into the finished product – especially if you’re making a salad dressing, mayo, or finishing sauce.

Q: Is there any difference between canola and vegetable oil?

A: Yes. If you check the ingredient declaration on your vegetable oil, you’ll probably find it’s actually soybean oil. Canola oil is derived from rapeseed plants. The name Canola comes from “Canadian Oil Low Acid. (The product was developed in Canada in the 70’s, and was designed to be low in erucic acid.) Both vegetable and canola oil are inexpensive and have a mild flavor – big benefits for home cooking.

A Food Science tid-bite: To delay oxidative rancidity in your kitchen, purchase cooking oils in smaller containers (or portion out large bottles, and store excess in the fridge for later use), seal in-use bottles with a tight-fitting cap, and store oils in a cupboard away from light and heat.

I had to make something with oil today while scheming my post. So, I whipped up a quick strawberry vinaigrette with strawberry balsamic coulis, a touch of red wine, vinegar, and…(enter trumpets)…olive oil.

Coulis is a beautifully easy recipe. Puree fruit (with a little sugar and fresh lemon juice), strain, and use! I know…strawberries definitely aren’t in season, but I had some leftovers in the fridge.
Emulsified strawberry vinaigrette, using olive oil.

2 thoughts

    1. Saturated fats have a chemical structure where a hydrogen atom is attached by a single bond to every available carbon atom. Since the molecule is holding all of the hydrogen it can, the fat is considered “saturated,” and is solid at room temperature – think lard. Mono and polyunsaturated fats are types of unsaturated fats, but differ in the number of double bonds in their makeup (saturated fats contain only single bonds). Unsaturated fats (like cooking oils) are more susceptible to oxidation.

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