Spoiled Rotten: How to Recognize Rancid Food

It's time for a thorough spring cleaning, and that also means tackling the pantry. To do this job right, look no further than your nose. From leftover walnuts to bags of whole wheat flour or the last dregs of olive oil, no container, bag or bottle should get by without a good sniff!

While teaching one of my university classes, I used some whole-grain barley flour (which I thought was fresh) to make scones. I noticed a strange odour while they were baking, but it was only when I sampled my handiwork that I recognized the unmistakable, bitter, cardboard-like taste of rancidity. Without revealing the problem to my students, I asked what they thought of my scones. Sure enough, some grimaced, but others simply brushed off the funky flavour, describing it as being the normal taste of whole-grain food. And therein lies the problem: Fresh barley flour tastes deliciously nutty but when it’s rancid—or when any ingredient is rancid, for that matter—it’s quite possible to (mistakenly) think that the odd taste is normal. Let’s take a closer look at rancidity, how to detect it, and especially how to prevent it.

How can you tell if something is rancid?

With your nose. Rancid foods have the same look and texture as when they were purchased, but their smell and taste have changed. The odour is akin to wet cardboard, oil paint, wood varnish or play dough. Some people are offended by it and know to throw the product out, while others think that’s just how the product normally smells.

There are a number of reasons why we’ve become accustomed to the taste of rancid food:

› Habit: Chopped nuts sold in grocery stores, for example, are so often rancid that we no longer know what fresh nuts really taste like.

› Super-sized formats sold in big-box stores: It takes us that much longer to get to the bottom of our giant bag of nuts or that big bottle of oil, so rancidity has more time to settle in.

› The use of monounsaturated or polyunsaturated oils: In recent years, these have been widely used by the food industry to replace hydrogenated fats (which tend not to go rancid). There is no doubt that these oils are much better health-wise, but they’re also more fragile—and susceptible to rancidity.

› Whole-grain foods: While they are good for your health, unfortunately whole-grain products go rancid much faster than we are used to: Brown rice, for example, has a much shorter shelf life than white rice.

What should I do with rancid food?

As soon as you ask yourself, “Does this smell normal?” there’s no doubt: Your food is rancid—beyond recovery. If you use it for cooking, it will lend a bitter taste and unpleasant odour to your dish.

How can I prevent food from going rancid?

Sooner or later, all foods that contain fat will go rancid, but to avoid waste:

› Buy in reasonable quantities, preferably at a store where there’s fast product turnover.

› Check the packaging date and best-before date, if there is one.

› Put nuts, seeds, whole flours, whole-grain cereals and other bulk purchases into sealable containers (avoid paper bags or thin plastic bags). Containers also protect against flour-loving insects.

› Make sure to close containers, bags or bottles after use.

What is rancidity exactly?

Simply put, it’s the oxidation of fat. Oxygen in the air attacks fat molecules and causes a series of chemical reactions that lead to the formation of new—and decidedly smelly—molecules. These reactions occur even more quickly in the presence of light and heat. Foods that contain polyunsaturated fats are particularly susceptible:

  • Whole flours
  • Seeds
  • Vegetable oils
  • Nuts
  • Brown rice

The problem can also extend to foods that contain the ingredients mentioned above:

  • Granola bars
  • Crackers
  • Chips

Product by product, here are our top tips:

NUTS: When in doubt, go for whole nuts, because they oxidize much more slowly than chopped nuts. Whole nuts will keep in the pantry for three to four months. Store all other nuts (chopped or ground) in the refrigerator (up to one year) or in the freezer (up to two years). You can add frozen nuts directly to recipes without having to thaw them.

ALMONDS: Whole, unblanched almonds will keep for about one year at room temperature, thanks to their high vitamin E (antioxidant) content and their skin, which protects against oxygen. Store sliced, chopped or ground almonds in the freezer.

SUNFLOWER AND PUMPKIN SEEDS: Shelled and roasted, they will keep for three months in the pantry, six months in the fridge and one year in the freezer. Raw sunflower seeds (that are not roasted) are much more stable and will keep up to a year in the pantry.

FLAXSEEDS: Whole flaxseeds resist rancidity and can be kept in the pantry for up to a year. Ground seeds will keep for three months in the refrigerator and six months in the freezer.

BROWN RICE: Since it contains both the bran and the germ (sources of polyunsaturated fat), brown rice is much more prone to rancidity than white rice. It can be kept for three months in the pantry, up to six months in the refrigerator or one year in the freezer.

WHOLE FLOURS: Factory-processed whole wheat flour (which is essentially white flour with the bran added) will keep for about six months in the pantry. Stone-ground, whole-grain flours, on the other hand, will keep for only three months. You can extend that by storing them for up to six months in the refrigerator or one year in the freezer. When ready to use, measure the necessary amount, then let it warm up to room temperature.

CEREAL FLAKES (OATS, BARLEY, SPELTS, ETC.): The good news here is that they will keep for up to one year without refrigeration. These grains undergo a gentle heat treatment that deactivates an enzyme that causes rancidity.

OILS: Keep them away from light and heat (and, obviously, never set them next to the stove!). How long an oil will keep after opening depends on its chemical makeup. Here’s how to store the most popular types of oil:

› Olive oil a monounsaturated oil rich in polyphenols (antioxidant molecules). It is less susceptible to rancidity than polyunsaturated oils and can be kept at room temperature for about one year after opening.

› Vegetable oils (canola, safflower, soy, sunflower, peanut, grapeseed, etc.) will keep for about six months after opening.

› Toasted sesame oil is rich in antioxidants and keeps well at room temperature for about six months after opening.

› Nut oils (especially walnut oil) are highly polyunsaturated and therefore quickly go rancid. Always store them in the fridge, for a maximum of three to four months after opening.

› Flaxseed oil is very fragile and will keep for four to six weeks in the fridge after opening.

For other oils (avocado, pumpkin seed, argan, etc.), look for the best-before date and storage information on the product label.

Is it dangerous?

Eating rancid food won’t make you sick, but the new molecules that form as oxidation occurs may lead to digestive issues. Rancid foods are also less nutritious because oxidation destroys the good fats and some of the vitamin content. Experts agree that eating rancid food or oils from time to time is probably not harmful, but they do suspect that regular consumption could contribute to the development of inflammatory diseases, cardiovascular illness and even certain cancers.


Christina Blais

For Christina Blais, explaining food chemistry to the masses is as simple as making a good omelet. Holding a Bachelor and Master degree in Nutrition, she has been a part-time lecturer for over 30 years in the Department of Nutrition at the Université de Montréal, where she teaches food science courses. She has been sharing the fruits of her experience with Ricardo since 2001, during his daily show broadcast on ICI Radio-Canada Télé. And diehards can also read her Food Chemistry on our website. You can follow her on Facebook at @Encuisineavecchristinablais.