rancid fats
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Rancid fats

We all know rancid fats are unpleasant. In fact, we use the word rancid to refer to other undesirable things. But what exactly is rancidity? What causes it? Is it bad for you? How can you prevent it?


Rancidity, in the chemical sense, is the “breakdown of fats by oxygen and light into small, odorous fragments” (1)

“When a fatty substance is exposed to air, its unsaturated components are converted into hydroperoxides, which break down into volatile aldehydes, esters, alcohols, ketones, and hydrocarbons, some of which have disagreeable odours.” (2)

Which foods can become rancid?

As you recall from my article on macronutrients, many foods besides the obvious (e.g. oils) contain fats. Out of those, the ones that contain unsaturated fats are more prone to rancidity. Of those, monounsaturated fats (MUFAs) are more stable, followed by polyunsaturated (PUFAs) ones. The omega-3 PUFAs are the most unstable (3).

Foods that can become rancid include: vegetable oils (including olive oil), nuts, seeds, fish and poultry. In turn, red meats and dairy foods (including butter) are more protected from rancidity because they contain more saturated fatty acids (SFA).

See the table below for the total PUFAs and MUFAs per 100g of selected foods (data source: 4). I have ordered the results PUFA content, so foods at the top are, potentially, more prone to rancidity than foods at the bottom. Note that some foods with low PUFA content have high MUFA content, so they will be more susceptible to rancidity than foods with more SFA.

Also watch out for packaged foods that contain fat as an ingredient as they can, too, be rancid by the time they reach your mouth. Remember they have been exposed to air, light and potentially high temperatures. In addition, they might be sitting on a supermarket shelf for months or years. A couple of rancid items I recently came across are nori (seaweeed) snacks and gluten-free shortbread cookies. If you can tell by taste of smell that the product is rancid, discard it asap.

Are rancid fats bad for you?

Apart from the off-odours and flavours, the oxidation of fat produces free radicals and chemical compounds that can react with proteins in our bodies and alter their function (3). This may result in detrimental effects in health.

How to avoid rancid fats


  • Avoid buying nuts from bulk containers
  • Try to buy nuts in their shell. If not possible, try to buy them whole and with the skin on instead of in pieces/slivered/sliced.
  • Store in a dark, cool place, or even better: in the fridge or freezer
  • Harold McGee recommends storing nuts in opaque glass jars instead of plastic bags to stop light and air from getting through (1).


  • Buy in dark bottles, small in volume to make sure you use them before they become rancid
  • As a general rule, unrefined oils (such as cold pressed extra virgin olive and avocado oil) are more stable than refined versions
  • Store in a dark, cool place
  • Don’t leave bottles open

Fish oil/omega-3 supplements

  • Liquid: buy in dark bottles and keep in the fridge
  • Capsules: buy in dark bottles and keep in the fridge or freezer


The following tips are based on Harold McGee’s recommendations (1):

  • If possible, avoid buying pre-ground meat and grind it yourself just before using
  • Try to buy meat in vacuum-sealed packaging
  • Wrap raw meat tightly in impermeable plastic wrap first, then foil to keep it dark. Store it in the coolest spot in the fridge or freezer and use it asap.
  • Pair meats with antioxidant-rich foods, such as rosemary and extra virgin olive oil


  1. McGee H. On food and cooking : the science and lore of the kitchen. New York: Scribner; 2004.
  2. The Editors of Encyclopaedia Britannica. Rancidity: Encyclopædia Britannica, Inc.; 2015 [Available from: https://www.britannica.com/science/rancidity.
  3. Vieira SA, McClements DJ, Decker EA. Challenges of utilizing healthy fats in foods. Advances in nutrition (Bethesda, Md). 2015;6(3):309S-17S.
  4. Food Standards Australia New Zealand (2014). AUSNUT 2011–13 – Australian Food Composition Database. Canberra: FSANZ. Available at www.foodstandards.gov.au

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