Fat can come from either animal or plant sources, and it may be solid or liquid.
All types of fat can become rancid. This happens when they are exposed to air, light, and heat.
"It's usually easy to detect because the food develops an unpleasant smell and taste. Some compare it to paint or turpentine, while others think it smells like dry grass," says Astrid Nilsson.
She researches fat at the food research institute Nofima.
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Reacts with oxygen
Fat is made up of different fatty acids. A typical fatty acid contains up to 22 building blocks called carbon atoms, linked together in a chain. The way the carbon atoms are connected determines whether the fatty acid is saturated or unsaturated.
Astrid Nilsson studies fat and food quality at Nofima.(Photo: Nofima)
Saturated fat is most commonly found in animal products such as butter, cheese, milk, and meat, although it is also present in coconut oil and palm oil. Unsaturated fat is found in plant-based oils like olive and rapeseed oil, as well as in nuts, seeds, and oily fish. These unsaturated fats are especially prone to reacting with oxygen.
"This starts a chemical reaction in the unsaturated fatty acids. The oxygen attacks the bonds between the carbon atoms. They become unstable and break down into smaller molecules called aldehydes and ketones. Together, these create the rancid smell and taste," explains Nilsson, who presented her research at a seminar at Nofima last week.
Toxic and odourless
Not all of the compounds produced during rancidity have a noticeable smell or taste.
"These substances may actually be of greater concern, because some of them have been shown to be toxic. Our research indicates that they form alongside the ones that produce an odour. So it appears that the rancid smell may indirectly protect us from the odourless compounds as well," says Nilsson.
Oxygen from the air enters the bottle the very first time it is opened. Each time you open and close it, a little more air gets inside. As the oil level drops, the air space at the top of the bottle grows larger.
"And every time you tilt the bottle to pour, the oil that has been exposed to air mixes with the rest of the oil," says Nilsson.
Lots of light and heat
It's not only air that starts the rancidity process. Light and heat also speed up the process.
"There's a reason why cod liver oil bottles are often green and why some manufacturers use frosted glass for their oil bottles," says Nilsson.
Butter and margarine are better protected against rancidity because air only comes into contact with the surface of the product. Even so, they can still go rancid.
"You can see this when a yellow layer forms on the butter or margarine. I usually scrape it off, because it can affect the taste. But consuming small amounts is not considered harmful," says Nilsson.
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Minus 80 degrees is ideal
Oil should be stored in a dark, cool place once the bottle has been opened. When Nilsson wants to stop rancidity in the lab, she freezes the oil at minus 80 degrees Celsius.
"That's the ideal temperature if we want to prevent rancidity completely, but it's obviously not practical in an ordinary home," she says.
Dark bottles provide better protection from light, but not from air.(Photo: Nina Kristiansen)
Many oils are sold in clear plastic bottles. These bottles may be less favourable than green or frosted bottles.
"Some types of plastic are denser than others, and some contain layers that block light more effectively. But these days we want to use less and thinner plastic. Multi-layer plastics can also create challenges when it comes to recycling," says Nilsson.
But are rancid oils and butter actually harmful to our health?
The body must defend itself against oxygen
Our bodies need oxygen, but they must also defend themselves against it.
The body's cells can be damaged by oxygen. To counter this, the body relies on a range of protective mechanisms, including antioxidants that we get through food.
Nilsson believes we also benefit from another form of protection: our ability to detect rancid food almost immediately through smell and taste. As a result, we are unlikely to consume large amounts of it.
"If you taste a single rancid seed on a crispbread, you throw it away," says Nilsson.
But she explains that people's sensitivity to rancid smells and flavours differs. This is partly because of how much rancid food they have been exposed to throughout their lives.
"Cured lamb, for example, can have a slightly rancid taste. Some people dislike it, while others love it," she says.
Nilsson has studied how the body responds to rancid fat. In a lab experiment, she exposed intestinal cells to varying amounts of highly rancid oils. The cells were able to cope up to a certain level.
In another experiment, healthy participants consumed rancid fish oil capsules for eight weeks. Blood tests taken afterwards did not reveal any significant changes.
"Even so, I recommend biting into one of these capsules from time to time to check whether the fish oil has become rancid," she says.
A Brazilian study found that some fish oil supplements sold in pharmacies and health food stores contained capsules that were so rancid they failed to meet international standards. The researchers therefore warned against storing them at room temperature for long periods.
There is no research indicating that consuming oils is harmful simply because they may oxidise iside the body, according to Nilsson.
"It's important to remember that oils also contain antioxidants, which help protect both the oil and us. Eating plenty of colourful foods, including berries, fruit, and vegetables, is another good way to increase antioxidant intake," she says.
