Preventing Deteriorative Changes in Fats and Oils

Have you ever opened a bag of chips or a bottle of cooking oil, only to be hit by a strange, stale, or “off” smell? Or perhaps you’ve tasted old nuts that have a distinct, unpleasant, paint-like flavor. That experience is the result of deteriorative changes in fats and oils, a process often simply called rancidity. Fats (which are solid at room temperature) and oils (which are liquid) are chemically active, and over time, they break down. This breakdown doesn’t just ruin the taste and smell of your food; it can also degrade its nutritional value and even produce harmful compounds. Understanding *how* this happens is the first step to preventing it and keeping our food fresh, safe, and delicious.

Autoxidation: The rancidity culprit 🔗

The most common and significant type of fat spoilage is autoxidation. As the name suggests, it’s a “self-fueling” reaction that requires oxygen. Think of it as the fat’s version of rusting; it’s a slow-burning chemical reaction that degrades the quality of the lipids. This process is a classic free-radical chain reaction, which food scientists break down into three distinct phases: initiation, propagation, and termination.

The initiation phase 🔗

This is the “spark” that lights the fire. All it takes is an “initiator” to knock a hydrogen atom off one of the fatty acid chains. This initiator can be energy from UV light (like sunlight hitting a bottle of oil), heat (from storage in a warm kitchen), or the presence of trace metals (like tiny particles of iron or copper from processing equipment). This action creates a highly unstable and reactive molecule called a free radical. This molecule is “unhappy” because it’s missing a hydrogen, and it will do anything to get one back.

The propagation phase 🔗

This is the chain reaction where the damage spreads exponentially. The new free radical immediately attacks a *different*, stable fatty acid, stealing *its* hydrogen atom. This stabilizes the first radical but creates a *new* one. This new free radical then reacts with oxygen (which is almost always present) to form a peroxy radical. This peroxy radical is also highly unstable and, in turn, attacks *another* stable fatty acid to steal its hydrogen. The cycle repeats: attack, create a new radical, react with oxygen, repeat. This cascade (propagation) builds up a large numberall of reactive compounds called hydroperoxides.

The termination phase 🔗

Eventually, the reaction fizzles out. This happens when two free radicals find each other and combine to form a stable, non-radical molecule. However, by the time this “termination” occurs, the hydroperoxides formed during propagation have already started to break down. These hydroperoxides are very unstable and decompose into a cocktail of other compounds, including aldehydes and ketones. It is these secondary products that have the strong, offensive odors and “off-flavors” that we associate with rancidity-the “cardboard” taste in stale crackers or the “painty” smell of old vegetable oil.

Factors influencing lipid oxidation 🔗

Since autoxidation is a chemical reaction, its speed is heavily influenced by its environment. Several factors can press the “gas pedal” on this process, accelerating spoilage. Controlling these factors is the key to food preservation.

Fatty acid composition 🔗

This is perhaps the most important factor. Fats are made of fatty acids, which can be saturated (like in butter and coconut oil) or unsaturated (like in olive and fish oil). The “saturation” refers to whether the carbon chain is fully loaded with hydrogen atoms. The sites of attack for oxidation are the double bonds found in unsaturated fatty acids. A monounsaturated fatty acid (like in olive oil) has one double bond. A polyunsaturated fatty acid (PUFA), like those in fish oil or flaxseed oil, has two or more. The rule is simple: the more double bonds a fat contains, the more susceptible it is to oxidation. This is why fish oil goes rancid much faster than lard.

Oxygen, temperature, and light 🔗

Oxygen is the fuel for autoxidation, so limiting its access is a primary preservation method. This is why chips are packaged in “nitrogen-flushed” bags (replacing oxygen with inert nitrogen gas) and oils are kept in tightly sealed bottles. Temperature acts as a catalyst; for every 10°C (18°F) rise in temperature, the rate of oxidation roughly doubles. Storing sensitive oils in a cool, dark place (or even the refrigerator) can dramatically slow down rancidity. Light, especially UV light, provides the initial energy for the “initiation” phase, which is why high-quality oils are often sold in dark glass or opaque tin cans.

Pro-oxidants and moisture 🔗

Pro-oxidants are substances that actively promote or accelerate oxidation. The most common culprits are trace metals, especially iron and copper. These metals can act as powerful catalysts for the initiation phase, creating free radicals at a much faster rate. Even microscopic amounts of these metals, perhaps from a metal scoop or a processing vat, can be enough to significantly shorten a fat’s shelf life. Moisture also plays a complex role. While completely dry fats are very stable, a small amount of water can actually increase oxidation by helping to dissolve the metal pro-oxidants, making them more mobile and reactive.

Lipolysis: Hydrolysis of fats 🔗

While oxidation gets the most attention, another deteriorative change, lipolysis, targets fats in a different way. “Lipo” means fat, and “lysis” means to break apart. Instead of oxygen, the attacker here is water, in a process called hydrolysis. A fat molecule (a triglyceride) is made of a glycerol “backbone” attached to three fatty acids. Lipolysis occurs when water breaks the ester bonds connecting the fatty acids to the glycerol, setting them loose. These “free fatty acids” (FFAs) can have a major impact on food quality.

Enzymatic hydrolysis 🔗

In many raw foods, this process is driven by enzymes called lipases. These enzymes are naturally present in foods like milk, grains, and oilseeds. For example, in raw milk, lipase will slowly break down the milkfat, releasing short-chain fatty acids like butyric acid. This is what gives certain cheeses (like Parmesan or Romano) their sharp, piquant flavor. But if uncontrolled in fresh milk or butter, it leads to a “soapy” or “goaty” off-flavor. Pasteurization (heating) is used to destroy these enzymes and prevent this type of spoilage.

Hydrolytic rancidity 🔗

Lipolysis can also be caused by heat and moisture alone, without enzymes. The classic example is deep-fat frying. When you drop moist food (like potatoes or chicken) into hot oil, the water escaping the food as steam attacks the hot oil. This is hydrolytic rancidity. The triglycerides in the oil break down, releasing free fatty acids. This build-up of FFAs is bad for several reasons: it lowers the oil’s smoke point (making it smoke at lower temperatures), causes the oil to foam, and can give the fried food an undesirable, sharp flavor.

Thermal decomposition 🔗

When you heat fats and oils to very high temperatures-especially past their smoke point-you get a different kind of breakdown: thermal decomposition. This is what’s happening when you see that acrid, blue-black smoke pouring from an unattended skillet. At these extreme temperatures (typically above 200°C or 400°F), the fat molecules themselves begin to crack and break apart. They can re-form into a wide variety of new, and often undesirable, compounds. These can include cyclic dimers, aldehydes, and ketones. These compounds not only create burnt, bitter flavors and acrid odors but also degrade the nutritional quality of the oil. This is why repeatedly re-using frying oil without filtering and careful temperature control is not recommended; these compounds build up over time, affecting both food quality and safety.

Antioxidants: Guardians of lipid quality 🔗

If autoxidation is the enemy, antioxidants are the heroes. An antioxidant is a molecule that can safely interrupt the free-radical chain reaction. They are “sacrificial” molecules; they work by generously donating a hydrogen atom to a free radical, neutralizing it and stopping it from attacking a new fatty acid. The key is that after the antioxidant donates its hydrogen, it forms a stable, unreactive radical that does not continue the chain reaction. This simple action can delay the onset of oxidation, extending the shelf life of food products from weeks to months.

Natural antioxidants 🔗

Many fats and oils come with their own built-in protection. The most common natural antioxidants are tocopherols, which are part of the Vitamin E family. These are abundant in most vegetable oils, with their primary purpose in the plant seed being to protect the oil from rancidity. This is one reason why unrefined, cold-pressed oils may have a more robust flavor profile and, in some cases, better stability, as they retain more of their natural tocopherols.

Added antioxidants 🔗

Sometimes, the natural antioxidants aren’t enough, or they are lost during refining. This is especially true for animal fats (like lard or tallow) which have very few natural antioxidants. In these cases, food manufacturers will add antioxidants to ensure a long shelf life. You’ve likely seen them on ingredient labels:

• BHA (butylated hydroxyanisole)
• BHT (butylated hydroxytoluene)
• TBHQ (tert-butylhydroquinone)
• Propyl Gallate

These synthetic antioxidants are very effective, even at tiny concentrations. They are commonly used in cereals, crackers, processed meats, and shortening to keep the fats stable and prevent those “stale” flavors from developing on the supermarket shelf. By stopping autoxidation before it can even really begin, these guardians of quality ensure that the food you buy is as fresh as the day it was packaged.

What do you think? After seeing how light, heat, and air affect your food, what’s one change you might make to how you store your cooking oils at home? Have you ever had to throw out food because it tasted “off,” and do you now have a better idea of what might have been happening?

References

1. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/autoxidation
2. https://onlinelibrary.wiley.com/doi/full/10.1111/1541-4337.12588
3. https://www.sciencedirect.com/science/article/abs/pii/S030881461830846X
4. https://www.ift.org/news-and-publications/food-technology-magazine/issues/2021/february/columns/food-safety-and-quality-frying-oil