6.3 Lipid oxidation and antioxidants

Lipid Oxidation

Lipid oxidation is the primary chemical process that makes fats go rancid, producing the off-flavors and off-odors that shorten a food's shelf life. Understanding how it works is essential for controlling it, and that's where antioxidants come in. This section covers the oxidation mechanism step by step, the products it generates, and the tools food scientists use to slow it down.

Rancidity and Free Radicals

Rancidity is the result of oxidative deterioration in lipids. When fats break down this way, they produce compounds that taste and smell unpleasant.

The process starts with free radicals, which are highly reactive molecules carrying an unpaired electron. That unpaired electron makes them unstable, so they aggressively seek out electrons from neighboring molecules. In foods, free radicals target the double bonds in unsaturated fatty acids, kicking off a chain reaction of oxidation.

Several factors generate free radicals in food systems:

• Light (especially UV)
• Heat (during cooking, frying, or storage at warm temperatures)
• Metal ions, particularly iron ($Fe^{2+}$) and copper ($Cu^{2+}$), which catalyze radical formation even at trace levels

Once the chain reaction gets going, it produces aldehydes, ketones, and other volatile compounds responsible for rancid flavors. Hexanal and pentanal are two commonly measured marker compounds.

The Autooxidation Process

Autooxidation is the spontaneous reaction between atmospheric oxygen ($O_2$) and unsaturated fatty acids. It proceeds through three distinct stages:

1. Initiation: A hydrogen atom is abstracted (pulled away) from an unsaturated fatty acid, usually at a carbon adjacent to a double bond. This creates a lipid free radical ($L\cdot$). The energy to start this step comes from heat, light, or metal ion catalysis.

2. Propagation: The lipid free radical reacts with $O_2$ to form a peroxy radical ($LOO\cdot$). That peroxy radical then abstracts a hydrogen atom from another unsaturated fatty acid, producing a lipid hydroperoxide ($LOOH$) and a new lipid free radical. This is the self-sustaining chain reaction: each cycle generates a new radical that feeds the next cycle.

3. Termination: The chain reaction stops when two radicals combine to form a stable, non-radical product, or when an antioxidant donates a hydrogen atom to neutralize a radical before it can propagate further.

The more double bonds a fatty acid has, the more susceptible it is to autooxidation. That's why polyunsaturated oils (like fish oil or flaxseed oil) go rancid much faster than saturated fats (like coconut oil).

Hydroperoxides and Decomposition Products

Lipid hydroperoxides ($LOOH$) are the primary products of autooxidation. They're tasteless and odorless on their own, but they're highly unstable. They readily break down into secondary oxidation products, and those are the compounds you can actually taste and smell.

Secondary products include:

• Aldehydes (e.g., hexanal, malondialdehyde)
• Ketones
• Alcohols
• Short-chain hydrocarbons

Heat, light, and metal ions all accelerate this decomposition, which is why rancidity develops faster in poorly stored foods.

Food scientists measure the extent of lipid oxidation using two common tests:

• Peroxide value (PV): Measures the concentration of hydroperoxides (primary products). A rising PV indicates early-stage oxidation.
• Thiobarbituric acid reactive substances (TBARS): Measures secondary products like malondialdehyde. TBARS reflects more advanced oxidation and correlates better with actual off-flavor development.

Using both tests together gives a fuller picture of where a product stands in the oxidation timeline.

Antioxidants

Antioxidants are compounds that delay or prevent lipid oxidation. They work through two main mechanisms: chain-breaking (donating hydrogen atoms to neutralize free radicals) and metal chelation (binding pro-oxidant metal ions so they can't catalyze radical formation). Food scientists use both natural and synthetic antioxidants depending on the application.

Natural Antioxidants

These are compounds naturally present in foods that inhibit oxidation. The major categories:

• Tocopherols (vitamin E): The most widely used natural antioxidants in the food industry. They're especially effective in vegetable oils and nuts, where they're already naturally present. Alpha-tocopherol is the most biologically active form, but gamma- and delta-tocopherols are actually more effective as antioxidants in food systems.
• Ascorbic acid (vitamin C): Commonly added to meat products to prevent oxidation and maintain the red color of myoglobin. It works as a chain-breaking antioxidant and also regenerates tocopherols, making the two more effective together than either one alone.
• Carotenoids (beta-carotene, lycopene): These pigments can quench singlet oxygen, a reactive oxygen species that initiates oxidation, particularly in light-exposed foods.
• Phenolic compounds (flavonoids, phenolic acids): Found in fruits, vegetables, herbs, and spices. Rosemary extract, for example, is widely used as a natural antioxidant in processed foods.

Synthetic Antioxidants

These are chemically synthesized compounds added to foods specifically to control lipid oxidation. They're effective at very low concentrations, which is one of their main advantages.

The four most common synthetic antioxidants:

BHA and BHT are often used together because they produce synergistic effects, meaning the combination is more effective than either one alone. TBHQ is considered one of the most effective synthetic antioxidants for frying oils.

Food safety authorities (such as the FDA and EFSA) regulate the use of synthetic antioxidants and set maximum permitted levels for each compound. These limits are based on toxicological studies, and manufacturers must stay within them.

Chelating Agents

Chelating agents don't neutralize free radicals directly. Instead, they bind pro-oxidant metal ions ($Fe^{2+}$, $Cu^{2+}$) into stable complexes, preventing those metals from catalyzing the initiation step of autooxidation. Think of them as a complementary defense that works alongside chain-breaking antioxidants.

Common chelating agents in food:

• Citric acid: Widely used in beverages, jams, and canned fruits and vegetables. It also contributes tartness, so it serves a dual purpose.
• EDTA (ethylenediaminetetraacetic acid): Often added to salad dressings, mayonnaise, and other emulsified products. It's particularly effective at very low concentrations.
• Phosphates: Used in meat and seafood products to control oxidation and improve water-holding capacity.

In practice, food scientists often combine a chain-breaking antioxidant (like BHT or tocopherol) with a chelating agent (like citric acid) to target both mechanisms of oxidation at once. This multi-pronged approach provides better protection than relying on a single antioxidant.