5 Lipids in Foods

Composition and fatty-acid structure

Most food fats and oils consist primarily of triacylglycerols, also called triglycerides. Each is made of glycerol joined to three fatty acids. Foods also contain smaller amounts of other lipids, including phospholipids and sterols.

A fatty acid has a carbon chain and a carboxyl group. Fatty acids are called saturated when their chains have no double bonds, monounsaturated when they have one, and polyunsaturated when they have more than one. Chain length and the number and arrangement of double bonds affect how a lipid behaves physically and chemically.

Double bonds in naturally occurring unsaturated fatty acids are often in the cis arrangement, which bends the chain and makes close molecular packing more difficult. In general, longer and more saturated fatty acids pack more readily and have higher melting points than shorter or more unsaturated ones. This helps explain why many oils are liquid at room temperature and many fats are more solid, although the exact behavior depends on the triacylglycerol mixture, temperature, and processing history.

Melting, crystals, and texture

Fats do not have a single, sharp melting point. Their different triacylglycerols melt and crystallize across a range of temperatures. When part of a fat is solid and part is liquid, solid crystals can form a solid-fat network that traps liquid oil. This network helps determine hardness, spreadability, and how a food melts in the mouth.

Cooling rate, mixing, and other processing conditions affect the size and arrangement of fat crystals. As a result, the same fat can contribute different textures under different conditions. Fat crystallization helps give spreads their structure and helps chocolate develop its characteristic firmness and snap.

Liquid oils and solid fats can also contribute tenderness, lubrication, and a smooth or creamy sensation. In baked goods, fat can coat flour particles and limit some gluten development, contributing to tenderness. Beating a solid fat with sugar can incorporate small air pockets that help give a baked product a lighter structure.

Emulsions in foods

Many foods combine fat and water as emulsions, mixtures in which droplets of one liquid are dispersed in another. Phospholipids such as lecithin have both a water-attracting region and an oil-attracting region, so they can help stabilize the boundary between the two liquids.

The two liquids can be arranged in different ways: mayonnaise is an oil-in-water emulsion, while butter is a water-in-oil emulsion. Droplet size and stability, along with the structure of the surrounding food, affect an emulsion’s texture and appearance.

Oxidation and food quality

Oxidation is a set of reactions in which lipids, especially unsaturated fatty acids, react with oxygen. Light, heat, and transition-metal ions can promote it. In a common pathway called autoxidation, a free-radical chain reaction begins, continues as radicals react with oxygen and other lipid molecules, and slows when radicals combine.

Early oxidation products include lipid hydroperoxides. These unstable compounds can break down into smaller substances, including aldehydes, ketones, and alcohols. Some of these substances produce rancid or otherwise undesirable odors and flavors. Oxidation can also reduce desirable nutrients and affect food color or texture.

In general, a fatty acid is more susceptible to oxidation when it has more double bonds. Oxidation can be slowed by limiting exposure to oxygen, light, and excessive heat, and by using appropriate packaging and antioxidants.

Hydrolysis and oxidation

Hydrolysis is a separate process from oxidation: it breaks triacylglycerols into glycerol and fatty acids, often with the help of water and enzymes. The two processes are not the same reaction, though both can change flavor and food quality.

Lipids, aroma, and flavor

Lipids contribute flavor in several related ways. They can have flavors of their own, and volatile compounds formed as fats break down during cooking or storage can add characteristic aromas. Fat also dissolves and carries many aroma compounds, influencing when and how strongly those compounds are released as food is eaten.

Lipids may interact with compounds formed by other cooking reactions, contributing to the overall flavor of cooked foods. These effects can be desirable, as part of cooked-food aroma, or undesirable when oxidation produces rancid notes.