Physical Properties of Lipids

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Water is the substance that makes life possible, which is why most organic molecules are soluble in water. Lipids are an exception, with the unique physical property of being hydrophobic, or insoluble in water. The physical properties of lipids give them an essential role in influencing the texture, appearance and healthfulness of the foods we eat. Thus, the food science industry has developed a number of criteria by which to measure and evaluate the physical properties of lipids.

The Facts

  • Lipids are one of the four major groups of organic macromolecules, along with proteins, carbohydrates and nucleic acids. All of these large molecules share the properties of being carbon-based and essential for life as we know it. Lipids are a diverse group of molecules that include fats, oils, waxes, phospholipids and steroids.

Structure

  • Like all organic molecules, lipids are composed of a chain of carbon atoms attached to other functional groups of atoms. Fats are composed of a glycerol (a three carbon alcohol) joined to three fatty acids.

Significance

  • Lipids are important in the food science industry, as lipids are a major source of dietary energy and affect the nutritional value, taste and texture of foods.

    Criteria important in food science include the solid fat content of the lipid, the cloud point and the smoke/fire/flash point.

Solid Fat Content

  • The solid fat content is the ratio of the lipid volume that is solid mass compared to the total mass of the lipid. Solid fat content affects spreadability, firmness, texture and stability of the lipid. Food manufacturers are interested in the solid fat content of products like butter and margarine.

Cloud Point

  • The cloud point is a measure of the temperature at which crystallization begins in an oil when cooled. Producing oils that don't form crystals at cool temperatures is of practical importance as it may increase the ability of some oils to be stored for prolonged periods.

Smoke/Fire/Flash Point

  • Understanding the smoke and fire points for particular lipids is important for selecting lipids that can be used at high temperatures. These points measure the effects of heating on the physical properties of the lipid and are an indicator of the amount of volatile organic material present in the lipid.

Saturated and Unsaturated Fats

  • Carbon's ability to form a maximum of four bonds with other atoms is important for understanding the difference between saturated and unsaturated fats.

    Saturated fats, like lard or butter, are solid at room temperature. This is due to the physical properties of the fatty acid tails in the lipid molecules. In a saturated fat, each carbon atom forms a single bond with hydrogen and other atoms in the molecule. This creates a fatty acid with a straight "tail," which allows many saturated fat molecules to be packed tightly together in a relatively small space.

    Unsaturated fats, like olive oil, are liquid at room temperature. In these fats, carbon atoms form double bonds that create a kink in the tail of the fatty acid. Kinks prevent tight packing of unsaturated molecules.

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