One Atom, Two Bonds

• At the chemical level, fat molecules are made up of carbon and hydrogen atoms. If each carbon atom is bonded to a pair of hydrogen atoms, it's referred to as a "saturated" fat because each possible connection is taken up with a hydrogen atom. If one or more of those connection points are attached to a second carbon atom instead, it's referred to as "unsaturated" because it doesn't have the maximum number of hydrogen atoms. Fats with one double carbon bond are called monounsaturated, and those with more than one are called polyunsaturated. Oils contain varying percentages of all three types of fat molecules.

Oxidation is the Culprit

• If the carbon and hydrogen atoms were left alone, the structure of the fat molecules wouldn't change over time. However, oxygen in the atmosphere can interact with fat molecules, breaking them down and changing their molecular structure. When that happens, the fats can quickly develop unpleasant flavors and odors. Saturated fats and monounsaturated fats resist oxidation relatively well, but polyunsaturated fats are very susceptible to oxygen damage. That's why corn oil is vulnerable.

About Corn Oil

• Among common cooking oils, corn oil has one of the lowest levels of saturated fat. That makes it heart-healthy, but less stable over time than other oils. It contains approximately 24 percent monounsaturated fats, and 59 percent polyunsaturated fats. This unusually high percentage of polyunsaturated fat makes corn oil among the most spoilage-prone of common cooking oils. It's best consumed within six months of purchase, especially if you live in a warm climate. Otherwise it can rapidly become unpleasant in taste and texture.

The Importance of Storage

• Light can speed oxidation, so it's best to keep your corn oil in a dark cupboard. For the best shelf life, the temperature in your pantry should be moderate and not change much. Some cooks prefer to store their corn oil in the refrigerator, which makes it appear thick and cloudy until it returns to room temperature. Once warmed, the oil returns to its original consistency and color. This provides an additional degree of protection against rancidity, and can extend the life of your corn oil by another month or two.