Melting Point of Fats

The Melting Point of Fats

Fatty acids are single molecules, each with a distinct melting point. Because triglycerides contain different fatty acids, food fats have a range of melting temperatures. Ultimately, four factors determine a fat’s melting point:

• The fatty acid’s degree of saturation
• The length of its carbon chain
• Its cis-trans configuration
• Its crystalline structure

Most plant oils contain more polyunsaturated than saturated fatty acids, which causes them to be liquid at room temperature. Animal fats tend to have more saturated fatty acids, causing them to be solid at room temperature. The length of the fatty acids can alter these general rules, as even saturated fats with shorter carbon chains can have lower melting points than those with longer ones. Butyric acid and stearic acid are saturated fatty acids found in butter. However, butyric acid has only four carbons and thus melts sooner than stearic acid, which is eighteen carbons long. Coconut oil is a saturated oil containing short fatty acids, which cause it to remain liquid at room temperature.

Another structural difference that affects melting point is the configuration of the fatty acid. A fatty acid with a trans configuration has a higher melting point than an identical fatty acid with a cis form at the double bond. For example, oleic acid, an 18-carbon fatty acid with one double bond in the cis form, has a melting point of 57°F (14°C), while the same fatty acid in the trans form is called eladidic acid and has a melting point of 111°F (44°C). Hydrogenation, a commercial process that adds hydrogen to the double bonds of the unsaturated fatty acids, changes the cis form to a trans form.

The fourth influence on the melting point of fats is the arrangement of the fatty acids on the triglyceride molecule. How they are packed, or crystallized, in the solid phase of the fat determines when the fat will melt. This principle is important to chocolate manufacturers. The larger the fat’s crystals, the higher the melting point, which allows chocolate to be held in the hand without melting.

Most fats exhibit polymorphism, the ability to exist in more than one crystalline form. Fat crystals are classified as alpha, beta prime, or beta. The melting point of fats rises as the crystal sizes increase from alpha to beta prime, and eventually to beta. The rate of cooling dictates the type of crystals formed. Rapid cooling results in unstable alpha crystals with a waxy transparent consistency. Alpha crystals are extremely fine and very unstable, melting readily and recrystallizing into the larger, more stable beta prime form. These beta prime crystals can be obtained by agitating the fat during cooling, which should be conducted at an intermediate rate. Beta prime crystals are best for food preparation, because they yield fine-textured baked goods and smooth-surfaced hydrogenated vegetable shortenings. Extremely slow cooling or long storage times form the most stable, or beta, crystals, which have an opaque look. Beta crystals, however, produce a sandy, brittle texture.