Lipids

Lipids are substances that do not dissolve in water, including both fats and oils. The major difference between fats and oils is their state at room temperature — fats are solid but oils are liquid. However, some tropical oils are solid at cooler room temperatures (around 21°C) but melt quickly in a warm room.

Chemical Structure

Lipids are triglycerides, consisting of 3 fatty acids bonded to propane-1,2,3-triol, more commonly known as glycerol.

Fatty acids are made up of carbon chains with 4-22 carbon atoms, they can either be saturated or unsaturated. Saturated fatty acids consist of carbon atoms bonded by single bonds only. If triglyceride molecules have a high saturated fatty acids content, they would have a high melting point. This is because the straight molecules allow them to pack closely together, forming stronger intermolecular bonds. Examples of lipids naturally high in saturated fatty acids include animal fats, tropical oils and cocoa butter which are solid at room temperature. Saturated fatty acids are linked to various health risks, such as raised blood cholesterol and increased risk of coronary heart disease.

In comparison, unsaturated fatty acids consist of carbon atoms bonded by double bonds and triple bonds. There are 3 bonded electron regions around each carbon atom involved in a double bond, so there would be a trigonal planar structure around them (imagine each atom positioned at each of the vertices of an equilateral triangle, with the carbon atom at the centre). Hence, the long chain of carbon atoms bend at every double bond between carbon atoms. Most of the fatty acids are cis fatty acids, meaning that the atoms are located on the same side of the double bond. Monounsaturated fatty acid contains only one double bond between 2 carbon atoms, but there might be more double bonds between carbon and atoms of other elements. Polyunsaturated fatty acid contains more than one double bond between carbon atoms. This makes them more curled and therefore less likely to crystallise into solid fat.

Omega-3 fatty acids are polyunsaturated fatty acids with the last double bond being 3 carbons in from the end of the carbon chain. Omega is the last letter in the Greek alphabet so the name omega-3 indicates the structure of the fatty acid. Whilst omega-6 fatty acids are polyunsaturated fatty acids with the last double bond being 6 carbons in from the end. A healthy ratio of omega-6 to omega-3 fatty acids is considered to be 2:1 or lower. Corn, peanut, safflower, cottonseed oil and soybean oil have extremely high ratios of omega-6 to omega-3. Whereas canola oil has a ratio of 2:1 and salmon, flaxseed and walnuts are high in omega-3s relative to omega-6s.

Effect of Heat on Solid Fats

Solid fats contain tiny fat crystals that consist of fat molecules packed closely together. Fats have a mix of different fatty acids, so each has a different melting point. Fats soften when some fat crystals have melted whilst others have not. For example, butter softens noticeably around 27°C when bonds between shorter fatty acids break. At 34°C, all bonds between fatty acids break, including the longer fatty acids causing butter to liquefy. As the mouth temperature is around 37°C, butter melts completely resulting in a pleasant mouthfeel, whilst those that do not melt completely at 37°C can give a waxy feel. The final melting point is the temperature at which all fat crystals have melted into liquid, for butter this is 34°C when no solid fat crystals are visible.

Processing Lipids

Hydrogenation is the process which convert unsaturated fatty acids into saturated fatty acids. Fats and oils are hydrogenated by being exposed to hydrogen gas at high temperatures in the presence of a catalyst such as nickel, which would be removed before hydrogenated fat is packaged and sold. As saturated fatty acids have a high melting point, fully hydrogenated fats are solids making them hard to work with, so fats are usually only partially hydrogenated. In addition, hydrogenation can delay oxidative rancidity, which is the breakdown of fatty acids into smaller fragments that have rancid off flavours. As double bonds require little energy to break, fatty acids with more double bonds undergo oxidative rancidity faster. Soybean oil is highly polyunsaturated so it is often hydrogenated to delay rancidity. However, there are numerous health risks to a diet consisting of a high percentage of hydrogenated lipids. Besides the health risks associated to consuming saturated fatty acids, there are also health risks posed by trans fatty acids produced in partial hydrogenation. Trans fatty acids are where hydrogen atoms bonded to the carbon atoms involved in the double bond are positioned on opposite sides. They increase bad cholesterol (LDL) and decrease good cholesterol (HDL), raising the risk of coronary heart disease even more significantly than naturally occurring saturated fatty acids.

Plastic fats are solid fats consisting of liquid oil trapped in a network of solid fat crystals, making them soft and mouldable like plastic. Their consistency depends on the temperature. As mentioned before, butter is plastic at room temperature, solid when refrigerated and is a liquid above 34°C. Similarly, all-purpose shortening is plastic at room temperature, but is also plastic when refrigerated. This gives them a soft and workable consistency over a wide range of temperature. Blending saturated palm oil with fully hydrogenated solid fat could achieve the desired plastic consistency.

Moreover, soybeans can be genetically modified so that they are naturally low in polyunsaturated fatty acids. An enzyme such as lipase can also be used to rearrange or to change the order of fatty acids. This process is called interestification which can produce trans fat-free plastic shortenings. They could alter how the fat solidifies and melts, producing structured fat. They could also be used to improve the plastic properties of lard.

Reference

• Figoni, P.L. (2010) How Baking Works: Exploring the Fundamentals of Baking Science. 3^rd ed. Providence, Rhode Island: Wiley.