Introduction

Natural oils and fats are liquids or semisolids consisting primarily of triacylglycerols (TAGs). In the literature, triacylglycerols are often referred to as triglycerides, even though this latter term is less accurate in representing the molecular structure of this class of compounds. The distinction between fats and oils is a nuance seen in their physical state at ambient temperature. The major sources of oils and fats are from animals and plants. Greater than 90% of commercial oils and fats used for human consumption are plant-derived vegetable oils. Unrefined natural oils and fats, after extraction from the source, comprise mostly TAGs containing less than 5% of minor components such as sterols, phospholipids, tocopherols, fatty acids, and partial glycerol esters. The minor components and their concentration in the crude oil depend on the origin and method of oil extraction. The crude oils are subjected to various processing steps such as degumming and alkali refining (to remove phospholipids and fatty acids), bleaching (to remove colored pigments and polar matter), and steam stripping or deodorization (to remove volatile components) to make them suitable for human consumption. Oils subjected to these steps are usually referred to as RBD (refined, bleached, and deodorized) oils and contain about 99% TAGs.

The major vegetable oils of commerce are soybean, cottonseed, canola, sunflower, corn, peanut, palm, palm kernel, and coconut. Other vegetable oils like olive, rice bran, safflower, sesame, and other specialty oils are not used extensively due to availability and cost. A typical chemical structure of TAG is shown in Figure 1.1. The TAG contains a glycerol backbone with three hydroxyls esterified to three long linear carboxylic acids called fatty acids. The glycerol portion of TAG is constant in all oils and fats. The type of fatty acid structure and the position of esterification on glycerol differ from one TAG to another. Glycerol is a prochiral molecule capable of forming two different TAG stereoisomers, when esterified with different fatty acid chains at the 1 and 3 positions. These stereoisomers, when differentiated from one another, are identified as stereospecifically numbered, sn-glycerol derivatives (Kodali et al., 1984, 1989a). Even though biological systems can recognize the isomeric sn-glycerol derivatives, the physical and chemical properties of these isomers are very similar. For all functional and practical purposes they are treated as one and the same.

Vegetable oils contain a mixture of specific TAG molecules of given concentrations. Fatty acid structures differ from each other in carbon chain length and the number of double bonds. Most of the naturally occurring fatty acids are even numbered, 4 to 24 carbon atoms long, because they are synthesized from two-carbon unit acetyl coenzyme A. More prevalent saturated fatty acids with no double bonds that occur in oils and fats are lauric (C12:0), myristic (C14:0), palmitic (C16:0), and stearic (C18:0) acids. The number in parentheses shows the number of carbon atoms corresponding to the fatty acid chain length. A zero after the number indicates that there are no double bonds. The predominant unsaturated fatty acids are oleic (C18:1), linoleic (C18:2), and linolenic (C18:3) acids. The numbers in parentheses show the carbon chain length followed by the number of double bonds. The position of double bonds in the chain and the double-bond configuration are also very important. In oleic acid, the position of unsaturation is at carbon-9, in linoleic it is at C9 and 12, and in linolenic it is at C9, 12, and 15. Most of the unsaturated fatty acids that occur in natural fats and oils, with few exceptions, have the double bonds in cis configuration. The fatty acids that contain a single double bond are referred to as monounsaturated and others with more than one double bond are polyunsaturated fatty acids. Even though there are hundreds of different fatty acids that occur in oils and fats, the fatty acids referred to previously are most common and abundant in natural oils and fats.

The predominant vegetable oils in commerce can be divided into three types based on carbon chain length: lauric, palmitic, and oleic. The lauric oils, mostly coconut and palm kernel oils, contain high levels of 12-carbon lauric acid. The common palmitic oil, palm oil, contains 16-carbon palmitic acid in high concentration. The lauric and palmitic oils are high in saturated fatty acids and are semisolids at ambient temperature. Because coconut, palm, and palm kernel oils are grown in hotter climates closer to the equator, they are referred to as tropical oils. Oleic oils predominantly contain 18-carbon fatty acids such as stearic, oleic, linoleic, and linolenic acids. The soybean, sunflower, canola, corn, and cottonseed oils belong in this category. The major types of edible oils that are consumed in North America are soybean, corn, and canola. Palm oil is not extensively used in the United States, but its use has been rising because it provides cost-effective trans-free solutions. Palm oil is more extensively used in Asian countries and, to a certain extent, in Europe. Soybean and palm are the two most abundantly available vegetable oils in the world, together accounting for 60% of the total worldwide vegetable oil production of close to 157 million tons per year. The use of various vegetable oils in a given geography mainly depends on what is grown in that area and the cost and availability. Their yields, production volumes, and major production geographies for the four major vegetable oils are given in Table 1.A. Oil palm is a perennial crop with the majority of oil coming from the fruit pericarp and minor quantities from the kernel. Because of the high oil yields and production volumes, palm oil tends to be more cost competitive compared to other oils. However, production of the other vegetable oils yields protein as a byproduct along with the oil, which has nutritional and economic value. Beyond the cost and availability, nutritional and functional characteristics of fats and oils play an important role in their selection and use.

Defining vegetable oil composition by individual TAG molecular structure and concentration, rather than overall fatty acid composition of the oil, is more accurate but very cumbersome. Vegetable oils are identified by gross fatty acid composition (by weight). The fatty acid compositions of most edible oils used in commerce are shown in Table 1.B. The fatty acid compositions of various vegetable oils are determined by plant variety and genetics. In a given variety, the fatty acid composition changes a little due to geography and environmental factors. Because of this variation, the fatty acid composition is often expressed as a range rather than as a single number. The fatty acid compositions in Table 1.B are given in average weight percent.