Chapter 1
Introduction

1.1 Setting the Stage

World production of major oilseeds has increased from 331 million tonnes to 573 million tonnes in the last decade, whereas the harvested area has increased from 186 million acres to 278 million acres in the same period. According to the United States Department of Agriculture (USDA), soybean oilseeds represent over half of the total production of oilseeds and are mainly grown in Brazil, USA and Argentina; followed by rapeseed (grown in European Union (EU)-27, Canada and China), cottonseed (China and India) and sunflower oilseeds (Ukraine, EU-27 and Russia). Figure 1.1 shows the total production of major oilseeds around the world in 2017/2018.

Figure 1.1 Total production of oilseeds 2017/2018 (Mt).

Adapted from United States Department of Agriculture

http://www.fas.usda.gov/psdonline

©United States Department of Agriculture [1]

The figures shown above translate from 96 million tons (Mt) in 2002/2003 to 199 Mt in 2017/2018 of actual oils, with an average growth rate of ~7 Mt a year, (Table 1.1) [1]. This increasing demand is associated with the needs to feed an increasing population and, more recently, to the demand for biodiesels as partial replacements for fossil fuels. Minor vegetable oils such as castor oil and linseed oil are almost solely used for industrial applications because they are not appropriate for consumption by humans or animals.

Table 1.1 Major vegetable oils: world supply and distribution (Commodity view) - (million metric tonnes) [1]

Palm oil and soybean oil are the most important (as well as the most widely exported) oils, followed by rapeseed and sunflower counterparts (Table 1.1).

The common structure of vegetable oils discussed here is that of aliphatic triglycerides (Scheme 1.1), in which the 'fatty acid chains' R1, R2 and R3 are most often identical, but can also vary, within a given molecule. The length of the fatty-acid chain is 14-22 carbon atoms, but most members bear 16 or 18 units. The other important feature of these linear aliphatic motifs is the possible presence of C=C unsaturations, which range from 0 to 3. More than 1,000 fatty acids have been identified, but only »20 are present in appreciable quantities in vegetable oils [2, 3].

Scheme 1.1 Generic structure of a natural triglyceride component of vegetable oils in which R1, R2 and R3 are fatty-acid chains

Vegetable oils comprise a mixture of triglycerides (albeit with one or two specific structures which usually predominate). These compositions vary according to plant species, crop type, season, and growing conditions [4]. Table 1.2 enumerates the most common fatty acids in the triglycerides of plant oils and Scheme 1.2 shows their structures.

Scheme 1.2 Structures of the most common fatty acids

Table 1.2 Most common fatty acids in vegetable triglycerides [5, 6]

Some fatty acids (e.g., lauric, myristic, palmitic, stearic) are saturated, whereas others are monounsaturated (e.g., oleic, erucic) or polyunsaturated (e.g., linoleic, linolenic). In most vegetable oils, the double bonds of the fatty-acid chains are in the cis configuration (e.g., oleic, linoleic), although trans counterparts may also be present (e.g., a-eleostearic, licanic). The double bonds are more often non-conjugated (e.g., in linoleic and linolenic motifs) but conjugated sequences are also encountered (e.g., in eleostearic and licanic structures). Some oils contain fatty acid esters with other moieties along their chains, such as ricinoleic, vernolic and licanic structures with hydroxyl, epoxy and carbonyl groups, respectively.

Isolation of vegetable oils from their seeds is carried out mechanically or by solvent extraction [7]. The mechanical process consists of submitting the beans, cells and oil bodies to shearing to liberate oil. Heat is generated during this procedure, which can induce a negative effect on the proteins therein. Advantages of the mechanical-isolation process reside in its low cost, low investment and safety in terms of environmental concerns because it does not involve solvents or hazardous substances. It is, nevertheless, marred by poor yields of oil extraction because the amount of oil left in the ensuing residues can be £7%.

The principle of this type of solvent extraction is based on diffusion of solvent through seeds and subsequent solubilisation of oil. The most common solvents used in this process are alkanes with low boiling points such as hexane. The key parameter of this process is the rate of diffusion of the solvent into the oil body. This process is more efficient than its mechanical counterpart but involves use of volatile organic solvents (though their recuperation is highly optimised).

After isolation, vegetable oils are refined to obtain high-quality products free from impurities such as phosphatides, free fatty acids, gummy substances, coloured bodies, tocopherols, sterols, hydrocarbons, ketones, and aldehydes [2].

Composition of vegetable oils is highly variable as a function of the associated species, which determines their possible applications as renewable feedstock. Table 1.3 provides the typical composition of some vegetable oils in terms of their fatty-acid residues. In the case of the more exotic castor, oiticica and tung oils, the main fatty-acid residues are ricinoleic (87.5%), licanic (74%) and a-eleostearic acids...