2.1.1 Background

Vegetable oils are lipid materials derived from plants and are in liquid phase at room temperature. Vegetable oils are mostly composed of triglycerides whose molecular structures are tri-esters of fatty acids based on glycerin backbones. Vegetable fats are classified basically in the same group as veg­etable oils, except that vegetable fats are solid at room temperature and are made of higher molecular weight materials. The distinction between veg­etable oils and fats may intuitively appear to be in their melting point dif­ferences. However, both vegetable oils and fats are actually mixtures of similarly structured triglyceride molecules, shown in Figure 2.1, as opposed to unimolecular substances; as such, a precise definition of melting point is impractical for them. Instead, melting point ranges are used to characterize vegetable oils and fats.

Vegetable oils have long been used by humans in a variety of essential end uses including food ingredients, cooking oil, heating, lighting, medicinal treatment, and lubrication. Some vegetable oils are directly edible, but many others are not. Examples of edible oils include sesame oil, corn oil, coconut oil, palm oil, sunflower oil, olive oil, peanut oil, rice bran oil, and soybean oil, whereas examples of inedible vegetable oils include linseed oil, tung oil, and castor oil which are used in lubricants, solvents, stains, paints, cosmet­ics, pharmaceuticals, and other industrial purposes. Basically, all edible oils and fats can be described as triglycerides where the acyl group (also known as alkanoyl group, RCO-) is a fatty acid moiety, which is from an aliphatic

y

FIGURE 2.1

Molecular structure of triglycerides.

carboxylic acid with an even number of carbon atoms that may contain one or more double bonds [1, 2]. Edible oils and fats always contain several differ­ent fatty acids, and inasmuch as almost all triglycerides in an oil or fat also contain various fatty acid moieties, the actual number of unique triglycerides in an oil or fat can be quite large. As such, a range or a distribution of melting points is used to characterize the oil and fat rather than a single unique value for its melting point. Because the triglyceride composition controls the physi­cal and chemical properties of the oil or fat, it is this composition that may have to be modified to alter the physical and chemical properties in order to meet the specific application requirements [1, 2]. Converting vegetable oil into conventional biodiesel via well-publicized and established transesterifi­cation reaction is a good example of this viewpoint.

Unsaturated vegetable oil molecules contain a number of unsaturated C=C double bonds in their molecular structures which can be hydrogenated by a relatively simple catalytic hydrogenation process. Hydrogenation of unsatu­rated vegetable oils can be achieved by bubbling or sparging the oil in the presence of a hydrogenation catalyst with hydrogen at high temperature and pressure. Precious metals such as platinum, palladium, rhodium, and ruthe­nium make highly active catalysts. However, the catalyst most commonly used is a powdered nickel catalyst (such as Raney nickel or Urushibara nickel) for economical reasons, as in most hydrogenation reactions. The nickel-based catalysts are less active and require higher pressures such as 60-70 atm. The reactor type used for such an operation is a three-phase flu­idized bed reactor. As the hydrogenation reaction converts unsaturated veg­etable oils toward saturated vegetable oils, both partially and fully saturated oils, the viscosity and melting point of the resultant oil increase. Although hydrogenation of vegetable oils can alter the oils’ physical properties and textures, the results of hydrogenation of vegetable oils are not necessarily beneficial, especially to human health. Partial hydrogenation of vegetable
oils results in the formation of a large amount of trans-fat in the resultant oil mixture, which is considered very unhealthy in edible oils [3]. Trans-fat is a common name for unsaturated fat with trans-isomer fatty acids and can be either mono — or poly-unsaturated in its structure. As well publicized, the negative health-related consequences and concerns of trans-fat consumption go far beyond cardiovascular risk to humans.