Oil bleaching, which is performed in order to prepare a sufficiently light-colored product of enhanced appear­ance and improved stability, is usually achieved by treating the crude or refined oil with powdered absorbent. These absorbents usually contain a calcium montmorillonite (fuller’s earth) or natural hydrated aluminum silicate (bentonite). Adsorption of color bodies, trace metals and oxidation products, as well as residual soaps and phospholipids remaining after washing neutralized oils takes place, if possible. Acid — activated clays are the major adsorbent used, although active carbons and synthetic silicas are also applied industrially with more specific goals. Thus, active carbons are used specifically to eliminate polycyclic aromatic hydrocarbons from some oils, especially fish oils and pomace oils, while synthetic silicas are quite efficient in adsorbing secondary oxidation products, phospholipids and soaps (LeiSn-Camacho et al., 2003). There are a number of good sources of material with more detailed descriptions of each process found online at the Lipid Library (Hardwood and Weselake, 2013), in "Proceedings of the World Conference on Oilseed Technology and Utilization" (Applewhite, 1993) and finally in, Edible Oil Processing (Hamm and Hamilton,

2000) .

Transesterification

Despite being energetically favorable, the direct use of plant or other biolipids in fuel engines is problematic as described earlier. Briefly, due to high viscosity (over 10 times higher than diesel fuel) and low volatility, they do not burn efficiently and can form deposits in the fuel injector of diesel engines. Furthermore, acrolein (a highly toxic substance) is formed through thermal decomposition of glycerol. Different ways have been considered to reduce the high viscosity of plant and other biolipids, but the principal method is to engage in chemical transesterification to produce biodiesel, which could be used in the common diesel engine with minor modification.

As mentioned previously, biolipids consist primarily of triglycerides, which are three hydrocarbon chains connected by glycerol. The bonds are hydrolyzed to allow the formation of FFAs, which are mixed and reacted with methanol or ethanol to form methyl (or ethyl) fatty acid esters. The use of methanol (methanol — ysis) is widespread and considered advantageous, as it is cheaper than ethanol (although in Brazil, ethanol 90 is plentiful) and has less azeotrophic qualities (Encinar et al., 2007). The same reaction using ethanol is more complicated as it requires a water-free alcohol, as well as a biolipid with low water content, in order to obtain good glycerol separation. Methanolysis can happen by heating 80—90% methanol with a small amount of cata­lyst. The received biodiesel after methanolysis is FAME and with ethanol to form fatty acid ethyl ester. The use of ethanolysis reaction using bioethanol has been discussed as being possibly more environmentally favorable as it would involve the use of a nonfossil fuel. Apart from this, ethanol is less toxic and slightly increases the cetane number of the biofuel. Although transesterification can proceed in the absence of catalysts, the reaction proceeds much too slowly to be economically viable and thus typically requires an acidic or alkaline catalysis. Among the most commonly used alkaline catalysts in the biodiesel industry are potassium hydroxide (KOH) and sodium hydroxide (NaOH) flakes, which are inexpensive, easy to handle and can be transported and stored easily. For this reason, they are preferred by smaller producers. Alkyl oxide solu­tions of sodium methoxide (NaOCH3) or potassium methoxide (KOCH3) in methanol, which are now commercially available, are the preferred catalysts for large continuous-flow production processes.

In the transesterification process, the effective species of catalysis is the methoxide radicals (CH3OO and the activity of a catalyst depends upon the amount of meth — oxide radicals (Komers et al., 2001a, b). For sodium or potassium hydroxide, the methoxide ion is prepared in situ by reacting methanol with hydroxide, a reaction that will also produce water that remains in the system. Hydrolysis of triglycerides and alkyl esters may occur due to the presence of this water, which further leads to the formation of FFAs and thus to a soap. Saponifica­tion may also occur if a strong base, e. g. NaOH or KOH, is present in the system by reacting with esters and triglycerides directly. All these problems can be avoided completely if sodium and potassium methoxide solutions, which can be prepared water-free, are applied (Singh et al., 2006).