Glycerol production from oils provides a special case where both fermentation and enzymes are used in the processing of a food technology co-product. The process forms a closed circle, or cycle (Fig. 9.2), and biocatalysis is involved in almost every step of the procedure. Oils and fats are produced from renewable sources (e.g. oil plants, i.e. biomass) and are used in cooking, particularly frying. The waste (used) frying oils - usually selectively collected - (chemical name: triglycerides) can be converted either into biodiesel (transesterification with methanol) or fatty acids (enzymatic hydrolysis). In both cases glycerol is formed as a co-product.
Biodiesel is mainly produced industrially by transesterification with methanol using alkali as a catalyst (Mittelbach and Tritthart, 1988).
However, it is a hazardous material and should be treated after usage. By applying enzyme catalysis, a 'green (waste-free) process' can be realised by, for example, using lipase enzyme in a solvent-free system (Bélafi-Bakó et al., 2002b). Although the high price of the enzyme currently makes the process unprofitable, intensive research work is being carried out to develop cost-effective industrial enzyme preparations.
Hydrolysis of oils results in fatty acids and glycerol. The main difficulty in the process is caused by the distinct solubility of both substrates (triglycerides and water) and products. Enzymatic hydrolysis by lipase has several advantages compared with conventional methods. Instead of high pressure and temperature, the enzymatic reaction takes place under mild conditions resulting in energy savings and products of better quality. If the hydrolysis is carried out in a membrane bioreactor, further advantages are added to the process: the two phases remain separated during the reaction, thus there is no need to use an emulsion system, and the separation of the products can also be solved by the membrane. Moreover, the membrane offers a suitable surface for immobilization of lipase, resulting in a more stable biocatalyst, which can be used for a longer time (Bélafi-Bakó et al., 1994).
Glycerol can be processed further into PD (Papanikolaou et al., 2000) or PHA (Doi, 1992) compounds that are raw materials for green polyesters (as mentioned previously). These polymers can easily be degraded biologically and in this way - after use - they become part of the biomass. Thus the circle is closed.
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