Bioconversion of Waste Materials to Industrial Products

The immobilization of enzymes is a technique extensively studied since the late 1960s (Silman and Katchalski, 1966). The knowledge base accumulated on enzyme and cells immobilization studies has grown to very large proportions (Klibanov, 1983; Ariga et al., 1993; Crumbliss et al., 1993; Champagne et al., 1994). This wealth of information is one of the primary reasons for the present advances in enzyme engineering. The introduction of immobilized enzyme systems into commercial use, which was slower than predicted, has been the result of numerous factors, such as the long time required for approval of new processes for use in food applications, the need to control microbial contamination in biological reactor systems and some enzyme characteristics that limit the economic success of the immobilization process. The engineering of enzymes with better characteristics will overcome some of the problems encountered that have prevented commercial processes from developing.

Solid substrate fermentation processes have been used by man for many centuries. The term `solid substrate fermentation’ (SSF) describes the microbiological transformation on and/or within the particles of solid matrix (solid substrate) where the liquid content, bound with them, is at the level corresponding to the water activity (a <sub>W</sub>) assuring growth and metabolism of cells, but not exceeding the maximal water-holding capacity of the solid matrix (Cannel and Moo-Young, 1980a; Durand et al., 1988; Paredes-López and Harry, 1988; Pandey, 1992). In other words, in SSF the moist water-insoluble solid substrate is fermented by microorganisms in the absence or near-absence of free water, resulting in semisolid or solid fermentation systems (Hesseltine, 1977a). `Solid state’ and `solid substrate’ fermentations are terms used by different workers but they are essentially the same. On the other hand, in liquid state fermentations (LSF), the substrate is solubilized or suspended as fine particles in a large volume of water. SSF were used long before the microbiological or biochemical processes involved were understood. Various SSF processes with histories reaching far back in time are still practiced today (Moo-Young et al., 1983; Steinkraus, 1983a). These traditional SSF systems deal with fermented foods (e.g. tempeh in Indonesia, miso in Japan, pozol in Mexico), mold-ripened cheeses, starter cultures for fermented brews, and silage and compost. The use of soy sauce koji, a fermented oriental food preparation used in China, Japan and south-east Asia goes back as far as 1000 Bc and probably 3000 Bc in China (Cannel and Moo-Young, 1980a; Pandey, 1992).

Newly heightened concerns or realizations about climate change, depletion of stratospheric ozone, environmental pollution, wildlife, shrinking bio-diversity, health, food safety, land degradation, pressures on nonrenewable resources and intergenerational justice, have vindicated the sustainable development ethos expressed by the Brundtland Commission in its report, ‘Our Common Future’. In practical terms, the new paradigm promoted the 4R strategy of Reduce, Re-use, Recover and Recycle wastes to conserve resources and reduce pollution by emulating, adopting and maximizing beneficial processes of an ecosystem. An ecosystem is maintained in essence by the cycling of carbon and mineral nutrients so that the losses of nutrients from the system are not greater than the relevant mineral ions gained through soil weathering and the nitrogen fixed by biological agents or the electrochemical force in lightning.

Vast quantities of agricultural and agro-industrial residues that are generated as a result of diverse agricultural and industrial practices represent one of the most important energy-rich resources. Accumulation of this biomass in large quantities every year results not only in deterioration of the environment but in a loss of potentially valuable material which can be processed to yield a number of value added products such as food, fuel, feed and a variety of chemicals. A few of these potential residues are listed in Table 5.1. While cellulose, hemicellulose and lignin are the major constituents of these lignocellulosic residues, minor quantities of protein, pectin, soluble sugars, vitamins and minerals are also present. The composition of these residues, which depends on the age of the plant, is also different in different parts of the same crop, such as stalks, stems, straws, hulls and cobs. A correct analysis of the carbohydrates and other chemical components of lignocellulosic residues is of utmost importance for the evaluation of effectiveness of pretreatment method, product yield and process design. The limitations involved in comparison of data from different laboratories have been discussed by Saddler and Makle (1990). Using recent methods for their analysis, the carbohydrate components are first hydrolysed to monomeric sugars which are then estimated by gas chromotography (GC) or high-pressure liquid chromatography (HPLC). Based on the monomeric sugars, the relative amounts of the corresponding polymers from which they are derived, i.e., cellulose and hemicellulose, are calculated stoichiometrically (Karr et al., 1991; Puls, 1993; Hayn et al., 1993). This procedure also allows the determination of relative amounts of soluble sugars in the lignocellulosic biomass. The composition of a few lignocellulosic residues is summarized in Table 5.2.

Starch and flour, presumably from barley or emmer (a primitive variety of wheat), have been utilized from antiquity for the production of beer and bread (Demain and Solomon, 1981). Reliefs obtained from the fifth dynasty Egyptian tombs from about 2400m representing scenes of baking and brewing are now kept in the museum of antiquities in Lieden. From these first applications, the use of starch has grown both in quantity and quality in a large number of applications.

Today’s society, in which there is a great demand for appropriate nutritional standards, is characterized by rising costs and often decreasing availability of raw materials together with much concern about environ-mental pollution. Consequently, there is considerable emphasis on the recovery, recycling and upgrading of wastes. This is particularly valid for the food and food processing industry in which wastes, effluents and by-products can be recovered and can often be upgraded to higher-value and useful products.

Whey is a by-product of cheese production, when fat and casein are removed, which is generated at a rate of about 9 kg for every 1 kg of cheese or 6 kg for every 1 kg of cottage cheese (91% of the milk processed). One hundred kilograms of whey are equivalent to the sewage produced by 45 people (Jelen, 1979). At this rate, a medium sized cheese plant discharging 50–150 X 103 kg of whey day^-1 constitutes a polluting strength equal to the sewage from 22 500–67 500 people on a daily basis (Modler, 1982). In many countries, including Canada, most of the whey is discarded as waste creating severe pollution problems owing to its biological oxygen demand (BOD) (35 000–40 000 ppm) (Kosikowski, 1976; Scott, 1981). This BOD is due mainly to the lactose which is preent at concentrations of between 4.5% and 5% (Kosikowski, 1976; Garoutte et al., 1983).

For the last 20 years researchers have been shifting their attentions from ways of disposing of lignocellulosic wastes to utilizing these wastes to produce useful products of higher value. Examples of these products are: single cell protein (SCP) (Chahal, 1991a) to help to meet the ever increasing demand for protein and for introducing new foods to the world; and liquid fuel (ethanol) because of the dwindling supply of fossil fuels and also to minimize the environmental effects of pollution generated by these fossil fuels (McIntyre, 1987). Complete utilization of lignocellulosic wastes is the challenge that researchers face to produce the desired products economically and to eliminate the pollution generated by them.

The pulp and paper industry is one of the most important sectors of the Canadian economy. Although beneficial, this industry ïs associated with numerous environmental problems. Every year this industry uses approximately 80 millions tons of chemical products and an enormous quantity of fresh water (100–170 m3 ton^-i of produced pulp). This industrial sector is one of the biggest water polluters and hence is potentially harmful to aquatic ecosystems. Increasing demands for improvement in pulp quality and environmental safety standards forces this industry to make changes continuously. At present, the pulp and paper industry more than ever needs new technologies in order to minimize the production of hazardous substances.

When compared with the rest of the food industry, it has been generally regarded that the fish processing industry has been late in introducing new technologies to its production operations, including the treatment and/or recovery of wastes. Recently, interest has been focused on the application of new technological methods to operations related to the seafood industry, with the objective of increasing its general efficiency. To this end, the effects of technology on the nutritional value of seafoods has been presented by Pigott and Tucker (1990). In this context, it has been evident that the application of biotechnology to the utilization of biomass from by-products or wastes of the seafood industry could bring about improvements in its overall economy (Martin and Patel, 1991).

Chemical insecticides have been used to control insect pests that damage plants in agriculture and forestry or that are vectors of human diseases, such as malaria, filaria, yellow fever and encephalitis, which represent serious health problems in many countries (Ejiofor, 1991). These products are efficient but their production costs are high and they are sources of environmental pollution (Carlton, 1990; Ejiofor, 1991). They are also harmful to non-target organisms thus creating problems for the environ-mental equilibrium. Their action can affect nervous systems by disruption or inhibition of certain metabolic functions (Fisher, 1993) becoming a risk to all kind of organisms. They can also be accumulated in the environment, contaminating surface and underground water, soils, agriculture products and reach the human food chain (Carlton, 1990).

It is widely accepted that the Romans made use of biological solubilization to recover copper from the Rio Tinto area of Spain nearly two thousand years ago. However, one of the first written descriptions of what was almost certainly biologically assisted metal recovery appeared in about 1510.