|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Possible applications of enzyme technology in rural areas|
Biocatalytic processes may be defined quite broadly, as illustrated in Table 1. Such processes include fermentation, enzyme catalysis, and whole cell systems It is interesting to reflect on the historical flow of this technology. Clearly, the earliest form of biocatalytic processes are represented by fermentation itself, both aerobic and anaerobic. In using such processes, the fermentation industry has developed, and continues to create and expand, new technologies related to pharmaceuticals, foods, and chemicals. However, as our understanding of microorganisms, and enzymes in particular, increased, it became possible to develop enzyme catalysis as a process technology in itself. This has taken place in the form of homogeneous catalysis using freely soluble enzymes for a wide range of applications in the food and pharmaceutical industries (1,2).
TABLE 1. Biocatalytic Processes
|Whole cell systems||cell suspensions|
Primarily as a consequence of the high cost of enzyme isolation and the sensitivity of enzymes to denaturation, technology has been developed for the immobilization of enzymes onto water-insoluble supports (3). This technology has opened the whole field of heterogeneous catalysis to enzyme catalysts. The use of immobilized enzymes has greatly minimized the cost of production by permitting repeated use of the enzymes, and has substantially increased the stability of the enzymes themselves. This technology has had major impact on the food and pharmaceutical industries and will clearly expand its importance in the years to come.
However, an interesting trend is occurring in the development of biocatalytic processes and that is the use of whole cell systems (4). The cells may be in the form of simple cell suspensions, or they may be packed in beds. Building on the development of immobilized enzymes, a technology has been developed to immobilize whole cells to achieve both stability and process control. In a sense, our development of sophisticated technology and our understanding of enzyme processes has allowed us to go full circle. The use of whole cell systems obviates enzyme isolation and immobilization, but presents the challenge of how to develop microbial cell systems with highly focused reactions and, returning to the original question: How can such systems be used to better life in rural areas?
The major class of enzyme reactions offering the most immediate possibilities for application to bioconversion in rural areas are the hydrolytic enzymes. The use of hydrolytic processes makes thermodynamic sense; such processes take advantage of the fact that hydrolysis of natural polymers in a predominantly aqueous environment has a free energy flow in the direction of hydrolysis. Such processes are usually carried out by one or a few enzymes, they do not require biological co-factors, and they permit de-polymerization under very mild conditions, thereby minimizing side-product formation. Furthermore, the substrates are readily available in rural communities. These substrates include cellulose, starch, hemicellulose, proteins, and many heteropolysaccharides that are present in plant materials.
The utilization of hydrolytic processes can be of benefit by improving current processes that are in use in rural communities; an example is the removal of mucilage from the coffee bean prior to drying (5). Or, it may be possible to use dextranases to eliminate the dextrans that form during prolonged harvesting times or improper storage of sugar-cane before sugar extraction. In a broader sense, it may be possible to use such hydrolytic processes to expand the available raw material base used in food or feed production. For instance, the hydrolysis of starch in products such as cassava, and the hydrolysis of cellulosic materials are possible routes for the production of sugar syrups for immediate use or for further processing.
In the remainder of this paper, I would like to focus on two applications of enzyme technology that might be suitable in rural areas but, more importantly, these two applications illustrate a broad class of possible use. In the first application, emphasis is on the use of enzymes in starch hydrolysis; in the second, it is on the use of whole cell systems in cellulose hydrolysis.