|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Possible applications of enzyme technology in rural areas|
The manioc plant has become of increasing interest as a consequence of its high starch content in tuberous roots that can be propagated in a wide variety of environments (6). Of the two varieties of manioc, the bitter and the sweet, the bitter varieties are of particular interest for cultivation for industrial use because of their higher starch content The starch content of these roots varies according to soil and climatic conditions, but is generally in the range of 20 - 40 per cent; the remaining 60 - 75 per cent is moisture with about 5 per cent of protein, cellulose, minerals, etc. (7). Manioc can provide a low-cost source of starch for food uses and is important for this reason, and it is of increasing importance to industry for the same reason. In many applications, it is desirable first to hydrolyze the starch to produce soluble sugars for subsequent use.
To this end, Lages and Tannenbaum have recently developed a process for using an extremely thermostable a-amylase for liquefaction of cassava starch followed by a high temperature treatment with amylogiocosidase (8). While starch hydrolysis is itself not new, the utilization of high temperature for hydrolysis has several advantages. These include rapid liquefaction of the starch and pasteurization of the product because of the elevated temperature.
In addition, it has been shown by Lages and Tannenbaum (8) that, following the thermoamylase treatment, the requirements for amyloglucosidase are greatly reduced, as is the time for complete conversion of the starch to sugar. The a-amylase employed was produced from Bacillus licheniformis (Thermamyl Liquid 60; Novo Industrials, Bagsvaerd, Denmark) added to either crude tapioca or cassava meal. Starting solutions of the starches were prepared by adding 33 g of material to enough water to give a total weight of 100 9. After preparation of these slurries, the a-amylase was added and the mixture was heated to 105°C for six minutes, after which the temperature was reduced to 95 C for two hours. At the completion of liquefaction, the temperature was reduced to 60 C and amyloglucosidase (AMG 150, liquid; Novo Industrials) was added.
The kinetics and conversion in such a process, in which the authors used an optimal amount of amyloglucosidase (0.5 per cent V/W) to achieve 100 per cent conversion in less than 24 hours, are illustrated in Figure 1. The resulting product from this hydrolysis can be used directly as a sugar syrup, or it can provide the substrate for subsequent fermentations to desired products, including single-cell protein and alcohol for fuel use (9).
The desirability of carrying out the starch hydrolysis before the production of single-cell protein is based upon the rationale that one can now use widely acceptable food yeasts, e.g., Saccharomyces cerevisiae or Candida utilis, rather than having to develop microbial species with amylolytic activity. These sugar syrups would also be quite suitable for making such products as baker's yeast. However, this approach to enzyme application implies the availability of the enzyme, which may, in fact, be the major limitation in the application of enzyme technology in the rural community.