13 Improving the Nutritional Quality of Ogi and Gari
T. G. Sokari
Ogi is a blancmange-like product processed by fermenting the slurry from wet-milled maize (or sorghum or millet). Used as both a weaning food for infants and as a breakfast food by adults, ogi is one of the most important food items in Nigeria. Yet it is nutritionally inferior to maize, which is deficient in certain essential amino acids, because of the maize-milling process that is an integral part of ogi production.
Cassava, another very important food crop, has the problem of possible nutritional complications because it contains the cyanogenic glucosides linamarin and lotanstralin. Although the cyanogens in cassava are hydrolyzed to hydrogen cyanide during processing by the endogenous enzyme linamarase (1,2), not all processes are equally effective. It has even been suggested that traditional processing techniques are unlikely to remove all the cyanide from cassava (3,4).
In view of this, studies were undertaken to increase the protein content of ogi relatively inexpensively and to develop a technique for processing cassava into gari that would eliminate cyanogens from the product or reduce them to innocuous levels.
PROCESSING OF OGI
The traditional technique for processing maize into ogi is summarized in Figure 1. Also shown in Figure I is an alternative to this procedure; a 20-minute boiling step is substituted for the normal 24- to 28-hour steeping of maize prior to wet milling (5). Cowpea can be combined with maize to increase the protein content of ogi.
Substituting 20 minutes of boiling for the traditional 24 to 28 hours of steeping prior to wet milling maize reduced processing time from 72-76 hours to about 24 hours.
There was, however, no significant difference ( p> 0.05) in the aroma, color, taste, and overall acceptability between the products obtained by the short-time processing and traditional processing (5). The same was also true for unenriched and protein-enriched ogi except for color (6).
CYANIDE REDUCTION DURING CASSAVA PROCESSING
Two foods processed from cassava (gari and ijapu) were studied. Adding water to grated cassava at the 75 percent (v/w) level and heating at 50°C for 6 hours resulted in linamarin reduction of >99 percent (Figure 2). The pH of the mash fell from 6.4 to 6.3 during the period (7). After dewatering, the mash was adjusted to a pH below 4 by equilibrating with a 3-day fermented cassava liquor (40 percent, v/w) at 50°C for 12 to 18 hours. The equilibrated mash was then dewatered and toasted (Figure 3). A panel of tasters who were familiar with gari but otherwise untrained could not differentiate between the product and traditionally processed gari. Both sets of products were equally acceptable to the panel.
The modified procedure for processing cassava into gari reduced the processing time from >96 hours to about 24 hours. Cyanogens were not detectable in the product by the method of Cooke (8,9).
The study of the traditional production of ijapu (Figure 3) was intended to aid in understanding the loss of cyanogens during cassava processing. About 54 percent of the cyanogens in raw cassava were lost after boiling peeled cassava, but a substantial proportion remained in the water (Table 1). After slicing the boiled cassava and steeping the slices, a substantial proportion of the cyanogens was again lost.
TABLE 1 Cyanide Content of Cassava During Processing Into Ijapu
Note: Cyano, Cyanohydrin; numbers in parenthesis, percent loss; ND, not determined
Much of the loss could, however, be accounted for in the steep water, and the proportion lost depended on the duration of steeping and the cassava: water ratio (Tables I and 2).
ROLE OF FERMENTATION
The reduction of >99 percent in the linamarin content of grated cassava within 6 hours of adding water, with little or no change in the pH of the mash, would imply that fermentation had nothing to do with the detoxication. Linamarin breakdown is essentially a hydrolytic process catalyzed by the endogenous enzyme linamarase (1,2). The results of the present study indicate that the addition of water aids in the hydrolytic process. Apparently not all of the water normally in raw cassava tuber is available for hydrolysis.
During the boiling of cassava for processing into ijapu, linamarase would be inactivated. Yet a substantial proportion of the linamarin in cassava was still lost, appearing to a large extent in the water used for boiling and for steeping (Tables 1 and 2). This would suggest that leaching could be an important factor in cyanide loss during cassava processing. This would be true not only during the boiling and steeping of cassava for ijapu production but also during the dewatering of grated cassava for gari production.
Cassava detoxication during processing is essentially an enzymic hydrolysis of cyanogens in cassava (1,2,8). Fermentation has little role
TABLE 2 Effect of Sliced Boiled Cassava: Steep Water Ratio on Cassava Detoxification
(a) Prior to slicing and soaking in water.
in this process and may even be antagonistic to it (10). Leaching is another important process for cyanogen reduction during cassava processing. Although fermentation does not aid in cassava detoxication during processing, it is important in flavor development (11,12) and preservation of the product.
1. Conn, E.E.1969. Cyanogenic glucosides. Journal of Agricultural and Food Chemistry 17:519-526.
2. Nartey, F. 1978. Cassava: Cyanogenesis, Ultrastructure and Seed Germination. Copenhagen, Denmark: Munksgaard.
3. Cooke, R. D., and E. N. Maduagwu. 1978. The effects of simple processing on the cyanide content of cassava chips. Journal of Food Technology 13:299-306.
4. Oke, O. L. 1983. Processing and detoxication of cassava. Pp. 329-336 in: Proceedings: 6th Symposium of the International Society for Tropical Root Crops, Lima.
5. Sokari, T. G., P. S. Karibo, and L. F. F. Manuel, 1991. Substitution of boiling for steeping in ogi production. Discovery and Innovation (In press).
6. Manuel, L. F. F. 1990. Microbiological, Nutritional and Sensory Evaluation of the Effects of Cowpea Fortification of Ogi. M.Phil. thesis, Rivers State University of Science and Technology, Port Harcourt, Nigeria.
7. Sokari, T. G., P. S. Karibo, and C. K. Wachukwu. 1991. Reevaluation of the role of fermentation in cassava detoxification during processing into foods. Proceedings: Workshop on Traditional African Foods, Dar-es-Salaam, Tanzania (In press).
8. Cooke, R. D. 1978. An enzymatic assay for the total cyanide content of cassava. Journal of the Science of Food and Agriculture 29:345-352.
9. Cooke, R. D. 1979. Enzymatic assay for determining the cyanide content of cassava and cassava products. Cassava Information Center, Centro Internacional de Agricultura Tropical, Cali, Colombia, 05EC-6, 14 pp.
10. Maduagwu, E. N. 1983. Differential effects on the cyanogenic glucoside content of fermenting cassava activities. Toxicology Letters 15:355-359.
11. Vasconcelos, A. T., D. R. Twiddy, A. Westby, and P. J. A. Reilly. 1990. Detoxification of cassava during gari preparation. International Journal of Food Science and Technology 25:198-203.
12. Dougan, J., J. M. Robinson, S. Sumar, G. E. Howard, and D. G. Coursey. 1983. Some flavoring constituents of cassava and of processed cassava products. Journal of the Science of Food and Agriculture 34:874-884.
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