15 Leaf and Seed Fermentations of Western Sudan
David B. Harper and M. A. Collins
Kawal, sigda, and furundu are fermented foodstuffs indigenous to the Kordofan and Darfur provinces of Western Sudan. All are produced by solid state fermentation of readily available plant materials of little or no economic value which, though unpalatable in its natural state (and indeed toxic in the case of kawal), contain protein rich in sulphur amino acids. In each case, fermentation yields a product that is not only organoleptically acceptable but also sufficiently highly regarded nutritionally by the local people to be employed as a meat substitute. As the sun-dried food can be stored indefinitely without deterioration, these fermentation processes represent a food preservation technique particularly well suited to the climate and conditions of this part of Africa. Biochemical and microbiological aspects of these fermentations and their nutritional implications have been investigated by Dirar (1), Dirar et al.(2), and Elfaki et al.(3).
Kawal is prepared from the fresh leaves of a wild and reputedly toxic legume, Cassia obtusifolia, which are pounded to a paste and packed into an earthenware zeer buried to the neck in the ground in a shaded location. A layer of green sorghum leaves is placed on the surface of the paste and the zeer fitted with a lid that is sealed with mud. At intervals of 3 days the vessel is opened, the sorghum leaves removed, and the paste remixed thoroughly by hand. The repacked paste is covered with fresh sorghum leaves and the zeer resealed. After 11 to 15 days, the strongly smelling black mass is removed, molded into small balls, and dried in the sun for 5 days. The dried kawal is usually consumed in a stew with onions, okra, or other local vegetables.
The seedcake remaining after oil extraction from Sesame indicum seed is the raw material for the sigda fermentation. The bitter, indigestible seedcake made from nondecorticated seed is often used only as animal feed. In the traditional sigda process the seedcake is ground to a paste with warm water. Kambo, a local form of potash from the dried leachate of the ash of the central stems of the sorghum seed head, is frequently, but not invariably, added (3 to 20 g/kg). The mixture is packed in an earthenware vessel sealed with a cotton cloth and a close-fitting lid to minimize access of air. The fermentation lasts 3 to 7 days at=30 C° with occasional remixing, addition of water if necessary, and resealing of the container, after which the product is molded into small balls and sun-dried. Like kawal, sigda is usually consumed in a vegetable stew. A similar fermented food, furundu, is prepared from the crushed seeds of karkade (Hibiscus sabdariffa) by a process almost identical to that employed for sigda.
The microflora of C. obtusifolia leaves (the substrate for the kawal fermentation) was dominated by four bacterial species, Bacillus subtilis, Lactobacillus plantarum, Propionibacterum sp., and Staphylococcus sciuri, and two yeasts, Candida krusei and Saccharomyces sp. Although the relative proportions of these organisms changed, all persisted in detectable numbers throughout fermentation. The principal species present during fermentation were B. subtilis and Propionibacterium sp., the other organisms comprising a comparatively small proportion of the population. No marked interspecial successional pattern occurred during fermentation.
The microflora of unfermented sesame seedcake was dominated by two bacterial species, Pediococcus sp. and Streptococcus sp., and two yeasts, Saccharomyces sp. and Candida sp. Pediococcus sp. was eliminated after the second day of fermentation, and the occurrence of the two yeasts was confined to the first half of the fermentation period. However, the homofermentative lactic acid bacterium Streptococcus sp. dominated the microflora throughout most of the fermentation. Additionally, the yeasts Debaryomyces sp. and Torulopsis sp. appeared in low numbers late in fermentation.
No detailed examination of the microflora during fermentation of furundu has been attempted, but the principal organism present in the final product was identified as a Bacillus sp.
PROTEIN CONTENT AND QUALITY
The crude protein content decreased only slightly, if at all, during fermentation of each substrate, indicating little loss of nitrogen during the process (Table 1). It is clear that the high sulphur amino acid content of all the fermentation substrates is largely retained in the fermented products, which compare favorably with the FAO reference protein in this respect (Table 2). The branched chain amino acids valine, leucine, and isoleucine also tend to be at a higher level in the protein of sigda and furundu than in the protein of their respective substrates. The other noteworthy feature is the markedly enhanced concentration of alanine in sigda and, to a lesser extent, in furundu, compared with the unfermented substrate. This increase is probably attributable to the transamination of pyruvate formed by oxidation of the lactic acid produced in the fermentation. Significantly, alanine concentration did not rise during the kawal fermentation where lactic acid production is negligible.
The overall protein quality of each of the fermented foods is determined by the content of lysine, which is limiting in the raw material for both the sigda and furundu fermentations and does not increase appreciably during fermentation. Nevertheless, the proteins of kawal and furundu, with chemical scores of 73 and 80, are of surprisingly good quality, whereas that of sigda, with a chemical score of 33, is no poorer nutritionally than the protein of the local staple cereal, sorghum.
MINERAL, CRUDE FIBRE, AND OIL CONTENT
Ash content of all fermented foods showed a substantial increase on that of the unfermented substrate, which, in part, reflects the mineral contribution made by clay scraped from the interior of the fermentation vessel during preparation but also, in the case of sigda and particularly furundu, the liberal addition of kambo (Table 1). The latter consists largely of potassium bicarbonate with smaller quantities of potassium chloride, silicate, and sulphate. C. obtusifolia leaves display an unusually high calcium content, which is believed to be critical in determining the course of fermentation (see below). Oil and crude fibre contents of the fermented foods was not significantly different from that of the unfermented substrates, suggesting that participation of these fractions in the fermentation process is unlikely.
CHANGES DURING FERMENTATION
The dominant role of lactic acid and the marked decrease noted in pH during the sigda fermentation contrast strongly with the high concentrations of volatile fatty acids (VFA) and minimal pH change observed in the kawal fermentation (Table 3 and Figure 1).
TABLE 1 Composition of Field Collected Kawal, Sigda, and Furundu Compared With That of Their Fermentation Substrates on a Dry Weight Basis
(a) ND Not detemmined
TABLE 2 Amino Acid Composition of Kawal, Sigda, and Furundu and Their Fermentation Substrates Compared With the FAO Reference Protein
Amino add concentration (g 16 g-1 N)
TABLE 3 Lactic Acid and Volatile Fatty Acid Content of Field Collected Kawal, Sigda, and Furundu (Mean and Range in g 100 g-1 Dry Matter)
Thus, by the eleventh day of the latter fermentation, VFA - mainly n-butyric (8 percent), acetic (5 percent) and n-propionic (9 percent) - comprised 15 percent of the fermentation mixture. However, the pH had not changed by more than 0.5 unit from the initial value. On the other hand, by the fifth day of the sigda fermentation, when a total acid concentration of 6 percent had been attained, the pH of the fermentation mixture had fallen to 4.0 from an initial value of about 6.0. This difference in the course of fermentation is almost certainly attributable to the stronger buffering capacity of the substrate of the kawal fermentation, C. obtusifolia leaves, which possess approximately double the calcium content of sesame seedcake. Conditions in kawal do not, therefore, favor the selection of acidoduric lactic acid bacteria.
In addition to these bacteria, the two yeast species present in unfermented sesame seedcake proliferated during the initial period of fermentation. Concomitantly, starch levels were observed to fall rapidly from 2 percent in the unfermented substrate to zero after the first two days of fermentation. As the only amylolytic organisms present, the yeasts presumably were responsible for degradation of starch, rendering it available to the lactic acid bacteria. The poorly fermentative yeasts Torulopsis sp. and Debaryomyces sp. isolated in the final stages of the fermentation can utilize lactic acid aerobically and may cause the decline in concentration of the compound during this period. The addition of kambo did not appear to have any significant effect on the course of the sigda fermentation, and it was concluded that this supplementation was probably practiced mainly on organoleptic grounds.
--, pH; -·-,VFA concentration; -O-, Am, lactic acid concentration; Am, -D- ammonia-N concentration
The VFA, primarily n-butyric and acetic acids, which are the principal products of the kawal fermentation, are characteristic of clostridial fermentation of plant material as is the accumulation of ammonia nitrogen. However, all attempts to isolate clostridial species from the fermentation mixture were unsuccessful, indicating that the microbial origin of VFA must be sought elsewhere. The formation of acetic acid can probably be ascribed to the heterofermentative B. subtilis, the co-dominant microorganism, whereas propionicacid is probably an end product of anaerobic fermentation by propio'Zi~acte ~ium sp. for which lactate is a preferred substrate. Utilization of lactate in this way could explain the low level of lactate in Karl, despite the substantial population of Lactoloacill~´s plantar´´m. The microbial pathway leading to formation of n-butyric acid is difficult to define, although its production may be characteristic of fermentation by this type of mixed culture as a whole rather than that by any single microorganism. n-Propanol (2.3 percent), n-butanol (01 percent), and ethanol (0.1 percent) were also detected in Karl toward the end of fermentation, though all were lost from the product during the drying phase. Formation of such alcohols is probably due to anaerobic fermentation of carbohydrate by the yeast species present.
The identification of Bacillus sp. as the principal microorganism in furundu when considered in the context of a final pH of 6.2 and the presence of both VFA and lactic acid in the fermentation mixture suggest that the furundu fermentation may be intermediate in character between those of sigda and kawal. Further investigation of thefuru,~du fermentation would be most instructive in this respect.
The sigda and furundu fermentations appear quite unlike the traditional oilseed fermentations practiced in Nigeria and elsewhere in West Africa where foods such as ogili and ogiri are fermented from castor oil seed (Ricinus communis) and melon seed (Citrullus vulgaris). There are even variations of the fermentation which use sesame seed and karkade seed known as ogiri-sara and red sorrel, respectively. During these West African fermentations, the pH increases to over 9.0 and ammonia production is frequently observed in the later stages. The fermentations are dominated by Bacillus s p., frequently Bacillus s´~btilis, an organism associated with spoilt sigda in Sudan. The principal reason for the difference would appear to be in the preparation of the seeds prior to fermentation, which in West Africa involves boiling for several hours in water until soft. Such pretreatment may alter the course of fermentation by two mechanisms - first, by rendering protein and polysaccharide more available for degradative attack by microorganisms, and second, by effectively eliminating much of the heat-sensitive indigenous microflora. The removal of amylolytic yeasts may well favor the selection of amylase-producing bacteria such as Bacillus sp. rather than lactic acid bacteria incapable of utilizing starch.
The three fermentations studied appear to afford a route by which unpalatable plant material or oilseed cake of little economic value can be converted into acceptable meaty-tasting food that is particularly rich in sulphur amino acids, which tend to be deficient in diets where access to meat or fish is limited. Phytic acid present in seeds can frequently hinder absorption of minerals in the gastrointestinal tract. As fermentation of plant products has been shown to reduce physic acid levels substantially, it is likely that the bioavailability of minerals in both sesame and karkade seed is increased in sigda and furundu.
1. Dirar, H. A. 1984. Kawal meat substitute from fermented Cassia obtusifolia leaves. Economic Botany 38:342-349.
2. Dirar, H. A., D. B. Harper, and M.A. Collins. 1985. Biochemical and microbiological studies on kawal, a meat substitute derived by fermentation of Cassia obtusifolia leaves. Journal of the Science of Food and Agriculture 36:881-892.
3. Elfaki, A. E., H. A. Dirar, M. A. Collins, and D. B. Harper. 1991. Biochemical and microbiological investigations of sigda - a Sudanese fermented food derived from sesame oilseed cake. Journal of the Science of Food and Agriculture 57.
[Украинский] [английский] [русский]