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Nutrient and Anti-nutrient Contents of Aspergillus niger -Fermented Cassava Products (Flour and Gari)
JOURNAL OF FOOD COMPOSITION AND ANALYSIS (2002) 15, 617–622 doi:10.1006/jfca.2002.1065 Available online at http://www.idealibrary.com on
SHORT COMMUNICATION Nutrient and Anti-nutrient Contents of Aspergillus niger-fermented Cassava Products (Flour and Gari) G. Oboh*, A. A. Akindahunsi*,1, and A. A. Oshodiw *Department of Biochemistry, Federal University of Technology, P.M.B. 704 Akure, Nigeria; and wDepartment of Chemistry, Federal University of Technology, P.M.B. 704 Akure, Nigeria Received August 14, 2001, and in revised form February 8, 2002
Pure strain of Aspergillus niger was isolated and cultured using potato dextrose agar and broth, respectively. This was subsequently used to ferment six portions of 1 kg each of cassava pulp for 72 h. Three portions of the fermented cassava were each sieved and fried in a hot pan to gari, while the other three portions were each sundried and milled to cassava flour, the forms in which cassava products are popularly consumed in Nigeria. The results of the proximate analysis revealed that there was a significant increase (Po0.05) in the protein content (flour (12.270.2%) gari (7.370.1)) determined by micro-Kjeldhal method as well as the fat content (flour (5.770.1%), gari (4.070.3%)) of the Aspergillus niger-fermented cassava products compared with unfermented and wildly fermented cassava products. However, there was no significant increase (P40.05) in the ash and crude fibre contents. The protein and fat contents were significantly higher (Po0.05) in the cassava flour than the gari. The mineral composition (Na, K, Ca, Zn, Fe and Mg) of the products was considerably low, though gari had higher Na, K, Ca and Zn than the flour. The Aspergillus niger-fermented cassava products had very low cyanide (flour (9.170.2 mg/kg), gari (4.170.2 mg/kg)) and tannin (0.2%) contents. The gari had a significantly lower (Po0.05) cyanide content than flour, while there was no significant difference (Po0.05) in the tannin content. The results of the present study show that Aspergillus niger, a cheap, non-pathogenic saprophyte, has the capability to increase the nutritional potential of cassava products by increasing the protein and fat contents and decreasing the level of cyanide and tannin (anti-nutrients) present in r 2002 Elsevier Science Ltd. All rights reserved. them. Key Words: nutrient; anti-nutrient; Aspergillus niger; cassava.
INTRODUCTION Cassava, a good famine relief crop that is eaten by all classes of people, originated from northeast Brazil. The crop has become widespread and assumed its current importance as a food crop during the 20th century. It is a crop of the low land tropics and it does well in a warm climate where the mean temperature ranges are 25–291C 1 To whom correspondence and reprint requests should be addressed. Tel.: +234-34-200090. E-mail: [email protected]
0889-1575/02/050617 + 06 $35.00/0
r 2002 Elsevier Science Ltd. All rights reserved.
618
OBOH ET AL.
and rainfall is 100–150 cm per year (Onwueme, 1978). The advantages it has over other root crops include its propagation, high yields, pest and drought resistance (O’Brien et al., 1991). Certain varieties contain large amounts of cyanogenic glucosides, which can be hydrolysed to hydrocyanic acid (HCN) by the endogenous enzyme linamarase when the plant tissue is damaged during harvesting, processing or other mechanical processes. Cassava is often considered an inferior food because the storage root is low in protein, essential minerals and vitamins. However, in many cassava-growing areas, its use as food helps to alleviate problems of hunger and carbohydrate intake deficiency and thus its importance in terms of food security in these areas cannot be overemphasized. Thus, processes for upgrading the protein content of cassava have been developed in some countries such as Canada (Reade and Gregory, 1975), Burundi (Vlavonou, 1988) and Nigeria (Akindahunsi et al., 1999). This study is a continuation of our study on nutrient enrichment and detoxification of cassava products using cheap, non-pathogenic, saprophytic fungi, Aspergillus niger.
MATERIALS AND METHODS Materials About 60 kg sample of a strain of sweet cassava storage roots (50 mg HCN/kg) was collected randomly from the research farm of the Federal University of Technology, Akure, Nigeria. The chemicals used were of analytical grade, and glass distilled water was used. Sample Preparation The cassava storage roots collected were randomly divided into six groups; they were peeled, crushed and pressed. About 1 kg portion from each group was inoculated with pure strain of Aspergillus niger (isolated, characterized and cultured by Prof. F. C. Adetuyi of the Microbiology Unit of Biology Department, Federal University of Technology, Akure, Nigeria) along with 730 mL of nutrient solution (containing urea (80 g), MgSO4 ? 2H2O (7 g), KH2PO4 (13 g) and citric acid (20 g)). They were allowed to ferment for 3 days (Vlavonou, 1988). Three portions of the fermented cassava were each sieved and fried in a hot pan to gari, while the other three portions were each sun-dried and milled to cassava flour. The natural and unfermented cassava products (flour and gari) served as control. The samples (and control) were kept at room temperature for subsequent analysis. Sample Analysis The proximate composition (ash, fat, and crude fibre) of the fungi-fermented cassava products were evaluated using the standard Association of Official Analytical Chemists (AOAC, 1984) method and the protein content was determined using the micro-Kjeldhal method (N 6.25). The tannin content was determined using the method of Makkar et al. (1993) while the cyanide content was determined using the method of De Bruijn (1971). The Zn, Ca, Mg, K and Fe contents were determined on aliquots of the solutions of the ash by established flame atomic absorption spectrophotometry procedures using a Perkin-Elmer atomic absorption spectrophotometer (Model 372) (Perkin-Elmer, 1982).
NUTRIENT AND ANTI-NUTRIENT CONTENTS OF CASSAVA PRODUCTS
619
Analysis of Data The data were analysed by Students t-test (Zar, 1984).
RESULTS AND DISCUSSION In view of the low protein content, lack of essential amino acids and high cyanide content of some cassava products, methods of upgrading the protein content of cassava and reducing the cyanide content have been developed in some countries. Fortification with proteinous plants such as soyabeans and melon has been reported (Odetokun et al., 1998). However, solid media fermentation of cassava products with certain fungi has the advantage of being able to increase the protein content and reduce the anti-nutrient content. Organisms reportedly used include Asgergillus fumigatus in Canada (Reade and Gregory, 1975), Rhizopus oryzae in Burundi (Vlavonou, 1988) and Nigeria (Akindahunsi et al., 1999). In the present study, Aspergillus niger, a natural flora involved in cassava fermentation (Okafor, 1998) was investigated for its potential to increase the protein content and to decrease the cyanide and tannin contents of cassava products. The proximate composition of the cassava products is shown in Table 1. The results revealed that Aspergillus niger fermentation of cassava increased the protein contents of both the flour (12.270.2%) and gari (7.370.1%), as well as the fat content of both the products (flour (5.770.1%), gari (4.070.3%)). However, there was no significant change in the crude fibre and the ash contents of the Aspergillus niger-fermented cassava products when compared with unfermented and wildly fermented cassava products. Furthermore, Aspergillus niger appeared to be more efficient when compared to Rhizopus oryzae which is already in use for protein enrichment of cassava products; the maximum protein content reported so far for Rhizopus oryzae was 10% (Vlavonou, 1988; Akindahunsi et al., 1999). The increase in the protein content of the cassava products could be attributed to the possible secretion of some extracellular enzymes into the cassava mash in an attempt to make use of the cassava starch as a source of carbon. Apart from this, the increase in the amount of the microbial biomass in the form of single-cell proteins may possibly account for the increase in the protein content of the Aspergillus nigerfermented cassava products (Akindahunsi et al., 1999). The reason for the unusual increase in the fat content of the cassava products could not be categorically stated. However, there could be possible transformation of carbohydrate to fat; Akindumila and Glatz (1998) reported that certain fungi could produce microbial oil during the course of fermentation. TABLE 1 Proximate composition (% dry matter) of Aspergillus niger-fermented cassava products (mean 7 SEM) Unfermented Sample Protein Crude fat Ash Crude fibre Note: n=3.
Wildly fermented
A. niger-fermented
Flour
Gari
Flour
Gari
Flour
Gari
4.470.1 2.670.2 2.170.1 3.870.2
3.670.1 3.670.1 1.970.2 3.770.2
4.770.1 2.270.0 2.170.0 2.870.1
3.570.2 4.170.2 2.770.3 3.170.1
12.270.2 5.770.1 4.570.1 3.070.3
7.370.1 4.070.3 1.970.1 4.470.4
620
OBOH ET AL.
It is obvious from the results in Table 1 that the protein content of the flour was significantly higher (Po0.05) than that of the gari. This could be attributed to the method of preparation of each of the products. During the processing of gari, which entails pressing, sieving, and frying of the fermented cassava, some of the protein may have leached off during pressing and burnt off during frying (Okafor, 1998). It is documented that pre-processing, processing and post-processing methods of preparation of cassava products determine the quality of the products (Akindahunsi et al., 1999). The mineral contents (Zn, Mg, Fe, Ca, Na and K) of the Aspergillus nigerfermented cassava products (Table 2) are considerably low when compared to other food crops such as fruit, mushroom, yam tubers and vegetables (Akindahunsi and Oboh, 1998, 1999). However, the gari had a significantly higher (Po0.05) Zn, Ca, Na and K contents than the cassava flour. This might be as a result of the fact that some of the metals in the frying pan used might have leached into gari (Akindahunsi et al., 1999). Table 3 shows the level of anti-nutrients (cyanide and tannin) which the plants probably use for defense. Tannins affect the nutritive value of food products by forming a complex with protein (both substrate and enzyme) thereby inhibiting digestion and absorption. They also bind Fe, making it unavailable and recent evidence suggests that condensed tannins may cleave DNA in the presence of copper ions (Aletor, 1993). It also imparts a dull colour to the processed products, which affects their market value (Hahn, 1992). The tannin contents of the Aspergillus nigerfermented cassava flour and gari (0.2%) are considerably low when compared to the usual level of tannin in cassava tubers (0.4–0.5%) (Hahn, 1992) and compared favourably with tannin content of Rhizopus oryzae-fermented flour (0.2%). This low tannin content could possibly explain why the cassava flour and gari are brightly coloured compared to the usual dull colour of cassava products (Hahn, 1992). The products could also be considered safe in terms of tannin poisoning in view of the fact that the level of tannins is far below the reported deleterious level of 0.76–0.90% (Aletor, 1993). The residual cyanide present in the Aspergillus niger-fermented cassava products (flour (9.170.2 mg/kg), gari (4.170.2 mg/kg)) was significantly (Po0.05) lower than that of the unfermented and wildly fermented cassava products. However, the residual cyanide level was very low, when compared with the usual cyanide content of cassava products in Nigeria (19.0 mg/kg (gari)–25 mg/kg (fufu)) (Akindahunsi et al., 1999; Oke, 1968) and the cyanide content of Rhizopus oryzae-fermented cassava products (17.2 mg/kg (flour)–13.5 mg/kg (gari)). This shows that Aspergillus niger is TABLE 2 The mineral composition (mg/100 g dry matter) of Aspergillus niger-fermented cassava products (mean 7 SEM) Unfermented Sample Zn Mg Fe Ca Na K Note: n=3.
Wildly fermented
A. niger-fermented
Flour
Gari
Flour
Gari
Flour
Gari
13.170.3 43.470.2 26.070.4 61.670.7 43.870.3 49.870.4
5.870.1 27.770.5 2.370.1 16.770.2 51.470.3 55.670.4
13.270.2 53.270.1 33.470.1 53.971.0 64.670.2 28.170.2
7.670.2 37.170.3 5.470.0 19.970.1 51.670.2 46.170.2
6.970.2 42.570.6 3.270.2 15.370.2 50.470.3 50.770.5
11.670.2 36.770.3 2.570.1 23.270.2 82.070.2 73.770.2
NUTRIENT AND ANTI-NUTRIENT CONTENTS OF CASSAVA PRODUCTS
621
TABLE 3 The anti-nutrient content of Aspergillus niger-fermented cassava products (mean 7 SEM) (dry matter) Unfermented Sample Cyanide (mg/kg) Tannin (%)
Wildly fermented
A. niger-fermented
Flour
Gari
Flour
Gari
Flour
Gari
21.370.3 0.270.0
14.670.3 0.270.0
9.170.1 0.170.0
7.170.2 0.170.0
9.170.2 0.270.0
4.170.2 0.270.0
Note: n=3.
capable of utilizing cyanogenic glucoside and the breakdown products, thus explaining why it is one of the natural flora involved in cassava fermentation during gari processing (Okafor, 1998). It is also evident from the results that Aspergillus niger is more efficient in cyanide detoxification than Rhizopus oryzae, thereby possibly explaining its higher efficiency in protein enrichment. The lower cyanide in gari could be attributed to the process of garrification (Aletor, 1993). The products could also be considered safe in terms of cyanide poisoning in view of the fact that the cyanide contents were far below the deleterious level of 30 mg/kg (Akinrele et al., 1962). From this study, it could be concluded that Aspergillus niger, a cheap, nonpathogenic saprophyte, would efficiently increase the protein content of cassava products and reduce the level of the tannin and cyanide (both high and low cyanide cassava varieties).
REFERENCES Akindahunsi, A. A. and Oboh, G. (1998). Chemical studies on two tropical wild yams Dioscorea bulbifera and Dioscorea manganotiana. Riv. Ital. Sostanze Grasse 75, 511–514. Akindahunsi, A. A. and Oboh, G. (1999). Effect of some post-harvest treatments on the bioavailability of zinc from some selected tropical vegetables. Riv. Ital. Sostanze Grasse 76, 285–288. Akindahunsi, A. A., Oboh, G., and Oshodi, A. A. (1999). Effect of fermenting cassava with Rhizopus oryzae on the chemical composition of its flour and gari. Riv. Ital. Sostanze Grasse 76, 437–440. Akindumila, F. and Glatz, B. A. (1998). Growth and oil production of Apiotrichum curvatum in tomato juice. J. Food Prot. 61 (11), 1515–1517. Akinrele, I. A., Cook, A. S., and Holgate, R. A. (1962). The manufacture of gari from cassava in Nigeria. Proceedings of the First International Congress Ed. Tech., London, pp. 633–644. Aletor,V. A. (1993). Allelochemicals in plant food and feedingstuffs: 1. Nutritional, Biochemical and physiopathological aspects in animal production. Vet. Hum. Toxicol. 35(1), 57–67. AOAC. (1984). Official Methods of Analytical Chemists, Arlington, VA, pp. 125–126, 132, 877–878. De Bruijn, G. H. (1971). A study of the cyanogenic character of cassava. Meded. Lanbouwhogesch. Wageningen 71 (13), 1–140. Hahn, S. K. (1992). Cyanide and Tannin, Traditional Processing and Utilization of Cassava in Africa. Nigeria. IITA, pp. 20–25. Makkar, H. P. S., Blummel, M., Bowwy, N. K., and Becken, K. (1993). Determination of tannins and their correlation with chemical and protein precipitation method, J. Sci. Food Agric. 61, 161–185. O’Brien, G. M., Taylor, A. J., and Poulter, N. H. (1991). Improved enzymatic assay for cyanogens in fresh and processed cassava. J. Sci. Food Agric. 56, 277–289. Odetokun, S. M., Aiyesanmi, F. A., and Esuoso, K. O. (1998). Enhancement of nutritive value of pupuru, a fermented cassava product, Riv. Hal. Sostanze Grasse LXXV, 155–158. Okafor, N. (1998). An integrated bio-system for the disposal of cassava wastes, bio-systems in zero emissions applications. Proceedings of the Internet Conference on Integrated Bio-Systems. http:// www.ias.unu.edu/proceedings/icibs.
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Oke, O. L. (1968). Cassava as food in Nigeria. World Rev. Nutr. Diet 9, 227–250. Onwueme, I. C. (1978). The Tropical Tuber Crops, pp. 118–159. John Wiley and Sons Ltd., New York. Perkin-Elmer (1982). Analytical Methods for Atomic Absorption Spectrophotometry. Perkin-Elmer Corp., U.S.A. Reade, A. E. and Gregory, K. F. (1975). High temperature protein enriched feed from cassava fungi. Appl. Microbiol. 30, 897–907. Vlavonou, B. M. (1988). In Cassava Processing Technologies in Africa: In Praise of Cassava (N. D. Hahn, Ed.), Nigeria, IITA/UNICEF, pp. 25–32. Zar, J. H. (1984). Biostatistical Analysis, 620p. Prentice-Hall, Inc., U.S.A.
SHORT COMMUNICATION Nutrient and Anti-nutrient Contents of Aspergillus niger-fermented Cassava Products (Flour and Gari) G. Oboh*, A. A. Akindahunsi*,1, and A. A. Oshodiw *Department of Biochemistry, Federal University of Technology, P.M.B. 704 Akure, Nigeria; and wDepartment of Chemistry, Federal University of Technology, P.M.B. 704 Akure, Nigeria Received August 14, 2001, and in revised form February 8, 2002
Pure strain of Aspergillus niger was isolated and cultured using potato dextrose agar and broth, respectively. This was subsequently used to ferment six portions of 1 kg each of cassava pulp for 72 h. Three portions of the fermented cassava were each sieved and fried in a hot pan to gari, while the other three portions were each sundried and milled to cassava flour, the forms in which cassava products are popularly consumed in Nigeria. The results of the proximate analysis revealed that there was a significant increase (Po0.05) in the protein content (flour (12.270.2%) gari (7.370.1)) determined by micro-Kjeldhal method as well as the fat content (flour (5.770.1%), gari (4.070.3%)) of the Aspergillus niger-fermented cassava products compared with unfermented and wildly fermented cassava products. However, there was no significant increase (P40.05) in the ash and crude fibre contents. The protein and fat contents were significantly higher (Po0.05) in the cassava flour than the gari. The mineral composition (Na, K, Ca, Zn, Fe and Mg) of the products was considerably low, though gari had higher Na, K, Ca and Zn than the flour. The Aspergillus niger-fermented cassava products had very low cyanide (flour (9.170.2 mg/kg), gari (4.170.2 mg/kg)) and tannin (0.2%) contents. The gari had a significantly lower (Po0.05) cyanide content than flour, while there was no significant difference (Po0.05) in the tannin content. The results of the present study show that Aspergillus niger, a cheap, non-pathogenic saprophyte, has the capability to increase the nutritional potential of cassava products by increasing the protein and fat contents and decreasing the level of cyanide and tannin (anti-nutrients) present in r 2002 Elsevier Science Ltd. All rights reserved. them. Key Words: nutrient; anti-nutrient; Aspergillus niger; cassava.
INTRODUCTION Cassava, a good famine relief crop that is eaten by all classes of people, originated from northeast Brazil. The crop has become widespread and assumed its current importance as a food crop during the 20th century. It is a crop of the low land tropics and it does well in a warm climate where the mean temperature ranges are 25–291C 1 To whom correspondence and reprint requests should be addressed. Tel.: +234-34-200090. E-mail: [email protected]
0889-1575/02/050617 + 06 $35.00/0
r 2002 Elsevier Science Ltd. All rights reserved.
618
OBOH ET AL.
and rainfall is 100–150 cm per year (Onwueme, 1978). The advantages it has over other root crops include its propagation, high yields, pest and drought resistance (O’Brien et al., 1991). Certain varieties contain large amounts of cyanogenic glucosides, which can be hydrolysed to hydrocyanic acid (HCN) by the endogenous enzyme linamarase when the plant tissue is damaged during harvesting, processing or other mechanical processes. Cassava is often considered an inferior food because the storage root is low in protein, essential minerals and vitamins. However, in many cassava-growing areas, its use as food helps to alleviate problems of hunger and carbohydrate intake deficiency and thus its importance in terms of food security in these areas cannot be overemphasized. Thus, processes for upgrading the protein content of cassava have been developed in some countries such as Canada (Reade and Gregory, 1975), Burundi (Vlavonou, 1988) and Nigeria (Akindahunsi et al., 1999). This study is a continuation of our study on nutrient enrichment and detoxification of cassava products using cheap, non-pathogenic, saprophytic fungi, Aspergillus niger.
MATERIALS AND METHODS Materials About 60 kg sample of a strain of sweet cassava storage roots (50 mg HCN/kg) was collected randomly from the research farm of the Federal University of Technology, Akure, Nigeria. The chemicals used were of analytical grade, and glass distilled water was used. Sample Preparation The cassava storage roots collected were randomly divided into six groups; they were peeled, crushed and pressed. About 1 kg portion from each group was inoculated with pure strain of Aspergillus niger (isolated, characterized and cultured by Prof. F. C. Adetuyi of the Microbiology Unit of Biology Department, Federal University of Technology, Akure, Nigeria) along with 730 mL of nutrient solution (containing urea (80 g), MgSO4 ? 2H2O (7 g), KH2PO4 (13 g) and citric acid (20 g)). They were allowed to ferment for 3 days (Vlavonou, 1988). Three portions of the fermented cassava were each sieved and fried in a hot pan to gari, while the other three portions were each sun-dried and milled to cassava flour. The natural and unfermented cassava products (flour and gari) served as control. The samples (and control) were kept at room temperature for subsequent analysis. Sample Analysis The proximate composition (ash, fat, and crude fibre) of the fungi-fermented cassava products were evaluated using the standard Association of Official Analytical Chemists (AOAC, 1984) method and the protein content was determined using the micro-Kjeldhal method (N 6.25). The tannin content was determined using the method of Makkar et al. (1993) while the cyanide content was determined using the method of De Bruijn (1971). The Zn, Ca, Mg, K and Fe contents were determined on aliquots of the solutions of the ash by established flame atomic absorption spectrophotometry procedures using a Perkin-Elmer atomic absorption spectrophotometer (Model 372) (Perkin-Elmer, 1982).
NUTRIENT AND ANTI-NUTRIENT CONTENTS OF CASSAVA PRODUCTS
619
Analysis of Data The data were analysed by Students t-test (Zar, 1984).
RESULTS AND DISCUSSION In view of the low protein content, lack of essential amino acids and high cyanide content of some cassava products, methods of upgrading the protein content of cassava and reducing the cyanide content have been developed in some countries. Fortification with proteinous plants such as soyabeans and melon has been reported (Odetokun et al., 1998). However, solid media fermentation of cassava products with certain fungi has the advantage of being able to increase the protein content and reduce the anti-nutrient content. Organisms reportedly used include Asgergillus fumigatus in Canada (Reade and Gregory, 1975), Rhizopus oryzae in Burundi (Vlavonou, 1988) and Nigeria (Akindahunsi et al., 1999). In the present study, Aspergillus niger, a natural flora involved in cassava fermentation (Okafor, 1998) was investigated for its potential to increase the protein content and to decrease the cyanide and tannin contents of cassava products. The proximate composition of the cassava products is shown in Table 1. The results revealed that Aspergillus niger fermentation of cassava increased the protein contents of both the flour (12.270.2%) and gari (7.370.1%), as well as the fat content of both the products (flour (5.770.1%), gari (4.070.3%)). However, there was no significant change in the crude fibre and the ash contents of the Aspergillus niger-fermented cassava products when compared with unfermented and wildly fermented cassava products. Furthermore, Aspergillus niger appeared to be more efficient when compared to Rhizopus oryzae which is already in use for protein enrichment of cassava products; the maximum protein content reported so far for Rhizopus oryzae was 10% (Vlavonou, 1988; Akindahunsi et al., 1999). The increase in the protein content of the cassava products could be attributed to the possible secretion of some extracellular enzymes into the cassava mash in an attempt to make use of the cassava starch as a source of carbon. Apart from this, the increase in the amount of the microbial biomass in the form of single-cell proteins may possibly account for the increase in the protein content of the Aspergillus nigerfermented cassava products (Akindahunsi et al., 1999). The reason for the unusual increase in the fat content of the cassava products could not be categorically stated. However, there could be possible transformation of carbohydrate to fat; Akindumila and Glatz (1998) reported that certain fungi could produce microbial oil during the course of fermentation. TABLE 1 Proximate composition (% dry matter) of Aspergillus niger-fermented cassava products (mean 7 SEM) Unfermented Sample Protein Crude fat Ash Crude fibre Note: n=3.
Wildly fermented
A. niger-fermented
Flour
Gari
Flour
Gari
Flour
Gari
4.470.1 2.670.2 2.170.1 3.870.2
3.670.1 3.670.1 1.970.2 3.770.2
4.770.1 2.270.0 2.170.0 2.870.1
3.570.2 4.170.2 2.770.3 3.170.1
12.270.2 5.770.1 4.570.1 3.070.3
7.370.1 4.070.3 1.970.1 4.470.4
620
OBOH ET AL.
It is obvious from the results in Table 1 that the protein content of the flour was significantly higher (Po0.05) than that of the gari. This could be attributed to the method of preparation of each of the products. During the processing of gari, which entails pressing, sieving, and frying of the fermented cassava, some of the protein may have leached off during pressing and burnt off during frying (Okafor, 1998). It is documented that pre-processing, processing and post-processing methods of preparation of cassava products determine the quality of the products (Akindahunsi et al., 1999). The mineral contents (Zn, Mg, Fe, Ca, Na and K) of the Aspergillus nigerfermented cassava products (Table 2) are considerably low when compared to other food crops such as fruit, mushroom, yam tubers and vegetables (Akindahunsi and Oboh, 1998, 1999). However, the gari had a significantly higher (Po0.05) Zn, Ca, Na and K contents than the cassava flour. This might be as a result of the fact that some of the metals in the frying pan used might have leached into gari (Akindahunsi et al., 1999). Table 3 shows the level of anti-nutrients (cyanide and tannin) which the plants probably use for defense. Tannins affect the nutritive value of food products by forming a complex with protein (both substrate and enzyme) thereby inhibiting digestion and absorption. They also bind Fe, making it unavailable and recent evidence suggests that condensed tannins may cleave DNA in the presence of copper ions (Aletor, 1993). It also imparts a dull colour to the processed products, which affects their market value (Hahn, 1992). The tannin contents of the Aspergillus nigerfermented cassava flour and gari (0.2%) are considerably low when compared to the usual level of tannin in cassava tubers (0.4–0.5%) (Hahn, 1992) and compared favourably with tannin content of Rhizopus oryzae-fermented flour (0.2%). This low tannin content could possibly explain why the cassava flour and gari are brightly coloured compared to the usual dull colour of cassava products (Hahn, 1992). The products could also be considered safe in terms of tannin poisoning in view of the fact that the level of tannins is far below the reported deleterious level of 0.76–0.90% (Aletor, 1993). The residual cyanide present in the Aspergillus niger-fermented cassava products (flour (9.170.2 mg/kg), gari (4.170.2 mg/kg)) was significantly (Po0.05) lower than that of the unfermented and wildly fermented cassava products. However, the residual cyanide level was very low, when compared with the usual cyanide content of cassava products in Nigeria (19.0 mg/kg (gari)–25 mg/kg (fufu)) (Akindahunsi et al., 1999; Oke, 1968) and the cyanide content of Rhizopus oryzae-fermented cassava products (17.2 mg/kg (flour)–13.5 mg/kg (gari)). This shows that Aspergillus niger is TABLE 2 The mineral composition (mg/100 g dry matter) of Aspergillus niger-fermented cassava products (mean 7 SEM) Unfermented Sample Zn Mg Fe Ca Na K Note: n=3.
Wildly fermented
A. niger-fermented
Flour
Gari
Flour
Gari
Flour
Gari
13.170.3 43.470.2 26.070.4 61.670.7 43.870.3 49.870.4
5.870.1 27.770.5 2.370.1 16.770.2 51.470.3 55.670.4
13.270.2 53.270.1 33.470.1 53.971.0 64.670.2 28.170.2
7.670.2 37.170.3 5.470.0 19.970.1 51.670.2 46.170.2
6.970.2 42.570.6 3.270.2 15.370.2 50.470.3 50.770.5
11.670.2 36.770.3 2.570.1 23.270.2 82.070.2 73.770.2
NUTRIENT AND ANTI-NUTRIENT CONTENTS OF CASSAVA PRODUCTS
621
TABLE 3 The anti-nutrient content of Aspergillus niger-fermented cassava products (mean 7 SEM) (dry matter) Unfermented Sample Cyanide (mg/kg) Tannin (%)
Wildly fermented
A. niger-fermented
Flour
Gari
Flour
Gari
Flour
Gari
21.370.3 0.270.0
14.670.3 0.270.0
9.170.1 0.170.0
7.170.2 0.170.0
9.170.2 0.270.0
4.170.2 0.270.0
Note: n=3.
capable of utilizing cyanogenic glucoside and the breakdown products, thus explaining why it is one of the natural flora involved in cassava fermentation during gari processing (Okafor, 1998). It is also evident from the results that Aspergillus niger is more efficient in cyanide detoxification than Rhizopus oryzae, thereby possibly explaining its higher efficiency in protein enrichment. The lower cyanide in gari could be attributed to the process of garrification (Aletor, 1993). The products could also be considered safe in terms of cyanide poisoning in view of the fact that the cyanide contents were far below the deleterious level of 30 mg/kg (Akinrele et al., 1962). From this study, it could be concluded that Aspergillus niger, a cheap, nonpathogenic saprophyte, would efficiently increase the protein content of cassava products and reduce the level of the tannin and cyanide (both high and low cyanide cassava varieties).
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