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MISE AU POINT D'UN FERMENT MIXTE DESTINE A LA BIOCONVERSION DES TUBERCULES DE MANIOC CYANOGENE

Abstract : Cassava (Manihot esculenta crantz) roots serve as food for more than 800 million people in the tropics. Despite this useful application, two drawbacks limit the utilisation of these roots as human food: These are its low protein content and potential cyanide toxicity. Traditional processing methods used by people in the rural areas to reduce these bottlenecks, always include a typical step of fermentation witch occurs spontaneously. This natural fermentation is plagued with many problems related to its dependence on the chance of inoculation from the environment. These problems include high levels of residual cyanide (100 to 170ppm), long softening time (4 to 5 days), non-reproducible quality and unhygienic products. To solve these problems, about one hundred microorganisms were isolated from dried fermented cassava tubers, on the basis of their ability to produce ?-glucosidase, ?-amylase, pectin hydrolase and polygalacturonase. These enzymes are involved in biochemical and physicochemical phenomena, which occur in cassava natural fermentation and are responsible for the degradation of cyanide compounds and tuber softening by degrading pectin, which constitutes the major part of cassava cell wall. Among these isolates 60% were identified as ?-amylase producers, 46% as ?-glucosidase producers, and 36% pectinase producers.
The ability of these selected microorganisms to produce enzymes was screened. Among the isolates, two microorganisms were found to be particularly interesting: lactic acid bacteria identified as Lactobacillus plantarum, showing ?-amylase activity of 17000±100UE/ml, and ß-glucosidase activity of 2000±300µmol CN-/mn; a mould identified as Rhizopus oryzae with an ?-amylase activity of 20000±500UE/ml and pectinhydrolase activity of 2,3±0,2 µmol COO-/mn/mg. These strains were selected and used as starter culture for cassava fermentation. The use of these strains as a mix starter culture (106fuc/g fresh cassava tubers of Lactobacillus plantarum and 103spores/g fresh cassava tubers of Rhizopus oryzae) for cassava retting rapidly softened the roots (12±3mm/5s after 48 hours of retting), and induced a rapid decrease in pH (3,5 after 24 hours of fermentation). However, natural fermentation was found to reduce more cyanide than starter fermentation. The rapid acidification observed with starter fermentation was found to be responsible of this low cyanide degrading activity by starter strains as low pH stops the reaction at the cyanohydrins step.
Studies on the effect of environment on starter strains show that Lactobacillus plantarum's optimal growth temperature was 40±5°C for an optimal pH of 6, while Rhizopus oryzae show an optimal growth temperature of 30±7°C for an optimal pH of 5. These strains also showed a good growth rate on Law et al., cassava medium (CTM).
With the aim of large-scale utilization of this starter culture, the microorganisms were tested for their ability to withstand drying conditions. Survival of Lactobacillus plantarum and Rhizopus oryzae after drying was determined under various support conditions. The support constituting starch and glycerol as protectants was found to be the best with more than 75±5% survivors after drying at 35°C under a hot air flow of 10 m3/h. The use of the dried mix starter for cassava retting permit the reduction of the total cyanide content by about 95?4%, as NaOH were added after 24 hours of fermentation to enable a drop in the pH level to 5. This enabled us to solve the bad effect of low pH on cyanide reduction observed by the fermentation of tubers using mixed starter cultures.
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Contributor : Roger Djoulde Darman <>
Submitted on : Saturday, July 23, 2005 - 12:57:10 PM
Last modification on : Thursday, July 23, 2020 - 4:30:05 PM
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  • HAL Id : tel-00007487, version 2

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Roger Djoulde Darman. MISE AU POINT D'UN FERMENT MIXTE DESTINE A LA BIOCONVERSION DES TUBERCULES DE MANIOC CYANOGENE. Sciences du Vivant [q-bio]. Université de Ngaoundéré, 2004. Français. ⟨tel-00007487v2⟩

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