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Effect of yam Dioscorea cayenenis-rotundata post-harvests treatments on yam chips quality

Dumaine F., Dufour D., Mestres C., Méot J.M., Bada C., Hounhouigan J.D.. 2002. In : Nakatani Makoto (ed.), Komaki Katsumi (ed.). Potential of root crops for food and industrial resources : Twelfth Symposium of the International Society for Tropical Root Crops (ISTRC), Sep. 10-16, 2000, Tsukuba, Japan. Tsukuba : Cultio Corporation, p. 394-394. Symposium of the International Society for Tropical Root Crops. 12, 2000-09-10/2000-09-16, Tsukuba (Japon).

In West Africa, a variety of late yarn with small tubers and white flesh (variety group kokoro) Dioscorea cayenensis-rotundata are used for the manufacture of dried chips intended for the preparation a paste called "amala or telibo-wo". In Benin, the yams are peeled and immersed whole in the water then brought gradually up to a measured maximum temperature of 66°C in the presence of foliages or barks. The maximum temperature measured in heart of the chips is 59°C. The chips are cooled in the bath and settings to be dried with the sun throughout one 6 to 8 days (13-15% dm). A temperature of bath close to 80°C causes a significant degradation of quality (significant exterior tanning, internal vitreous aspect, cracks during chips drying, very hard chips difficult to crush). This operation of "pre-blanching" prevents an Internal blackening during drying and gives a superficial pinkish colour compared to test without pre-blanching. Some of the plants used in the decoctions confer colours yellow, brown or red. Whatever are the plants used, after crushing and sitting, the flours do not present significant differences in colour (beige). Some of the decoctions confer to the amalas colours yellow to orange or reinforce the brownish colour obtained without plant. The sensory evaluation of the amalas, obtained in standardised conditions, highlights a lower bitterness of the samples (with decoction of plants) compared to the control (without plant). The role of the decoctions on the sensitivity of the chips to the attack of the insects was studied. The chips to be tested are placed In a hostile environment in the presence of strongly infested chips by insects (Dinoderus sp., Rhyzopertha dominica, Tribolium castaneum, Sitophilus zeamais, Palorus sp...). The rate of infestation is regularly estimated by counting of the number of galleries (holes on the surface), appeared in the course of time, by kg of initial dry matter. An attack of insect is observed on the control batches (without plant) and on the tests (peels of yam, Lophira lanceolata and Cochlospermum tinctorium) in the first week whereas all the other tests remain intact. On the day fifty eight, for the first time some galleries appeared on the test containing dried teak leaves: Tectona grandis (7 galleries per kg of chips, is less than one hole by chips). The test in the presence of Piliostigma thonningii show also a weak rate of infestation (130 galleries/kg). Calotropis procera, Cochlospermum tinctorium and the peels of yam oscillate between 280 to 340 galleries/kg. The tests in the presence of Bridelia ferruginea and Lophira lanceolata have a behaviour similar to the control without plant (520 to 870 galleries/kg). The exponential growth rates for the various tests seem identical since the chips are infested. On the other hand we observe an initial delay of the infestation due to the decoctions and in particular Tectona grandis and Piliostigma thonningii with respectively an appreciable beginning of development at 55 and 20 days under the test conditions (great infestation of the environment). Trials of validation of this experimental test are currently carried out In real conditions of storage to the market of Cotonou. These experiments highlight that the process variables control of the chips manufacturing makes it possible to notably improve quality of the amalas but also to reduce the post harvest losses....

Mots-clés : igname; produit séché; qualité; brunissement; technologie après récolte; blanchiment; lutte anti-insecte; afrique occidentale; chips

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