Effect of planting seasons, varieties and harvesting times on biochemical properties of cassava (Manihot esculenta Crantz) roots

Study on planting seasons, varieties and harvesting times affecting biochemical properties of cassava roots was conducted on six cassava varieties ; Rayong 1, Rayong 60, Rayong 90, Rayong 5, Kasetsart 50 and CmR33-57-81. The data collection was harvested at 6, 8, 10 and 12 months after planting. This experiment was studied on Warin soil series at Kham Talaesau Farm, Nakhon Ratchasima in early (May) and late (October) rainy seasons, 1996. The results of this study indicated that Rayong 90 and Kasetsart 50 gave high average starch content both in the early and late rainy planting seasons. Planting in early and late rainy seasons gave highest starch content at 8 and 12 months after planting, respectively. There were significant differences in the average content of starch between planting seasons. Biochemical properties of cassava roots like lipid and protein decrease starch viscosity and cause the rancidity of cassava starch. Phenolic compounds caused darkening of cassava starch. Moreover, the toxic cyanide and dense fiber imposed difficulties in industrial starch production process. The results of the study revealed that Rayong 90 gave low lipid, protein and fiber content and also Rayong 5 gave low cyanide and phenolic compounds content. Both lipid and phenolic compounds content were low at 6-8 months after planting. Protein and fiber content were low at 12 and 10 months after planting, respectively. Cyanide content was low during the rainy season and high during the dry season. Soil moisture content negatively correlated with cyanide content. There were no significant differences of the average contents of lipid, cyanide and phenolic among planting seasons. Amylose content and amylose size which determine physicochemical properties of starch were not different among cassava varieties. Harvesting cassava at early rainy season caused the reduction of amylose content and size, due to their regrowth as a result of amylose hydrolysis in which glucose was used as the energy source to produce their vegetative growth.