Processing of raw Cassava (Manihot esculenta) - Experiments and Modelling

Abstract: The objective of the present thesis was to understand how the quality and performance of dried cassava products could be developed by drying and other processing steps. This goal was achieved in several stages. First, a survey of the composition of six cassava varieties from Bolivia, was performed, which resulted in the report of chemical and mineral compositions of the tubers. The mineral compositions of the tubers were compared to the elemental composition of the soil in order to identify depleted and enriched elements. In a second step, an enzymatic assay for the determination of lower sugars in cassava was developed. This technique rendered it possible to measure the content of free glucose, maltose and sucrose in six varieties of cassava. The third step implied the development of a process for partial hydrolysis of non-gelatinized cassava starch. In this study, the kinetics of the enzymatic hydrolysis of raw cassava starch were described as a heterogeneous system, and the investigation involved leaching of amylose during starch granule swelling, and morphological characterizations of starch granules using SEM as well as light microscopy. Results included a model describing the kinetics of the enzymatic hydrolysis and which assumed a blocklet structure of the starch granule and reveals that the enzymatic attack occurs mainly at the interior starch-water interface of the granule. Fourth, a model describing the kinetics of the drying of cassava starch was put forward. From this investigation, a quasi steady state model was developed based on heat and mass transfer. Finally, the impact of high pressure homogenization of dissolved starch was studied, which involved the degradation and conformational changes of starch as caused by the process. The characterization was performed with AsFlFFF with MALS and RI detection. Upon high pressure homogenization, the molar mass, mean square radius and hydrodynamic radius were found to decrease. The process also induced an increase and scaling with size in the apparent density of the macromolecules in addition to alterations in the molecular conformation. Furthermore, homogenization caused an increase in the overlap concentration. During the investigations constituting this thesis, native cassava of Bolivia was discovered to be a fairly tasteless starch-rich product with a low degree of sweetness. In order to be used as a base for drinks, as is the case for chive, an interesting concept could be to partly hydrolyze the starch. In granular form its kinetics was described. Moreover, granular starch can be efficiently dried. The hydrolysis may restrict the viscosity-generating properties of the starch, and a critical parameter is the overlap concentration. The present study has shown that this parameter is not very dependent on the molar weight of the starch.