| Starch digestibility is an essential factor influencing the nutrition function of starchy food and is of great significance for the human health. As an important source of starch in southern China, different structural changes in different cassava starch biosynthesis growth stages can lead to differences in the digestion, pasting and rheological properties of starch. Thus, changing the cassava starch structure by controlling the growth stages of cassava starch for improving starch quality has been attracted great attention. Cassava starch A, B and C were selected in the current paper and their digestibility under different growth stages was evaluated. Combined with the changes at the chain structure and aggregation structure of cassava starch with different digestibility investigated by modern analytical techniques on polymer structure, the key multi-scale structure features influencing the cassava starch digestibility were obtained and their pasting and rheological properties were also analyzed in order to provide theoretical basis for selecting cassava starches with different digestibility and their corresponding cassava growth stages.Using Englyst In-Vitro method for the simulation of the human digestive process, the effect of growth stages on the cassava starch digestibility was investigated and the A rapid digestion cassava starch, B slow digestion cassava starch and C resistant cassava starch were obtained. It is found that the A-10 cassava starch had the highest rapid digestion cassava starch content, B-10 cassava starch had the highest slowly digestible starch content and C-9 cassava starch had the highest resistant starch content.By the methods of iodine complex, fluorescence assisted capillary electrophoresis, gel permeation chromatography coupled with multi-angle light scattering and Fourier transform infrared spectroscopy, the chain structural changes of the cassava starch with different digestibility during the growth stages were analyzed. The results indicated that as for the same digestion type of cassava starch, the slowly digestible starch content increased as amylose content decreased, and the resistant starch content increased as amylose content increased. A chains with DP 6-12 and B2 chains with DP 25-36 were mostly changed during the different growth stage of the process of branched starch chain synthesis. It is concluded that a large amount of A chains and B1 chains with DP 13-24 contributed to the formation of resistant starch, while the combined action between the large amount of A-chains and B1 chain and the smaller amount of B2 and B3 + chains with DP ≥37 contributed to the formation of slowly digestible starch. Moreover, the cumulative weight fractions at higher molecular weight of these three cassava starches increased gradually and then gradually decreased as the growth stage increased. Starch molecules with the molecular weight between 1× 107 to 2 × 107 g / mol tended to form resistant starch, and the molecular weight between 2× 107 to 4 × 107 g / mol tended to form slowly digestible starch. Cassava starch molecular short-range ordering was conducive to form slow digestion starch and resistant starch, and the higher the degree of ordering, the more conducive to resistant starch. Meanwhile, the different variations of aggregation structure at different growth stages of these cassava starches with different digestibility were revealed by the methods of small-angle X-ray scattering and X-ray diffraction. The results showed that with different growth stages, semi-crystalline lamellar thickness of the structure of cassava starch granules, the thickness of the crystalline lamella and amorphous lamella and crystallinity were changed. The higher thickness of semi-crystalline lamellar structure and crystalline lamellar structure, the higher degree of crystallinity and the smaller thickness of the amorphous lamellar structure, the more conducive to the formation of the SDS and RS. Hence, the digestibility of cassava starch was controlled collectively by its amylose content, amylopectin branching structure, molecular weight and its distributions, molecular short-range order, crystallinity degree, and semi-crystalline lamellar structure.Based on the relationship among the multi-scale structures and digestibility of cassava starch, the effects of different growth stages on the pasting properties and rheological characteristics of cassava starch were analyzed. It is indicated that C-9 cassava starch with high resistant starch content showed excellent paste properties with lower peak temperature and peak viscosity, stronger resistance to shear at high temperature, good cold paste stability, less retrogradation but with a higher gelatinization temperature; B-10 cassava starch with high slowly digestible starch content showed better paste properties with lower pasting temperature, lower peak viscosity, higher resistance to shear at high temperature, less retrogradation but with poor cold paste stability. All the pastes of A rapid digestion cassava starch, B slow digestion cassava starch and C resistant cassava starch belonged to pseudoplastic fluid, and showed shear thinning, thixotropic and gelling properties at different growth stages.The key multi-scale structure features of A rapid digestion cassava starch, B slow digestion cassava starch and C resistant cassava starch were revealed through the investigation of the changes at chain structures and aggregation structures during growth stages of cassava starch with different digestibility. Combined with the pasting and rheological properties, cassava starches with different digestibility and processing properties and the corresponding cassava growth stages were obtained. The results of this research provide a new approach for the regulation of starch digestibility and the improvement of the nutrition and quality of starchy food. It is greatly significant for promoting the development of the starch science and the food nutrition and health. Moreover, the results can guide applications and expand range of applications of cassava starch in southern China and provide the basic datas for cassava starch value added application. |
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