Gelatinization Behavior Of Starch At Low And Medium Water Content And Its Effect On Starch Digestibility

Starch digestion is closely related to human nutrition and health. Up to now, there is little information about the relationshiop between gelatinization behavior and its digestibility under water-limited conditions, while most of starch in food is cooked or processed with limited water. In this study, we investigated the effect of water content and heating time on gelatinization behavior and in vitro digestibility for wheat and yam starches.We also investigated the relationship between the degree of gelatinization (DG) and its in vitro digestibility for different polymorph starches. In present study, we used DSC to determine the thermo properties and to calculate DG of each starch samples. ATR-FTIR,LCM-Raman and XRD were used to character the short- and long-range molecular orders of the starch samples. SEM and LM were used to observe the morphology of starch samples after different heat treatments. All the in vitro enzymic digestion of starch samples were determined according to the modified Englyst method. The main results obtained were as follows:After heating for the same time at 100 ?,the degree of gelatinization of wheat and yam starches increased with increasing water content, and the disruption of multi-scale structures of both starches became increasingly severe with increasing water content. When the temperature is high enough (100 ?), starch gelatinization occurs within a short period of heating (for example, 5 min), while extending heat treatment did not further disrupt the multi-scale structures and increase the in vitro enzymatic digestibility of both starches with the same water content. So water content plays a more important role in determining the gelatinization behavior and in vitro enzymatic digestibility of starch than the duration of heating.The multi-scale structure and in vitro enzymatic digestibility of wheat starch samples with DG ranging from zero to 100% were evaluated. As DG increased, the granular morphology and long- and short-range molecular orders of the starch were disrupted increasingly in a gradual way. Compared with native starch, gelatinization increased in vitro digestibility and the first order kinetic constant (k) greatly, while for gelatinized starch samples of different DGs (from 6% to 100%), digestibility increased only slightly. We conclude that DG or degree of ordered structure in wheat starches is not the major rate-limiting factor for in vitro digestibility, we propose that the access/binding of enzymes to starch dominates the digestion process,and structural features are not the rate-limiting factor for starch digestion.We also investigated the multi-scale structure, morphology and in vitro enzymatic digestibility of potato starch samples with different DGs. Compared with native starch,freeze drying could disrupt both long- and short-range molecular orders, and the in vitro digestibility increased greatly after freeze drying. As freeze-dried potato starch was compared, disruption of the multi-scale structures in granules occurred progressively with increasing DG. Besides, in vitro digestibility and the first order kinetic constant (k)increased with increasing DG The higher in vitro digestibility for higher DG starch samples suggest that DG or degree of ordered is a key factor of the digestibility of potato starch.