Morphometric and crystallinity changes on jicama starch (Pachyrizus erosus) during gelatinization and their relation with in vitro glycemic index

Abstract

The effect of gelatinization on the in vitro glycemic index (GI) of starch obtained from jicama tubers was investigated, and the relationships between the starch crystallinity, granule morphology, and the GI were measured. Samples were prepared by heating an aqueous dispersion of starch (1:3 w/v) at different baking process temperatures (60, 65, 70, 75, or 80°C). Native starch granules showed spherical and polyhedral shapes, with a morphometric aspect ratio (AR) of 0.89–1.0 and sizes ranging from 3 to 21 μm. During the thermal process a change in the AR (0.21–0.88) and size (3.0–46 μm) was observed, as well as the formation of agglomerates. Native jicama starch exhibited a CA-type X-ray diffraction pattern, while those thermally treated showed a transition from the CA-type to CB-type, decreasing their crystallinity (20.8–11.5%) at higher temperatures. The gelatinization degree of the starch samples and the glycemic index showed the largest values (97.4% and 98.6% respectively) at the highest temperature (80°C), while the crystallinity percentage followed an inverse correlation to temperature (11.5%, 80°C), indicating that the thermal history affected the behavior of the starch enzymatic digestion. The in vitro glycemic index correlated positively with the gelatinization degree based on aspect ratio (R2 = 0.977), DSC gelatinization enthalpy (R2 = 0.969), and crystallinity (R2 = −0.988). Based on these results a mathematical model is proposed to determine the glycemic index as a function of the aspect ratio. However, more studies are required to validate this information.