Hydrodynamic radii of solubilized high amylose native and modified starches by pulsed field gradient NMR diffusion measurements

Over the last decade, there has been an increase in the application of pulsed field gradient nuclear magnetic resonance (PFG NMR) to characterize food materials. In this work, PFG NMR was used to examine the impact of chemical modification on properties of solubilized high amylose starches obtained from heated (121 °C for 15 min) starch suspensions (0.5% w/w). The starches examined were high amylose maize starch (HAMS) and high amylose maize starch chemically modified with acetate (HAMSA), propionate (HAMSP) and butyrate (HAMSB) at degree of substitution (DS) of 0.2. The hydrothermal treatment solubilized 3.36–4.96% of the starch in all samples, corresponding to concentrations of 10.36–10.91 μM on glucose equivalents basis. The hydrodynamic radii of the starches were 30 Å (HAMS), 45 Å (HAMSB), 60–70 Å (HAMSA & HAMSP). Contrary to expectations, the radii did not increase with the molecular weight of the fatty acid attached to the starch. Rather, the trend in the observed hydrodynamic radii was related to the physical organization of the starch molecules in higher DS granular dried starch (DS = 0.32–0.40) as observed by others using X-ray diffraction and small angle X-ray scattering, suggesting the preservation of the nano-structure of starch molecules upon solubilization. Upon storage (7 days at 25 °C), there was retrogradation of HAMS but not of modified HAMS. Given the correlation between retrogradation and food spoilage, the inhibition of retrogradation in the soluble fraction of modified HAMS may increase the stability of starch containing foods.