Molecular order and structure in enzyme-resistant retrograded starch

Molecular features underlying the resistance to amylolytic hydrolysis in cooked and cooled gels of wheat, amylomaize V and amylomaize VII starches have been investigated using a combination of physicochemical techniques. X-ray diffraction and C-13 CP/MAS NMR spectroscopy indicate levels of crystalline and double helical order to be 25-30% and 60-70%, respectively, in enzyme-resistant retrograded starches. The width of features in diffraction patterns and NMR spectra indicate smaller and/or less perfectly arranged B-type double helical aggregates than found in native potato or amylomaize VII starch. Differential scanning calorimetry in excess water shows a broad endothermic transition from below 100 to c. 170 degrees C which is interpreted in terms of double helix melting. Consistent with a broad melting endotherm, (linear) chain lengths present in enzyme-resistant starches cover a range of degree of polymerisation (DP) from less than 10 to c. 100 as determined by high performance anion exchange chromatography (HPAEC). This dispersion of chain lengths coincides with the range expected from previous studies for double helices (minimum required DP of 10) with no major intervening amorphous regions (maximum DP similar to 100). HPAEC analysis also shows a periodicity in chain length for DP multiples of 6 above DP18 for all three enzyme-resistant retrograded starches. A model is proposed to account for this observation based on restricted enzyme access to potential substrates arranged in double helical aggregates. In general, enzyme-resistant retrograded starch reflects features both of aggregated/gelled amylose (high double helix content; low crystallinity, DP range from junction zones of DP 10-100), and the consequence of enzyme action on such a structure (periodicity of six units from accessibility of enzyme to aggregated substrate).