Comparative NMR relaxometry of gels of amylomaltase-modified starch and gelatin

Abstract

With the aim of generating gelatin-like starch gel functionality, starches extracted from normal potato, high amylose potato, maize, waxy maize, wheat and pea and oxidized potato starch were modified with amylomaltase (AM) (4-α-glucanotransferase; E.C. 2.4.1.25) from Thermus thermophilus. Gel characteristics after storage for 1 and 10 days at 20 °C of 12.0% gels were assessed by monitoring proton relaxation for the resulting 51 enzyme-modified starches and two gelatins using low-field 1H nuclear magnetic resonance (LF NMR) relaxometry. Discrete and distributed exponential analysis of the Carr–Purcell–Meiboom–Gill (CPMG) LF NMR relaxation data revealed that the pastes and gels contained one water component and that the spin–spin relaxation time constants ( T 2) and distributions differed with respect to starch type and enzyme modification. Typically, AM modification resulted in starches with decreased T 2 relaxation time and a more narrow T 2 distribution indicating a more homogeneous water population. In contrast, treatment with a branching enzyme (BE) (EC 2.4.1.18) combined with AM increased T 2 relaxation time and a broadened T 2 distribution. As evaluated by the principal component analysis (PCA), long chains of amylopectin generated hard gels and decreased T 2 relaxation time at both day 1 and day 10. Especially at day 10, T 2 relaxation time could be predicted from the amylopectin chain length (CL) distribution. Reconstructed amylopectin CL distribution required to emulate gelatin LF NMR data suggest the importance of combined fractions of long (DP 60–80) and short (DP 10–25) amylopectin chains.