Functions of fenugreek gum with various molecular weights on the gelatinization and retrogradation behaviors of corn starch—1: Characterizations of fenugreek gum and investigations of corn starch/fenugreek gum composite system at a relatively high starch

Gelatinization and retrogradation behaviors of corn starch were investigated in an aqueous system in the presence or absence of fenugreek gum with various molecular weights. Weight-average molecular weight Mw of fenugreek gum samples ranged from 7.5×104 to 20.7×105 g/mol, whereas z-average root-mean-square radius of gyration Rg ranged between 16.3 and 122.3 nm. The Flory exponent of 0.59 determined by the relationship between Mw and Rg and the Mark–Houwink–Sakurada exponent of 0.67 determined by the relationship between Mw and intrinsic viscosity [η] suggested semi-flexible polymer chains with some chain stiffness for the molecules of fenugreek gum. The addition of fenugreek gum (0.5 w/v%) increased peak viscosity of the composite system (15% starch) during gelatinization when C[η] (C: concentration of the gum) and the Mw of the gum were larger than 2.38 and 87.0×104 g/mol, respectively. It rather shifted the onset of viscosity increase to higher temperatures without altering the endothermic enthalpy change during gelatinization detected by differential scanning calorimetry. The addition of fenugreek gum decreased the rate constant of the composite system, representing the relationship between rheological creep compliance and storage time at 4 °C for 14 days. This effect of the gum to inhibit the structural hardening of the composite system during long-term retrogradation (14-days time scale) was enhanced markedly when C[η] and the Mw of the gum were equal to or larger than 4.05 and 18.3×105 g/mol, respectively. These critical C[η] and Mw values for fenugreek gum were compared with corresponding data for guar gum, an alternative galactomannan with a different galactose/mannose ratio, and the functions of galactomannans were discussed on the gelatinization and retrogradation behaviors of corn starch in an aqueous system.