Functions of fenugreek gum with various molecular weights on the gelatinization and retrogradation behaviors of corn starch--2: Characterizations of starch and investigations of corn starch/fenugreek gum composite system at a relatively low starch concentration; 5w/v%

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. The addition of fenugreek gum (0.5w/v%) increased peak viscosity of the composite system (5% starch) during gelatinization when coil overlap parameter C[η] and weight-average molecular weight M w of the gum were larger than 2.38 and 87.0x10⁴ g/mol, respectively, shifting the onset of viscosity increase to lower temperatures. It increased dynamic mechanical loss tangent tan δ of the composite system after storage at 4°C for 24h and also an exponent n determined by the power-law regression between dynamic complex viscosity η * and frequency ω, η *=Kω n , indicating the lowering of structural hardening of the composite system during short-term retrogradation (24-h time scale). This effect of fenugreek gum was less M w dependent when C[η] and the M w of the gum were equal to or larger than 4.05 and 18.3x10⁵ g/mol, respectively, relating to the decrease in the amount of amylose (or amylose-like components) leached out starch granules during gelatinization. The addition of fenugreek gum decreased dynamic storage modulus G' of the composite system after storage at 4°C for 24h and also a constant K when C[η] and the M w of the gum were equal to or smaller than 4.05 and 18.3x10⁵ g/mol, respectively, without altering the rate constant k determined rheologically by the time-course of G' during short-term retrogradation. The addition of fenugreek gum increased syneresis of the composite system after storage at 4°C for 24h when C[η] and the M w of the gum were equal to or smaller than 0.65 and 11.1x10⁴ g/mol, respectively, while decreased when C[η] and the M w of the gum were equal to or larger than 4.05 and 18.3x10⁵ g/mol, respectively, according to the rheological data. These critical M w 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.