Steady and oscillatory shear behaviour of semi-concentrated starch suspensions

The viscoelastic moduli G’ and G” of aqueous suspensions with 40% (w/v) normal corn starch (NCS) and waxy corn starch (WCS) were determined by oscillation rheometry. The oscillatory shear flow experiments at heating from 30° to 75 °C and maintaining at this temperature showed changes from a behaviour predominant viscous (G”>>G’) to predominant elastic (G’>G”) for both starches at 60.5 °C for WCS, respectively 70,85 °C for NCS, WCS having higher values of G’ and G” as NCS. After the gelatinisation temperature was attired, NCS showed no significant changes, both moduli remaining relatively constant. Peaks of both moduli G’ and G” were obtained for WCS at its maintaining at 75oC, these changes being attributed to the changes in the amylopectin structure in the absence of amylose for this starch type. The frequency influenced the results; analysis at constant low frequency (10 s-1) gave big oscillations during the measurements and made the analysis impossible, whereas frequencies as 50 s-1 or 100 s-1 gaves reproducible and similar results. The shear flow measurements realised at angular frequencies ω from10-1 to 103s-1 at 25oC showed that changes from a behaviour predominant elastic (G’>G”) to predominant viscous (G”>>G’) occurred when ω attired the values 10 s-1 for WCS and 3 s-1 for NCS. The calculation of the ‘Power-Law’ parameter B showed that NCS forms a physical gel structure, whereas WCS behaves as a covalent gel in the frequency domain 10-1 to 10 s-1 and as physical gel in the frequency domain 10 to 102 s-1.