The effect of increasing soaking time on the properties of premixing starch-glycerol-water suspension before melt-blending process: comparative study on the behavior of wheat and corn starches

The reduction in the torque consumed during the preparation of thermoplastic starch to the minimum value was achieved by reaching equilibrium state of the premixing suspension of starch-glycerol-water before melt-blending process. The study was conducted on two kinds of cereal starches, wheat and corn. Microscope, FT-IR and XRD analytical methods were employed to follow the change in starch structure during soaking time (aging time) until reaching the equilibrium. Microscope photomicrographs exhibited a relative and partial swelling of the starch granules simultaneously with progressive breaking down of the granules with soaking time until 10 days. Thereafter, no additional breaking down was observed in starch granules. FT-IR analysis showed that shifting of -OH group, which refers to the formation of H-bonds between starch and plasticizers, started from the first day of soaking and continuously increased until the suspension reached the equilibrium state after 10 days of aging. XRD analysis shows that the A-type, which cereals starch characterized by, still be conservative during the soaking. Nevertheless, the absence of some peaks and the progressive growth of new other peaks, which refer to the V-H-type and B-type, were recognized. As microscope and FT-IR, XRD results show, after 10 days of soaking, no measurable change happened in diffractograms that means the equilibrium state was reached. This study demonstrated that higher amylose-content wheat starch compared with lower amylose-content corn starch showed greater torque requirement to get plasticized, formed stronger H-bonds with plasticizers and presented higher crystallization degree when soaking in plasticizers mixture.