Physicochemical characteristics of phosphorylated cross-linked starch produced by reactive supercritical fluid extrusion

Cross-linking of starch blends by phosphorylation using reactive supercritical fluid extrusion was investigated. A starch blend was mixed with 9% (starch basis) sodium trimetaphosphate and extruded at 60–70 °C with 0.1 M or 0.2 M NaOH solution and 0–1.5% (dry feed) supercritical carbon dioxide (SC-CO2). Physicochemical and morphological characteristics of extruded starch phosphates were evaluated and compared with unextruded and extruded starch controls. Degree of substitution (DS = 0.05) and incorporated phosphorus content (P = 0.92%) was highest in the starch phosphates produced with 0.2 M NaOH, indicating the highest level of cross-linking. Cross-linking of starch at highest reaction pH was also evidenced by its lowest water solubility and highest proportion of distarch monophosphate investigated by 31P NMR. Introduction of phosphate cross-links into the starch restricted the molecular structure mobility, leading to an increase in the gelatinization temperature (by up to 7 °C). SC-CO2-based expansion provided light weight and non-porous skin starch foams with excellent water resistance which would be desirable properties for their utilization as a biodegradable material.