Abstract
Incorporating fiber at high levels (>10%) into direct-expanded products with acceptable texture is challenging. Fundamental explanations for the interaction of starch and fiber and the cause of expansion reduction need further understanding for the effective incorporation of fiber into expanded products. This study aims to explain how cellulose content impacts the physicochemical properties of starch-based extrudates and the long-range and short-range molecular changes of starch. Mixtures of cornstarch (50% amylose) and cellulose were extruded using a co-rotating twin-screw extruder. Thermal and pasting properties of the raw mixtures were evaluated, and the physicochemical properties and microstructure of extrudates were determined. Long-range and short-range molecular changes of starch-cellulose mixtures before and after extrusion were observed by X-ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. The expansion ratio of extrudates reduced significantly as the cellulose content increased and had a strong negative correlation with crystallinity. Cell structures of starch-cellulose extrudates had a smaller and more uniform pore size but possessing a more ruptured matrix. FTIR spectra suggested that there was no covalent bonding interaction between starch and fiber after extrusion. Extrusion reduced the overall crystallinity compared to the raw mixtures. XRD showed that the crystallinity of the starch-cellulose extrudates increased as the cellulose content increased, and the XRD peaks representing cellulose remained unchanged. Cellulose could interfere with starch chain reassociation through intermolecular hydrogen bonding during the expansion process. Phase separation of starch and cellulose is likely to occur at high cellulose content, which could be another reason for the reduced expansion.