The effect of sorbitol and sweet sorghum to carrageenan ratio on the physicochemical properties of sweet sorghum/carrageenan bioplastics

Abstract

The use of synthetic plastic films has raised several environmental issues. The use of bioplastics is therefore expected as an alternative. In this study, sorghum grain extract was explored as the source for bioplastic films considering the prediction of massive availability of sorghum grains as by-products of bioethanol production. Sorghum grain extracts are rich in carbohydrate and protein, the two principals of bioplastic making. Therefore, the ability to re-utilize sorghum grain extracts is further expected to reduce the production cost. In addition to sorghum grain extracts, additives were involved in the process, which were carrageenan and sorbitol. Carrageenan was used as a gelling agent during the bioplastic’s productions while sorbitol was involved as the plasticizer. The effect of sorbitol and the ratio of sorghum extract to carrageenan on the physical and mechanical properties of the bioplastic film were investigated. The concentration of sorbitol was varied at 0, 2, 4, and 6%; the ratio of sweet sorghum extract to carrageenan was varied as 5:5, 6:4, 7:3, and 8:2 at sorbitol concentration of 4%. The alkaline method was used to solubilize milled sorghum grain. The solution casting method was used to produce the bioplastic film. The film thickness was dependent on the concentration of the sorghum extract. The sorbitol incorporation reduced the tensile strength of the film while the concentration of the sorghum extract did not significantly alter the tensile strength. Improved flexibility was observed as the sorbitol concentration was increased. Water vapor transmission rate of the films is in the range of 100–147 g/m2 day, lower than polymethylpentene (PMP), a polyolefin petroleum-based plastic film, indicating the potential of the bioplastic films for future application. This further indicates the possible biorefinery of sorghum by-products as raw materials for bioplastic production.