Structural modification of fiber and starch in turmeric residue by chemical and mechanical treatment for production of biodegradable films

Abstract

The dye extraction from turmeric rhizomes (Curcuma longa L.) is generated a residue with high starch content that in nature does not form film. Therefore, we decided evaluate how mechanical treatment (ball or cryogenic mill) and chemical treatment with alkali (NaOH 2.5% (1, 4, or 8 h) and bleaching with NaClO or H2O2 at 25 or 45 °C affect the chemical structure of the starch and fibers in turmeric residue, and its filmogenic capacity. Ball milling decreased the turmeric residue particle size more effectively favoring the chemical procedures. Only two types of chemical treatment consisting in alkaline treatment for 4 h and bleaching with NaClO and H2O2, respectively, for 4 h, at 25 °C yielded turmeric residues with filmogenic capacity. The chemical treatments oxidized the starch granules causing to lose their crystal structure as verified by DRX, and removed amorphous fibers such as lignin and hemicellulose increasing cellulose content in turmeric residue. FTIR analyses also revealed that the starch granules were oxidized. As bleaching agent, NaClO caused greater starch oxidation (the highest carboxyl and carbonyl groups contents) affording films with the best mechanical and functional properties. Although chemical treatment reduced the turmeric residue phenolic compounds, the films still presented antioxidant activity.