Lignocellulosic fractions from rice and coffee husks to improve functionality of biodegradable films based on starch and poly-lactic acid

Abstract

This Doctoral Thesis has focused on the isolation and characterisation of cellulosic materials and active extracts from coffee and rice husks, and their incorporation into starch films and starch-PLA compatibilised blend films in order to improve their functional properties as food packaging materials. Cellulose fibres were obtained through alkali and bleaching treatment with a final yield of 41 and 53 g fibres/100 g husk, respectively for rice and coffee husks. Cellulose nanocrystals were isolated from the bleached fibres by acid hydrolysis, with a yield of 5% with respect to bleached fibres, in both cases, with high crystallinity (90-92%), thermal resistance and aspect ratio (L/d: 20-40). The active compounds were obtained by hydrothermal extraction (180 ºC, 9.5 bar) with yields of 17 -18 g/100 g husks. They exhibited antioxidant properties (EC50: 5.37-5.29 mg extract solids/mg DPPH) and antibacterial activity against L. innocua (MIC: 48-52 mg powder/mL) and E. coli (MIC: 50-66 mg powder/mL), which were quantified in terms of the minimal inhibitory concentration. Cellulosic material from rice and coffee husks were incorporated into thermoplastic starch films (TPS) by melt blending and compression moulding. The elastic modulus increased by 186 and 121% when 1 wt% of cellulose nanocrystals (CNC) from rice and coffee husks, respectively, was incorporated into the matrix. Likewise, cellulose fibres (CF) were incorporated into TPS films at 1, 5 and 10 wt%. Both CF increased the film stiffness while reducing its stretchability. However, CF from coffee husk better maintained the film ductility at 1 and 5 wt%. The water vapour permeability of TPS films was not reduced in composites, although oxygen permeability was lowered by about 17%. When active extracts were incorporated into starch films, they improved the tensile properties; the elastic modulus increased by about 350%, while films became less stretchable. The cellulosic fibres from both residues were more effective as reinforcing agents in films containing extract solids than in net starch films. Starch-PLA blend films were also studied using grafted polycaprolactone with maleic anhydride and/or glycidyl methacrylate (PCLMG or PCLG) as compatibilisers. The effect of both the PLA ratio in the blend (20 and 40% with respect to starch) and the amount of both compatibilisers (2.5 and 5%) on the film properties was analysed. The analyses of microstructure, thermal behaviour and functional properties (mechanical, optical and barrier) of the films led to the conclusion that substituting 20% of the starch by PLA, and incorporating 5% of PCLG would be a good strategy to obtain films suitable for food packaging. The effect of the addition of cellulosic fillers (CF and CNC) and antioxidant aqueous extract from coffee husk to compatibilised starch-PLA blends was also studied. The antioxidant properties of the films were tested through their efficacy at preserving sunflower oil from oxidation. Significant differences were observed in the functional properties of the films when CNC was incorporated by two different methods. The reinforcing effect of cellulosic materials in S-PLA blends was less noticeable than in starch films, probably due to the overlapping of the PLA reinforcing effect. The antioxidant extract did not improve the mechanical performance in the blends, but conferred antioxidant capacity suitable for food packaging applications.