MICROSCOPIC STRUCTURE AND THERMAL PROPERTIES OF CASSAVA STARCH BIODEGRADABLE FILMS INCORPORATED WITH GRAPE RESIDUES

The application of fruit by-products with functional properties in biodegradable packaging for food is viable and sustainable. The present work aims to evaluate the microscopy structure and thermal properties of biodegradable polymeric films produced with cassava starch added with flour from by-products of the wine industry. The films were produced by casting method with 5 wt.% cassava starch about water, 20 wt.% glycerol and 0, 0.5, 1, 2, and 4 wt.% of flour about starch. To prepare the film-forming solution the mixture was magnetically stirred for 10 minutes for homogenization, then subjected to ultrasonic treatment at 60°C for 5 minutes, heated to 75°C, and kept under magnetic stirring for 20 minutes. The curing process was carried out in an oven at 40 °C for 24h. Ultrasonic waves interact with plant material, changing its physical and chemical properties through cavitation, agitation, and heating. Films characterizations were carried out through thermal tests by differential scanning calorimetry (DSC) and morphology by transmission optical microscopy (TOM). By microscopy, it was verified that there was a good incorporation of the grape flour in the cassava starch, being a continuous film and without evidence of voids in the interface between the matrix and the particles. This type of structure is fundamental for the development of a package. In addition, the particles in the film have different shapes and colors, this is because flour from by-products of the wine industry contains different parts of the grape, such as grains and skins. Broad endothermic peaks referring to melting were observed in all samples. In starch films, these events occur as result of amylose recrystallization and/or to a lesser extent amylopectin linear fractions during film drying. The samples with 2 and 4 wt.% of flour showed a melting temperature similar to the pure matrix (120 °C). On the other hand, the films with fewer residues had a reduction in the melting point, reaching 9% for the sample of 0.5 wt.% of flour in relation to the pure matrix. The glass transition temperature increased in all films compared to the pure matrix. The biggest increase of 14% of the sample with 4 wt.% of flour in relation to the control sample. Rigid particles such as flour from by-products of the wine industry can act as obstacles to the mobility of polymer molecules in the starch matrix and/or can change the interaction near the surface of the particles and thus increase the glass transition temperature. In this way, the biodegradable polymer film produced with cassava starch added with by-products of the wine industry flour showed important changes in thermal and structural properties and indicate potential application as a biodegradable material for food packaging.