RESUMO
Quinoa starch nanocrystals (QSNCs), obtained by acid hydrolysis, were used as a reinforcing filler in cassava starch films. The influence of QSNC concentrations (0, 2.5, 5.0, 7.5 and 10%, w/w) on the film's physical and surface properties was investigated. QSNCs exhibited conical and parallelepiped shapes. An increase of the QSNC concentration, from 0 to 5%, improved the film's tensile strength from 6.5 to 16.5 MPa, but at 7.5%, it decreased to 11.85 MPa. Adequate exfoliation of QSNCs in the starch matrix also decreased the water vapor permeability (~17%) up to a 5% concentration. At 5.0% and 7.5% concentrations, the films increased in roughness, water contact angle, and opacity, whereas the brightness decreased. Furthermore, at these concentrations, the film's hydrophilic nature changed (water contact angle values of >65°). The SNC addition increased the film opacity without causing major changes in color. Other film properties, such as thickness, moisture content and solubility, were not affected by the QSNC concentration. The DSC (differential scanning calorimetry) results indicated that greater QSNC concentrations increased the second glass transition temperature (related to the biopolymer-rich phase) and the melting enthalpy. However, the film's thermal stability was not altered by the QSNC addition. These findings contribute to overcoming the starch-based films' limitations through the development of nanocomposite materials for future food packaging applications.
RESUMO
Quinoa starch (QS) acid hydrolysis was investigated, focusing on the kinetics and physicochemical properties of nanocrystals production as a function of temperature (30, 35 and 40 °C). Waxy maize starch (WMS) was hydrolyzed at 40 °C for comparison. QS presented different hydrolysis percentages at 30 °C (63%), 35 °C (73%) and 40 °C (91%), on the fifth day. QS showed faster hydrolysis (first-order rate constant, k = 0.59 day-1) than WMS (k = 0.39 day-1) at 40 °C. Material produced at 30 °C was micrometric-sized and irregularly-shaped while that at 35 and 40 °C, was nanometric-sized and conical and parallelepiped-shaped. The hydrolysis temperature increase did not affect the crystallinity index of quinoa starch nanocrystals (QSNC), whereas zeta potential and Fourier transform infrared spectroscopy band intensities increased, and thermal transition peak temperature and thermal stability decreased when hydrolysis temperature increased. QSNC were produced at 35 and 40 °C with yields of 22.8% and 6.8%, respectively. At 40 °C, QSNC presented smaller sizes than WMS nanocrystals, but also lower yield and crystallinity index.