Evaluation of enzymatic degradation based on the quantification of glucose in thermoplastic starch and its characterization by mechanical and morphological properties and NMR measurements

Abstract

Biodegradable polymers represent a promising solution to the environmental problem of plastic waste disposal. Among the candidate polymers, starch, a low-cost natural polymer, can be processed as a thermoplastic. In this work, thermoplastic starch containing glycerol (20, 30 or 40 wt%) was prepared by extrusion. The mechanical properties of the blends were assessed by tensile stress at break, elongation at break and Young's modulus. Nuclear magnetic resonance (NMR), through proton spin-lattice relaxation ( T 1H) measurements, was used to understand the molecular dynamic of the samples. The morphology of the films was assessed by electron scanning microscopy (SEM). The susceptibility to enzymatic degradation was assessed in films incubated with α-amylase in acetate buffer. The loss of mass was determined after 1, 2, 3, 6, 9, 12 and 24 h of enzymatic digestion. The results obtained showed that all of the mechanical properties were improved by the addition of 20% glycerol, but decreased with higher concentrations of glycerol. The amount of free glucose released into the solution during digestion was quantified using a commercial glucose kit. Starch containing 40% glycerol degraded faster than the other blends, because it facilitated the efficiency of the gelatinization process and also the phase separation, as shown by the proton spin-lattice relaxation time. The amount of glucose released was directly proportional to the glycerol content of the samples. These results suggested a short lifetime for this blend, which is in agreement with the values of the T 1H relaxation parameter that suggests a decrease of the average molecular weight.