Abstract
Bio-based polymers have been suggested as one possible opportunity to counteract the progressive accumulation of microplastics in the environments. The gradual substitution of conventional plastics by bio-based polymers bears a variety of novel materials. The application of bioplastics is determined by their stability and bio-degradability, respectively. With the increasing implementation of bio-based plastics, there is also a demand for rapid and non-elaborate methods to determine their bio-degradability. Here, we propose an improved pH Stat titration assay optimized for bio-based polymers under environmental conditions and controlled temperature. Exemplarily, suspensions of poly(lactic acid) (PLA) and poly(butylene succinate) (PBS) microparticles were incubated with proteolytic and lipolytic enzymes. The rate of hydrolysis, as determined by counter-titration with a diluted base (NaOH), was recorded for two hours. PLA was hydrolyzed by proteolytic enzymes but not by lipase. PBS, in contrast, showed higher hydrolysis rates with lipase than with proteases. The thermal profile of PLA hydrolysis by protease showed an exponential increase from 4 to 30 °C with a temperature quotient Q10 of 5.6. The activation energy was 110 kJ·mol−1. pH-Stat titration proved to be a rapid, sensitive, and reliable procedure supplementing established methods of determining the bio-degradability of polymers under environmental conditions.
Highlights
Hydrolysis rates of poly(lactic acid) (PLA) and poly(butylene succinate) (PBS) were determined with protease from Bacillus licheniformis, Proteinase K from Tritirachium album, and lipase from Candida antarctica
For each of the two polymers (PLA and PHB), the hydrolysis rate was compared between the three enzymes by a 1-factorial analysis of variance (ANOVA) followed by Tukey’s HSD test
Compared to natural substrates such as proteins, carbohydrates, or lipids, the enzymatic hydrolysis of bioplastics is slow. This is due to the physical characteristics of the bioplastic particles
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Specific enzymes hydrolyze the polymer chain and release oligo- and monomers These degradation products may be assimilated and metabolized to water and carbon dioxide [8]. Various standard tests and methods are available for the analytical assessment of the biodegradability of bioplastics [16] These comprise the carbon dioxide forming test (Organisation for Economic Co-operation and Development, OECD 301b), differential scanning calorimetry (International Organization for Standardization, ISO 11357), weightloss measurements (ISO 13432), or Fourier transform infrared spectroscopy [17]. We adapt the pH-Stat titration method for routine analysis to determine the biodegradability of bio-based plastics under relevant environmental conditions with support of specific proteolytic and lipolytic enzymes. Parameters for an improved performance and reliability of the method were tested and procedural difficulties identified
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