Development of films based on bacterial biopolyesters produced from apple pulp waste

Abstract

The first objective of this thesis was to use apple pulp waste, a leftover from the fruit processing industry, for the cultivation of Pseudomonas citronellolis NNRL B-2504, Cupriavidus necator DSM 428 and a co-culture using both strains, culminating on the production of medium-chain-length polyhydroxyalkanoates (mcl-PHAs), short-chain-length polyhydroxyalkanoates (scl-PHAs) and a natural biosynthesized blend, respectively. PHAs are biopolyesters with versatile material properties, applicable in several fields. The soluble fraction of the apple pulp waste, rich in fructose, sucrose and glucose, was used for the batch bioreactor cultivation of P. citronellolis and C. necator. The first cultivation, P. citronellolis, reached a polymer content in the biomass of 30wt.% with a volumetric productivity of 0.61 g/L.day. The polymer was mainly composed of 3-hydroxydecanoate (68%) and 3-hydroxyoctanoate (22%), and had a molecular weight of 3.7×105 Da. It presented melting and thermal degradation temperatures of 53 and 296 °C, respectively, and a crystallinity degree of 15%. C. necator DSM 428 achieved a volumetric productivity of 1.59 g/L.day, reaching a polymer content 44 wt.% . The homopolymer, P(3HB) was obtained, with a molecular weight of 5.0×105 Da and a crystallinity degree of 43%. Moreover, the polymer had melting and thermal degradation temperatures of 179 and 293 °C, respectively. Finally, a third batch production using a co-culture was performed. The production presented a 34 wt.% of polymer content with a volumetric productivity of 0.96 g/L.day. The polymer produced was a physical mixture of mcl-PHA and P(3HB). These results demonstrated that apple pulp waste is a suitable feedstock for the production of different biopolymers with distinct physical-chemical properties. The aim of the second part of this thesis was to prepare films based on the PHAs produced from apple pulp waste, followed by their characterization, both physical and chemical. All films presented similar thermal, crystallinity and chemical features when compared to the neat polymer, which indicates that the procedure did not significantly impact on the polymers’ characteristics. Gas permeation tests were performed for both oxygen and carbon dioxide. mcl-PHA films presented the best results for both O2 and CO2 (1.1×10-09 and 5.3×10-09 cm3.cm/cm2.s.cmHg, respectively). The mechanical tests performed for all PHA films revealed that mcl-PHA and blends demonstrated higher deformation until break, and P(3HB) had superior tensile strength.