Production of Medium-Chain Length Polyhydroxyalkanoates by Pseudomonas citronellolis Grown in Apple Pulp Waste

Background and objective: Apple pulp waste generated by the fruit processing industry is a sugar-rich material with great potential to be used as a feedstock for production of valueadded microbial products. The aim of this work was to use this feedstock for the cultivation of Pseudomonas citronellolis and production of medium-chain-length polyhydroxyalkanoates, a natural elastomer. Material and methods: The solid fraction of the apple pulp waste was discarded and the soluble fraction, rich in fructose (17.7 g l^-1), glucose (7.5 g l^-1) and sucrose (1.2 g l^-1), was used for the batch bioreactor cultivation of Pseudomonas citronellolis NRRL B-2504. Results and conclusion: Pseudomonas citronellolis reached a polymer content in the biomass of 30% wt and a volumetric productivity of 0.025 g l^-1 h^-1. The polymer was mainly composed of 3-hydroxydecanoate (68% mol) and 3-hydroxyoctanoate (22% mol), with minor contents of 3-hydroxydodecanoate (5% mol), 3-hydroxytetradecanoate (4% mol) and 3-hydroxyhexanoate (1% mol), and had a molecular weight of 3.7×10^5 Da. It presented glass transition and melting temperatures of -12 and 53°C, respectively, and a thermal degradation temperature of 296°C. The polymer’s films were dense, ductile and permeable to oxygen and carbon dioxide. These results demonstrated that apple pulp waste is a suitable feedstock for the production of a biopolymer with properties that render it a promising alternative to some synthetic petrochemical-derived polyesters.