Current trends in biodegradable polyhydroxyalkanoates.

The microbial polyesters known as polyhydroxyalkanoates (PHAs) positively impact global climate change scenarios by reducing the amount of non-degradable plastic used. A wide variety of different monomer compositions of PHAs has been described, as well as their future prospects for applications where high biodegradability or biocompatibility is required. PHAs can be produced from renewable raw materials and are degraded naturally by microorganisms that enable carbon dioxide and organic compound recycling in the ecosystem, providing a buffer to climate change. This review summarizes recent research on PHAs and addresses the opportunities as well as challenges for their place in the global market.

Characterization of new isolated Ralstonia eutropha strain A-04 and kinetic study of biodegradable copolyester poly(3-hydroxybutyrate-co-4-hydroxybutyrate) production.

A new isolated bacterial strain A-04 capable of producing high content of polyhydroxyalkanoates (PHAs) was morphologically and taxonomically identified based on biochemical tests and 16S rRNA gene analysis. The isolate is a member of the genus Ralstonia and close to Ralstonia eutropha. Hence, this study has led to the finding of a new and unexplored R. eutropha strain A-04 capable of producing PHAs with reasonable yield. The kinetic study of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] production by the R. eutropha strain A-04 was examined using butyric acid and gamma-hydroxybutyric acid as carbon sources. Effects of substrate ratio and mole ratio of carbon to nitrogen (C/N) on kinetic parameters were investigated in shake flask fed-batch cultivation. When C/N was 200, that is, nitrogen deficient condition, the specific production rate of 3-hydroxybutyrate (3HB) showed the highest value, whereas when C/N was in the range between 4 and 20, the maximum specific production rate of 4-hydroxybutyrate (4HB) was obtained. Thus, the synthesis of 3HB was growth-limited production under nitrogen-deficient condition, whereas the synthesis of 4HB was growth-associated production under nitrogen-sufficient condition. The mole fraction of 4HB units increased proportionally as the ratio of gamma-hydroxybutyric acid in the feed medium increased at any value of C/N ratio. Based on these kinetic studies, a simple strategy to improve P(3HB-co-4HB) production in shake flask fed-batch cultivation was investigated using C/N and substrate feeding ratio as manipulating variable, and was successfully proved by the experiments.

Production and characterization of biodegradable terpolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-4-hydroxybutyrate) by Alcaligenes sp. A-04.

The production of the terpolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-4-hydroxybutyrate), P(3HB-co-3HV-co-4HB), by Alcaligenes sp. A-04 was investigated to determine the superior biodegradable polymer properties over those of poly(3-hydroxybutyrate), P(3HB), and its copolymers. The highest terpolymer content of 68% (w/w) was produced by Alcaligenes sp. A-04 at 60 h by shake-flask cultivation. The terpolymer with 93 mol% 4HB mole fraction units was produced when the cultivation time was extended to 96 h. Moreover, it was found that Alcaligenes sp. A-04 could utilize 1,4-butanediol for the synthesis of 3HB and 4HB monomers as well as the sodium salt of 4-hydroxybutyrate. The terpolymer content was 30% (w/w) and the composition was P(33%3HB-co-16%3HV-co-51%4HB). Next, terpolymers with 4HB mole fraction units ranging from 50 to 90 mol% were produced by varying the medium composition and cultivation time. The thermal and mechanical properties of the resulting terpolymers were different from those of the copolymers with a similar mole fraction of monomer units. The terpolymer P(4%3HB-co-3%3HV-co-93%4HB) showed an elongation of 430%, a toughness of 33 MPa, and Young's modulus of 127 MPa similar to those of low-density polyethylene. The terpolymer P(11%3HB-co-34%3HV-co-55%4HB) showed Young's Modulus of 618 MPa similar to that of polypropylene.

Model predictive controller for biodegradable polyhydroxyalkanoate production in fed-batch culture.

The aim of this study is to develop a model predictive controller (MPC) accompanied with a metabolic reaction model controller for controlling ethanol and n-pentanol concentrations and the mole fraction of monomer units in the production of poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate), P(HB-co-HV), a biodegradable copolyester. The controller consists of two parts: one is for alcohol concentration control and the other one is for mole fraction control, and is based on the concept of metabolic flux distribution control. For control of alcohol concentration, conventional proportional and integral (PI) controller and feedforward/feedback controller did not function sufficiently because the large sampling interval of the biosensor led to a severe overshoot of concentration. A single-input and single-output (SISO) MPC is constructed for control of ethanol concentration in the growth phase, whereas a multi-input and multi-output (MIMO) MPC is constructed for control of both alcohol concentrations in the production phase. Specific ethanol consumption rate was estimated by the MPC using the past time series data of ethanol concentration. By means of simulations and experiments, the weighting parameters of the noise filters in the MPC were well adjusted. Ethanol and n-pentanol concentrations were well controlled by the MPC, compared with PI controller and feedforward/feedback controller. As a result, P(HB-co-HV) production was maximized with a given value of mole fraction of 3HV units at the end of cultivation.