Crystallization of an Aromatic Biopolyester

Production of Polyhydroxybutyrate (PHB) and Factors Impacting Its Chemical and Mechanical Characteristics

Plastic pollution is fueling the grave environmental threats currently facing humans, the animal kingdom, and the planet. The pursuit of renewable resourced biodegradable materials commenced in the 1970s with the need for carbon neutral fully sustainable products driving important progress in recent years. The development of bioplastic materials is highlighted as imperative to the solutions to our global environment challenges and to the restoration of the wellbeing of our planet. Bio-based plastics are becoming increasingly sustainable and are expected to substitute fossil-based plastics. Bioplastics currently include both, nondegradable and biodegradable compositions, depending on factors including the origins of production and post-use management and conditions. Among the most promising materials being developed and evaluated is polyhydroxybutyrate (PHB), a microbial bioprocessed polyester belonging to the polyhydroxyalkanoate (PHA) family. This biocompatible and non-toxic polymer is biosynthesized and accumulated by a number of specialized bacterial strains. The favorable mechanical properties and amenability to biodegradation when exposed to certain active biological environments, earmark PHB as a high potential replacement for petrochemical based polymers such as ubiquitous high density polyethylene (HDPE). To date, high production costs, minimal yields, production technology complexities, and difficulties relating to downstream processing are limiting factors for its progression and expansion in the marketplace. This review examines the chemical, mechanical, thermal, and crystalline characteristics of PHB, as well as various fermentation processing factors which influence the properties of PHB materials.

Potential and Prospects of Continuous Polyhydroxyalkanoate (PHA) Production

Together with other so-called “bio-plastics”, Polyhydroxyalkanoates (PHAs) are expected to soon replace established polymers on the plastic market. As a prerequisite, optimized process design is needed to make PHAs attractive in terms of costs and quality. Nowadays, large-scale PHA production relies on discontinuous fed-batch cultivation in huge bioreactors. Such processes presuppose numerous shortcomings such as nonproductive time for reactor revamping, irregular product quality, limited possibility for supply of certain carbon substrates, and, most of all, insufficient productivity. Therefore, single- and multistage continuous PHA biosynthesis is increasingly investigated for production of different types of microbial PHAs; this goes for rather crystalline, thermoplastic PHA homopolyesters as well as for highly flexible PHA copolyesters, and even blocky-structured PHAs consisting of alternating soft and hard segments. Apart from enhanced productivity and constant product quality, chemostat processes can be used to elucidate kinetics of cell growth and PHA formation under constant process conditions. Furthermore, continuous enrichment processes constitute a tool to isolate novel powerful PHA-producing microbial strains adapted to special environmental conditions. The article discusses challenges, potential and case studies for continuous PHA production, and shows up new strategies to further enhance such processes economically by developing unsterile open continuous processes combined with the application of inexpensive carbon feedstocks.

Simultaneous biosynthesis of two copolymers in Pseudomonas putida GPo1 using a two-stage continuous culture system

The bacterial production of medium-chain-length polyhydroxyalkanoate (PHA) in continuous two-stage cultivation was investigated. Two chemostats were linked in sequence and Pseudomonas putida GPo1 was supplied with different concentrations of octanoic acid in the first and 10-undecenoic acid in the second reactor. PHA isolated from the second bioreactor was a blend of two copolymers typically consisting of poly(3-hydroxyoctanoate-co-3-hydroxyhexanoate) (PHO) and poly(3-hydroxy-10-undecenoate-co-3-hydroxy-8-nonenoate-co-3-hydroxy-6-heptenoate) (PHUE) with structural (monomeric) purities of 85-95 mol %, indicating simultaneous PHA accumulation and degradation inside the cell. The maximum PHA content was found to be 53.8% of total cell dry weight, which is very high for mcl-PHA.