Vecchiato V and Basnett P*
School of Life Sciences, University of Westminster, UK
*Corresponding author: Pooja Basnett, School of Life Sciences, University of Westminster, London, W1W 6UW, UK
Submission: November 06, 2020;Published: December 04, 2020
ISSN: 2770-6613 Volume1 Issue3
Polyhydroxyalkanoates (PHAs) are a group of biopolymers synthesized via bacterial fermentation. PHAs have emerged as a potential alternative to conventional plastics owing to their biodegradability and sustainable production process. The purpose of this review is to highlight key factors involved in the economical production of PHAs.
Keywords: Polyhydroxyalkanoates; Fermentation; Extremophile; Downstream recovery
Abbreviations: CO2: Carbon Dioxide; PHAs: Polyhydroxyalkanoates; PP: Polypropylene; PE: Polyethylene; WWF: Worldwide Fund for Nature
In the year 1957, an Italian company Montecatini started the production of a revolutionary
plastic material, known as Moplen Polypropylene, that became part of our daily lives. This
discovery and commercial production of plastic led Giulio Natta and Karl Ziegler to be
awarded with the Nobel Prize for Chemistry in 1963 .
Plastic is widely used in many different applications, since it is cheap, durable, and resistant; however, its negative impact on the environment cannot be ignored. According to WWF, 8 million tonnes of petroleum based plastic material escape the collection system and are dumped into the oceans every year. By the end of 2050, the production is expected to reach around 33 billion tonnes  and there could be more plastic in the sea than fish . To overcome plastic pollution, concerted efforts focused on recycling and reducing the use of single use plastic is being made; nevertheless, the idea of a circular economy based on a sustainable, non-toxic and biodegradable plastic, derived from renewable feedstock is the need of the hour. Among the biopolymers, polyhydroxyalkanoates (PHAs) have recently gained attention, as a green alternative to oil-based plastic. PHAs are polyesters of hydroxyalkanoates (HAs), biosynthesized by bacteria as a carbon and energy storage.
Plastic is widely used in many different applications, since it is cheap, durable, and resistant; however, its negative impact on the environment cannot be ignored. According to WWF, 8 million tonnes of petroleum based plastic material escape the collection system and are dumped in our oceans every year. Moreover by the end of 2050, the production is expected to reach around 33 billion tonnes  and there could be more plastic in the sea than fish . To overcome plastic pollution, concerted efforts focused on recycling and reducing the use of single use plastic is being made; nevertheless, the idea of a circular economy based on a sustainable, non-toxic and biodegradable plastic, derived from renewable feedstock is the need of the hour. Among the biopolymers, polyhydroxyalkanoates (PHAs) have recently gained attention, from various industries, as a green alternative to oil-based plastic. PHAs are polyesters of hydroxyalkanoates (HAs), biosynthesized by bacteria as a carbon and energy storage. These biopolymers are produced under specific conditions with an increasing amount of carbon to essential nutrients (such as nitrogen, phosphate, or oxygen) ratio . Although these biopolymers have been well known since 1926, they do not offer a cost competitive performance yet, compared to the petroleum-based plastic, such as polypropylene (PP) or polyethylene (PE), which cost US $0.60-0.87/lb, while the cost of PHAs is US $2.25-2.75/lb . This is due to its high production cost and low productivity coupled with instability in molecular weight and consequential properties . Recent advancement has improved three fundamental steps in the large-scale production of PHAs such as the use of cheap carbon source, use of novel bacteria species and the implementation of new recovery processes.
For a circular, environmentally friendly pathway, use of cheap carbon substrate to produce high quality PHA is fundamental. Agriculture, food, dairy or cosmetic industries are among the major suppliers of cheap and renewable carbon waste . To obtain maximum yield of PHAs, while maintaining its quality, an idyllic 100% content of carbon in the raw material is required, therefore the impact of pre-treatment costs is prominent. The conversion from whey into galactose or glucose, for example, is necessary to improve the fermentation efficiency of PHAs . Purification process is essential when it comes to obtaining crude glycerol from biodiesel, although the cost is partly counterbalanced by the maximum yield obtained, which is 75%, so far . Another promising and eco-friendly feedstock for the production of PHAs is using the emission of biogas, such as CO2. Biopolymers were produced, in this case, through a dual system heterotrophic–autotrophic cultivation, where CO2 was used as inorganic carbon substrate and glucose as the organic counterpart . In addition to using cheap carbon sources, the choice of the bacterial strain is key in the economical production of PHAs. Recent studies, that lie under the concept of “Next-generation industrial biotechnology (NGIB)”, have focused on working with extremophiles (such as halophiles and thermophiles), as a potential solution to overcome high costs related to sterile fermentation operations . Extremophiles have reduced risk of mesophilic microbial contamination, which allows operation under semi-sterile or even non-sterile conditions. It also enables production in a continuous mode for long periods. The use of extremophiles could replace the use of sterile water for the media preparation with sea water. For industrial-scale production, these factors are positive in terms of efficiency and cost reduction [11,12].
For the economical and environmentally friendly production of PHAs, developing new strategies of downstream recovery, which represents half of the total process cost is essential. Currently halogenated solvents, like chloroform, are mainly used for extraction; therefore non-toxic, cost-effective and eco-friendly alternatives have been considered including microwave assisted extraction, thermo-separating aqueous two-phase extraction (ATPE), or using recyclable solvents, like 1,2-propylene carbonate . These strategies have led to high-purity polymers; however, polymer recovery has been low . In addition to the cost and environmental impact, the composition and properties of the PHA material, the molecular weight and level of purity must also be considered during downstream processing .
The principal characteristics of PHAs include thermo-plasticity, hydrophobicity, high molecular weight, biodegradability. Their industrial applications range from being used as coating materials to packaging, agricultural sheets and inks . Due to their nontoxic nature, they have been explored for a variety of medical applications such as biocontrol agents, drug carriers, biodegradable implants and in regenerative medicine . As a result, PHAs are a promising group of biopolymers, tailored for a broad spectrum of applications. They could play a key role in the fight against plastic pollution and replace conventional plastics soon [17,18]. Further research should be done in the areas of fermentation strategies, cost-effectiveness while maintaining zero carbon footprint to meet the increasing industrial demand.
© 2020 Pooja Basnett. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and build upon your work non-commercially.