History

Polyhydroxyalkanoates (PHAs) comprise a range of different biodegradable biopolymers—bioplastics or bio-polyesters synthesized by various microorganisms [1]. They were discovered, in 1888, by Beijerinck as cellular inclusions of bacteria and other microorganisms [2]. In 1926 Maurice Lemoigne, a French scientist, was the first to synthesize a PHA (named poly-3-hydroxybutyric acid) using a bacterial strain of Bacillus megaterium [3]. Later on, in 1958, Macrae and Wilkinson claimed that the bacterial cells play a role as cellular factories for PHAs synthesis and storage [4]. Imperial Chemical Industries (ICI), UK, initiated commercial production of PHAs in 1961. In 1980, ICI synthesized a copolymer of PHA via fermentation, while in 1983, De Smet et al. [5] reported synthesis of a medium chain length polyhydroxyalkanoate (mcl-PHAs) which in turn became a milestone in the synthesis of PHAs. Also in 1980, the researchers had started investigations on the physiochemical properties of PHAs [6] and later on in 1990, scientists used them for the first time in the biomedical field. By 2010, the PHAs market had reached up to 1010 US$ with a growth of 170–180 Kt, annually [7].

Introduction

Plants have always been used as a major source of polymers such as starch, cellulose, flax and rubber for thousands of years. During the past few decades, plant polymers had extensively been replaced by petrochemical based polymers on mass scale production [8]. At the end of the twentieth century, it was realized that these synthetic polymers are non-biodegradable, persistent, and continually accumulate in the environment which ultimately represents a threat to life on Earth. During the past two decades environmental protection agencies had started awareness programs for both the public, in general, and manufacturers, in specific, on the importance of preserving and protecting the nature from toxic products and hazardous wastes [9]. These concerns had compelled scientists and researchers to further explore new approaches to synthesize and use eco-friendly bioplastics such as PHAs as alternative materials to plastics [10].

PHAs are biodegradable linear thermoplastic polyesters, which can be used as alternative polymers to synthetic ones. They are synthesized by different bacterial strains (both Gram positive and Gram negative) cultivated on different carbon sources like sugars, alkanoic acids, alkanes, alkenes and other renewable carbon sources [11]. During the last two decades, PHAs have attained much attention because of their diverse features such as hydrophobicity, elastomeric nature and biodegradability under a wide range of environmental conditions. They also constitute a natural part of renewable carbon cycle and are used as alternatives to synthetic polyesters. Nevertheless, there are still some limitations in their bulk scale production including low yields and high production costs [12].

Some 150 different types of PHAs congeners with different structures (varying side chains or functional groups) and properties had been synthesized using different microbial strains. PHAs can be synthesized though microbial fermentations both at laboratory and pilot plant scales followed by appropriate downstream processing [13]. They are classified according to their chemical unit structure. The polymers containing repeating units with 3–5 carbon atoms are known as short chain length PHA (scl-PHA) and a polymer with repeating units of 6–13 carbon atoms are known as medium chain length PHA (mcl-PHA), while those with more than 13 carbon atoms are known as long chain length PHA (lcl-PHA). A copolymer like poly(3-hydroxybutyrate-co-4-hydroxyhexanoate) could be produced by simply mixing poly(3-hydroxybutyrate), a scl-PHAs, and poly(4-hydroxyhexanoate), a mcl-PHAs monomers under desirable fermentation conditions [14]. A general structure of PHAs is shown in Fig. 1.

Fig. 1
figure 1

A general structure of PHAs

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PHAs find a large number of applications in different fields of science and technology. For instance, a blend of poly(hydroxybutyrate) P(HB) and poly(hydroxyoctanoate) P(HO) is used as food additive which had been approved by the US food and drug authority (FDA) [15]. PHB has also been used as a component in making plastic accessories, coating paper, biodegradable bottles, electronic accessories, pharmaceuticals, garments, upholstery, and packaging materials [16]. In the field of medicine, PHAs allow us to control the drug release and targeted drug action whereas PHB had been used to produce different medical devices [17]. A drug delivery system can be a sustainable way to administrate certain drugs by minimizing their toxic effects. An mcl-PHA had been used to administrate drugs through a transdermal drug delivery system. There are three basic components of a drug delivery system i.e., a carrier, a target moiety and an active part of the drug. Genetically engineered bacterial strains produce PHAs and allow us to utilize them in vascular grafting, blood vessel and heart valve development. P(3-HAs) has also been suggested for use as a component of biofuels to reduce global warming [18]. The wide range of applications of PHAs in different fields of life is listed in Table 1.

Table 1 Applications of PHAs in various fields and their examples
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In this review, we present recent developments in the bacterial production of PHAs on various carbon sources. We focused on the challenges in PHAs production in different types of bioreactors and operational regime using various bacterial strains. A particular emphasis is dedicated to current advantages and limitations of different bioreactor scale setup while concentrating on the efforts devoted to upgrade these processes to industrial scales. Finally, we endeavour to identify the most suitable downstream processing technology.

Financial obstacles in PHAs commercialization

Commercialization of a product mainly depends on its production cost, potential applications and market requirements. In the past decades, magnificent efforts had been made to improve PHAs yield both at laboratory and pilot plant scales [7]. The main challenges in the commercialization of PHAs had been their high production cost and low product yields. PHAs produced from different sources require different tools like fermenter, autoclave etc. with different fermentation approaches which increase the production cost ultimately limiting their commercialization. The inability to achieve optimal bacterial growth conditions and production yields for PHAs had been a major disadvantage in this context [19]. In addition, PHAs have some disadvantages in their mechanical properties including; incompatibility with conventional thermal processing, limited functionalities, susceptibility to thermal degradation and production cost which limits their competitiveness and application as a feasible biomaterial when compared with synthetic plastics. The possibility of commercialization of PHAs might be enhanced using mixed microbial culture (MMC) with cheap carbon sources like waste frying oils and crude oils. The main obstacle in the commercialization of PHAs had been their production cost which could be reduced using edible oils as carbon sources with non-sterilized fermentation conditions [11, 20].

Sustainable solutions for PHAs production

The economics of PHA production mainly depends upon substrate cost and compatibility to use product in similar to those produced using expensive growth substrate, efficiency of production, and downstream processing [21]. Recent studies on PHAs production not only consider the sustainability of polymer, but also its cost-effectiveness. Nitrogen and carbon sources get significant share in the production cost. Using cheap nitrogen sources such as urea and sodium nitrate reduces the cost of production significantly [10]. The utilization of industrial wastes as carbon source to produce a competitive cost effective PHA has also been reported as a promising approach for decreasing overall costs [2]. The economics of whole process is based on three approaches; large scale aerobic batch fermentation, use of cheap carbon source soybean and other waste frying oils, and simplification of the downstream recovery process to achieve an economical PHAs production approach [22]. PHAs bacterial sources and their chemical structures are listed in Table 2.

Table 2 Polyhydroxyalkanoates bacterial source and their chemical structure
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Other ways to meet financial barrier is though value addition by inserting new functionalities in PHAs to overcome their inherent limitations, improve their desirable properties and enhance potential applications [20]. This is usually achieved by blending the bio-polyesters with other natural biodegradable polymers, including cellulose derivatives, poly lactic acid (PLA), starch, lignin and poly(caprolactone). The introduction of different functional groups via chemical routes has been described with regard to the two main synthesis approaches, graft co-polymerization and block co-polymerization [23]. Grafting of different functional groups on side chains of linear bio-polyesters introduces many additional properties, which makes them sustainable with desired properties and controlled polymer structures. For instance, grafting of chitosan on PHA produces a copolymer P(H-co-chitosan) which is a sustainable biopolymer with some important applications in medical field. A cost effective production has been reported, carried out using MMC, under unsterilized conditions [20, 24].

PHA production in shake flasks

Shake flask fermentations are normally carried out in laboratories mainly to explore feasibility. Here, the information is often vogue and the experimental expenditures are high with large number of trails typically required to identify a set of optimum fermentation conditions. Such fermentations are usually carried out in Erlenmeyer flasks provided with minimal media, carbon source and any precursors at specific pH levels and incubated at certain temperature and shaking speed for a certain period of time. During shake flask fermentations, the process is followed up based on certain parameters such as carbon source or any other nutrients utilization, biomass formation and fermentation kinetics. In the previous studies, bacterial production of PHAs had mostly been carried out using glucose as carbon source. Later on, Pseudomonas sp. grown on MMC had been employed with volatile fatty acids (VFAs) as carbon source and also used as precursor for PHAs production with higher productivity [25]. The optimized fermentation conditions had been used to produce PHAs with efficient recovery. Different ratios of carbon and nitrogen sources had been used to optimize product yields. An ammonia free fermentation media were used to accumulate PHAs with VFAs. During the first phase of fermentation, glucose was used as carbon source after inoculating the media in Erlenmeyer flask and cell biomass was harvested via centrifugation at 6000 rpm and 4 °C with 20–40% g/g of product yield. The shake flask study was used to determine the feasibility of fermentation process with different parameters. The main disadvantage of shake flask investigations is the wide variation in the quality of produced product [26].

Kinetics of PHA production

The fermentation kinetics were previously investigated by determining the product yields with respect to substrate consumption. The kinetics of production determines how much PHAs cell dry mass (CDM) are produced with respect to time and expressed as g/L/h. A linear increase in PHAs yield was observed following a zero order reaction kinetics until the carbon source growth substrate was depleted [27]. The kinetics of PHAs production for different bacterial strains with large number of substrates had been established e.g., Azotobacter beijerinckii with glucose as growth substrate produced 0.09 g/L/h [28]. Burkholderia cepacia bacterial strain was grown on glycerol as energy substrate produced 0.103 g/L/h [29], Ralstonia eutropha with butyrate, propanoic, lactic and acetate acid as growth substrates produced 0.001–0.037 g/L/h [30]. Alcaligenes eutrophus grown on potato starch and saccharified wastes as growth substrates produced 1.5 g/L/h [31]. Hydrogenophaga pseudoflava with sucrose and lactose as growth substrates produced 0.02–0.12 g/L/h [32]. Pseudomonas frederiksbergensis used terephthalic acid as growth substrate and produced 0.004 g/L/h [33], Pseudomonas putida KT 224 produced 0.006 g/L/h of PHAs using glucose as growth substrate [34]. Haloferax mediterranei produced 0.05–0.2 g/L/h of PHAs using vinasse as growth substrate [35] and the same bacterial strain with glycerol produced 0.12 g/L/h [36]. Further kinetics of PHAs production with different bacterial sources, substrates and production scales are presented in Table 3.

Table 3 PHAs producing microbial strains, growth substrates and their kinetics
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Bioreactor scale production of PHAs

Bioreactor scale production of biopolymers is the most suitable production technique on industrial scale using large working volume and avoiding many restrictions encountered under shake flask conditions. Because, PHAs biosynthesis constitutes a multiple phase process, both the feeding strategy and bioreactor operation mode need to be adapted via optimization of operation conditions. Industrial bioreactor scale production of PHAs can also be operated under both fed batch and continuous fed batch feeding strategies. In the following text, various types of bioreactors and their conditions have been described and analyzed for enhanced PHA production opportunities based on the available literature.

Single batch production

During batch production, desirable amounts of different nutrients are added in the fermentation media. The single batch production process is simple but gives low productivity. The maximum allowed concentration of both carbon and nitrogen sources are added at the beginning of batch fermentation processes which restrict nutrient addition during PHAs production. In batch productions of PHAs, the amount of nitrogen source are typically added at 0.2–5 g/L and carbon source at 1–30 g/L at the beginning of fermentation process [37]. Moreover, the excess amount of carbon source added in the presence of other growth limiting nutrients such as N, P, S, and K. The single batch fermentation process is analogous to other PHAs production processes with minimum conversion toward biomass, CO2, PHAs and other metabolites; thus resulting in low overall conversion of growth substrate carbon source to PHAs cell biomass yield. Considering hypothetical biotransformation of biogenic carbon to PHAs, the yield was calculated to be below 0.4 g/g which indicates that such processes are not economically feasible. The single batch fermentation scale production of PHAs, therefore, suffers limitations which need to be addressed. The main set back of batch scale production is that the nutrients are not further provided which inhibited the bacterial growth, thus restricting the production of PHAs. If the fermentation period is extended above the optimum period (e.g., 7 days) then the degradation of PHAs starts. A list of single batch PHAs production processes using different bacterial strains is shown in Table 4.

Table 4 Single batch scale production of PHAs
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Sequence batch fermentation production processes for PHAs

In sequence batch bioreactor experiments, two or more reactors are connected in series to perform different functions simultaneously to produce PHAs. Albuquerque et al. [38] reported a study of two-configurations system used to produce PHAs from organic activated sludge waste as substrate. In the first configuration, nitrogen (as ammonium) was limited by simple conversion to nitrate and bacteria started to respond these conditions and produced PHAs. Both aerobic feasting stage (ammonia conversion to nitrate) and anoxic famine stage (denitrifying) conditions drive internally stored PHAs as carbon source [38].

First configuration: In the first configuration, continuous feeding with fix interval of 5 min had been carried throughout the fermentation process until desired cell mass was achieved. The fermentation medium was allowed to settle down for 15 min and then fermentation process was further run under aerobic conditions for an hour which was followed by anaerobic fermentation condition to produce PHAs [38].

Second configuration: Here, the fermentation process had been carried in two bioreactors which were connected in a series. In one of the bioreactors, nitration took place and in second bioreactor, biomass is optimized. The nitration was carried out under aerobic fermentation conditions. After that, the fermentation medium was further processed in the second reactor under aerobic feast and anoxic famine reaction conditions to accumulate high cell mass for PHAs inclusions [38].

Fed batch production of PHAs

During fed batch cultivation, the substrate is added through a pulse feeding when its concentration drops below its optimum value without removing any culture media. In the case of PHAs production, both carbon and nitrogen sources are added at periodic intervals according to their consumption by microorganisms. In fed batch fermentation process, these substrates are added to avoid any depletion during the production process until the desired cell biomass yield is achieved. Some researchers used fed batch process to produce 0.41 g/g of PHAs using Pseudomonas putida KT 2440 through feeding octanoic acid as a carbon source [39].

The fed batch production had also been used to examine the effect of different carbon to nitrogen (C/N) ratios on product yield. For that purpose, nutrient enrich media had been prepared using the following recipe: MgCl2·6H2O, KCl, Fe2+, NaHCO3, C6H12O6, NaCl, CaCl2, MgSO4·7H2O, NH4Cl, KH2PO4, NaBr and 1 mL of trace elements solution. An aliquot amount of 60 mL of inoculum was added in the nutrient enrich media of sequence batch reactor after 48 h of cultivation. The pH of fermentation media was maintained at 7.5–7.8 using 1 M NaOH and 1 M HCl solutions. The amount of dissolved oxygen in the nutrients enriched media was 100% at 40 °C for 72 h. The maximal substrate consumption, CDM and PHAs yield decreased with increase in C/N ratio. The yields of 0.970, 0.955, and 0.890 g/L of CDM were reported, respectively, using 0.47, 1.95, and 7.64 g/L of nitrogen concentrations [13].

Continuous production process

The fermentative production processes are typically carried out in continuous stirred tank bioreactors (CSTR) where process parameters like agitation speed, air flow, pH, substrate, product concentration and temperature are controlled to investigate PHAs yield. The PHAs production using MMC under continuous feeding strategies had been established. Such a process produced PHAs with improved composition on monomeric level and results in a more sustainable process. Continuous PHAs production process is clearly better than the batch production in terms of composition, flexibility and productivity of bio-polyesters [38]. Pseudomonas putida had also been reported to produce 80% (g/g) PHAs, at both shake flask and bioreactor scale fermentations. The pH of fermentation media at lab scale were kept neutral using 2M NaOH and 4M H2SO4 solutions at 30 °C, 250 rpm and 20% of dissolved oxygen (DO) throughout fermentation process. At pilot scale, CSTR fermentation with different working volumes has been carried out to produce PHAs. In CSTR, the fermentation parameters were kept at pH 7 (using 30% H2SO4and 2 M NaOH solutions), temperature 30 °C, rate of agitation 100–1000 rpm and aeration 0–30 L/min to produce 0.53 g/g of PHAs. The continuous production of PHAs had been carried out at single, double and multiple stages to facilitate the bacteria during fermentation process. In continuous production, each and every aspect regarding the fermentation process proceeded more accurately than fed batch mode. In fed batch process, nutrients were further added once their depletion inside fermention media to produce higher PHAs yields [40].

One stage continuous stirred tank reactor

The efficiency of a single stage continuous stirred tank reactor in terms of flexibility in polyester composition and productivity. The schematic diagram used is shown in Fig. 2a. Single stage continuous stirred tank reactors (single SCSTRs) are not productive for PHAs cell biomass production, because intercellular products of secondary metabolism only boost PHAs production under limited nutrients growth conditions. Hence, it is not possible to continuously supply these nutrients throughout the fermentation process, that’s why optimization is needed for both intracellular PHAs mass and high biomass formation. Atlic et al. [41] reported on the validity of single SCSTR using propionic acid as growth substrate, Cupriavidus necator bacterial strain and glucose as carbon source producing 0.33 g/g P(3-HB-co-3-HV) co-polyesters. The amount of cell biomass and cell dry mass produced in the single SCSTR was not competitive with fed batch process using the same bacterial production strain [41].

Fig. 2
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Schematic diagrams of continuous tank bioreactors a single stage, b two-stage and c three stage

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Two-stage continuous stirred tank reactor

The two-stage continuous stirred tank reactor fermentation process is used to produce PHAs with maximum yield which is not possible under shake flask conditions and with single SCSTR. A schematic diagram for this is shown in Fig. 2b. During two SCSTR fermentation process, different functions are carried out in two bioreactors, simultaneously. Regarding the cell biomass accumulation for the bacterial strain and substrate ratio, it had been observed that autocatalytic growth was much higher than the PHAs accumulation process. Due to this reason, it is impossible to carry out such process with sufficient productivity at single SCSTR scale. Therefore, it was consequently demonstrated that two-stage continuous stirred tank reactor (two SCSTR) process is better for such type of production. Here, in the first CSTR, a higher amount of cell biomass could be produced with higher density and continuously transferred to second CSTR, where large amount of carbon source are provided continuously, under limiting nitrogen substrate to stimulate PHA production [42].

The efficiency of two SCSTR has been demonstrated in the literature. For instance, the bacterial strain of Alcaligenes lata had been used to produce 0.38 g/g of P(3-HB) and 0.55 g/g of P(3-HB-co-3-HV) a copolymer using propionic acid (as precursor), residual sucrose (as carbon source) in the minimal nutrients media. In the first CSTR, the nitrogen source and precursor were completely utilized by the bacterial cells. The fermented broth containing residual sucrose was continuously transferred to the second CSTR, but no additional nutrients were added to facilitate the conversion of sucrose as growth substrate to increase PHAs content in the bacterial cell biomass [43]. Hence, in this phase, bacterial cell was allowed to complete the fermentation process to maximize the PHAs yield while depleting the sucrose (i.e., as carbon source). In the context of two SCSTR, the optimized bacterial strain and substrates were used to produce different scl-PHA, mcl-PHA as homo and hetero polymers. Some researchers cultivated the bacterial strain of P. putida GPo1 in two SCSTR with gaseous substrate n-octane as carbon source and produced a blend of two different polymers with block structures [21]. The block copolymer of poly(3-hydroxy-10-undecenoate-co-3-hydroxy-8-nonenoate-co-3-hydroxy-6-heptenoate) was produced at 0.63 g/g. This observation is in contrast to other reports, which claimed that block PHAs copolymers were only produced when adding sufficient amount of growth substrate during the intracellular accumulation of PHAs [44].

Multi stage continuous stirred tank reactor

In the case of multi stage CSTR, three or more bioreactors are connected with each other using plastic tubing. The schematic diagram of continuous stirred tank bioreactor is shown in Fig. 2c. PHAs can be produced with exponential growth rate under continuous growth conditions as used in two SCSTR. The culture condition can be used throughout the process until the desired cell biomass is achieved. The concentrated substrate was loaded to avoid any dilution of nutrients during continuous production condition in three SCSTR [45].

Factors affecting the production of PHAs

The bacterial production of PHAs through fermentation is influenced by different factors such as, strain used, growth substrate, C/N ratio, pH, DO and sodium chloride which are further elaborated in the following subsections.

Bacterial strains for PHAs production

More than 300 types of different Gram positive, Gram negative and archaea bacteria had been reported to produce PHAs. Different carbon source and CDM, PHAs yields and production kinetics are shown have been reported in Table 3.

Growth substrates

The carbon source is the most crucial and major factor during bacterial production of cellular metabolites like PHAs. Several types of carbon sources such as alkanes, polysaccharides, glycerides, acids, edible oils, gases, industrial waste, agricultural waste, carbohydrates, alcohols, petroleum products, animal waste, benzene and its derivatives had been investigated for PHAs production. The carbon source is used as basic food component to fulfil energy requirements and to act as biogenetic substrate for microbial growth and precursor for PHAs biosynthesis. In the above context, there are three basic functions for the carbon source within the bacterial cells such as cell maintenance, cell biomass synthesis and polymerization of PHAs molecules [46]. The small amount of growth substrate was used to restrict the molecular size of polymer during fermentation. Because bacteria have thick and rigid cell wall, it does not allow large polymeric molecule to be transported into cells. Different concentrations of glucose (as carbon source) had been used to optimize cellular growth and product yield. The results demonstrated a proportional increase up to a point after which glucose exhibited an inverse effect on microbial growth. The effect of growth substrates on PHAs yield is entirely based on nutrients media [47].

The selectivity of growth substrate not only depends upon their cost but also on their feasibility in the fermentation process. The selectivity of carbon source as growth substrate depends also on the bacterial strain used. The bacterial strain of P. aeruginosa shows compatibility for PHAs production with large number of growth substrates like oils, polysaccharide’s and acids [48].

Carbon sources as growth substrates generally belong to three main groups viz. triglycerol, sugars and hydrocarbons. The growth substrate used include carbohydrates such as sucrose [49], lactose [50], starch [51] and lignocellulose [52]. Other substrates such as ethyl alcohol [11], methanol [53], methane [54] and triglycerol containing compounds such as animal fat [55], plant oils [11], fatty acid [56], glycerol [57] and waste frying oil [58] had also been reported. Hydrocarbons had shown the highest affinity to produce PHAs including alkane, alkene, alkyne and a host of other hydrocarbons [59]. The use of fats, salts, ashes, whey as carbon sources had also been reported to have a large impact on PHAs yield hence decreasing the production costs up to 50%. Both culture conditions and the substrate have shown significant effects on both quality and quantity of the PHAs produced [60]. A number of different carbon sources with specific bacterial strains used in PHAs production are listed in Table 3.

Nitrogen sources and limitation

Several organic and inorganic nitrogen sources had been investigated for suitability and improvement of PHAs yield including different nitrogen sources such as amides [61], ammonium sulphate [26, 40], ammonium nitrate and ammonia nitrogen [40, 62], ammonium bicarbonate [63], ammonium carbonate, ammonium chloride [13], polyamide poly-γ-glutamate (PGA) [64], urea, nitrates [65] and sodium nitrate [27]. The amount of nitrogen sources (like organic and inorganic ones) had also shown linear relation with the bacterial growth, i.e., the number of bacterial cells increased with increasing nitrogen contents in the culture media. The PHAs stored inside bacterial cell and nitrogen source concentration had an inverse effect on cell biomass yield [66]. Lower concentration of nitrogen source showed better PHAs accumulation inside the bacterial cell, but higher values increased the biocatalyst activity to increase the number of bacterial cell in the fermentation media [38]. The effect of organic and inorganic nitrogen sources on PHAs production are listed in Table 5.

Table 5 Different organic and inorganic nitrogen sources for PHAs production
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Carbon to nitrogen ratio

The effect of C/N ratio on PHA yield had also been reported elsewhere [68]. The concept of C/N ratio originated from the biological law according to which limitation of carbon and nitrogen sources controls the molecular size of PHAs and the number of bacterial cells. A continuous increase in C/N ratio promotes the accumulation of bacterial cell mass with inverse effect on growth kinetics (yield with respect to time) of PHAs [11]. The depletion of nitrogen in nutrient media promotes the accumulation of PHAs. The highest percentage of PHAs within cell dry mass was produced as 47.22% at 35 C/N ratio with Haloferax mediterranei [13]. In another study using C. necator, it was reported that the C/N ratio caused a prominent effect on PHAs composition and accumulation where most of the cell mass formation occurred during first 12 h of incubation [69]. The effect of C/N ratio on PHAs is determined from production kinetics of PHAs cell mass [68]. Increasing the nitrogen concentration had a positive effect on growth rate, however, negative on PHA yield. During fermentative production of PHAs, the excess amount of carbon source is typically used with limited nitrogen source. When investigating growth-associated production of PHAs using A. lata, it was observed that PHAs cell biomass yield weren’t enough until sufficient amount of both carbon and nitrogen sources were provided. This C/N ratio generates high active biomass productivity with sufficient PHAs yield [13].

Effect of pH

The pH value of extracellular environment greatly affects the bacterial enzymatic activity. A fermentation medium at neutral pH produces more PHAs than both under acidic and basic environments. In the case of pure culture media, any fluctuation in neutral pH slows down the fermentative process, with a reduction in cellular activities of enzyme, i.e., Pha C and Pha Z, thereby affecting the growth rate, and ultimately the cell biomass. The overall, the effect of pH on PHAs production was totally depended on the composition of media. A general trend of fluctuations in pH value is usually observed for neutral fermentation media. After loading with a substrate, the pH of a fermentation medium increases in the early stages of incubation, and then according reinstates [70]. A similar effect of pH fluctuation had also been observed both in continuous and fed batch fermentations [71].

Dissolved oxygen demand

The gaseous requirement in most of the cells is atmospheric oxygen, which is essential for the bio-oxidative respiration process. Oxygen plays a vital role in adenosine triphosphate (ATP) formation and produces energy which is utilizable in vital cellular activities. The dissolved oxygen (DO) acts as carrier gas in the fermentation medium during PHAs production [72]. Under the anaerobic conditions, the bacterial cells which lack the enzyme CoA for the respiration. Both metabolic activity and the amount of DO were reported to increase during continuous feeding and pulse feeding fermentation as process proceeded to PHAs production. In an MMC media, the amount of DO had reverse effect on metabolic activity in the bacterial cells [73]. This reversal effect of both pH and DO was observed in MMC when pH of the fermentative media was less than 8 [74].

Concentration of phosphorus

Phosphorus in the fermentation media plays an important role in the synthesis of proteins, carbohydrate and fats with some additional roles in cells repair and maintenance of growth rate. The amount of phosphorus present in the nutrients enrich media also acts as buffer to resist any change in neutral pH throughout the fermentation process. Various concentrations of phosphorus had been used to optimize the fermentation condition with respect to product yield. The optimized concentration of phosphorus should be used because at higher concentration a reverse effect on product yield had been reported [75, 76].

Production with PHAs precursor addition

The PHA co-polyesters are biosynthesized via metabolic pathways using a variety of monomers as precursors. Over 150 different types of precursors have been reported for PHA co-polyesters formation. However, most of them were expensive, which limit their utilization at industrial scale. The chemical nature and concentration of these precursors have to be controlled carefully to avoid any inhibitions, toxicity and to ensure the formation desired product. For instance, butyrate, citrate and acetate species can act as precursors for PHAs production. Likewise, R. eutropha showed compatibility with specific precursors of propionate for poly-3-hydroxybutyrate-co-3-hydroxyvalerate, and ϒ-butyrolactone, 1,4-butanediol and 4-hydroxybutyrate for poly(3-hydroxybutyrate-co-4-hydroxybutyrate). Fatty acids with odd number carbon atoms have also been used as precursors in PHA co-polymers production [77]. Different organic acids such as propionic acid and alcohols like methanol had also been used as precursors in the biosynthesis of PHA copolymers. For example, the late addition of propionic acid to the fermentation media while using glucose as main substrate produced poly-3-hydroxybutyrate-co-3-hydroxyvalerate as co-polyester. Lignocellulosic carbon sources mainly present in the liquefied wood have also been used as precursor in PHAs production [78]. The effect of these nutrients on PHA production was studied individually through optimization. The addition of these precursors increased the PHAs yield 30–35% on using butyrate, citrate and acetate as precursors [8]. The addition of precursor facilitates the formation of co-polymer PHAs with remarkable properties which allow using them in different field of life. A co-polymer of PHAs showed different properties from the parent polymers in different aspect like drug delivery, durability, and many others [79].

Addition of sodium chloride

The external stress factors such as increased temperature or ionic species (NaCl) cause a stress response leading to enhanced PHAs production. The amount of NaCl as 9 g/L was added additionally into nutrient enriched media. The NaCl had been used as a cheap, non-toxic, sustainable, simple and nonreactive external stress factor which had been reported to increase PHAs productivity up to 30%(w/w) with C. necator. The results also demonstrated that the nutrients media supplemented with NaCl produced 6% (w/w) more cell biomass as compare to negative control. The product yield increased at a rate of 65–77% (w/w) in the presence of NaCl. This might be attributed to lower DO in the media at higher salinity stress [80].

Production of PHAs using gases

In recent years, PHAs had been produced from industrial by-product gases such as methane CH4 or CO2. These greenhouse gases can be fixed and used as carbon sources with additional advantages of cost-effectiveness and sustainability [81]. A pretreatment usually assists the utilization of these gases as carbon source to produce PHAs. Continuous fed batch production process also accounts for PHAs production using gaseous carbon sources, CH4 (by a methanotrophic bacterial strain) and CO2 (autotrophs bacteria) [82]. In the case of Cyanobacteria sp., the availability of substrate to bacteria is limited by the solubility of these greenhouse gases, in the aqueous fermentation media. Both the solubility and availability of substrate under these conditions are influenced by parameters such as pH value, size of gas bubbles, and the temperature. A sufficient supply of these gaseous growth substrates to cell biomass is possible in a continuous mode [83]. The plants are natural sources to fix the anthropogenic gases like CO2. In recent days different approaches have been considered to fix them using metal organic framework (MOF).

Downstream recovery processes

The downstream recovery process (DsRP) should be an economical eco-friendly process in which toxic solvents are not used [11]. In DsRP, organic solvent such as methylene, chloroform, carbonate, propylene and dichloroethane are used for ultimately degrading the cell wall and cell membrane to recover the PHAs [67]. Polymers recovery methods also contribute to production costs of polymers produced. The main purpose of DsRP is to minimize production cost, increase the purity of product, obtain greater yield, avoid cell disruption and minimize the use of toxic solvent. In the case of DsRP, different parameters like purity of product, substrate used and properties required must be taken into consideration before a PHAs recovery [84]. A solvent free recovery method (like sodium hypochlorite recovery method) could also be used to recover PHAs by simple dissolving the non-PHAs cellular mass; however, this method reduced the molecular mass of polymer. The polymers used in medical field must be of high purity with no impurities [85, 86]. Some downstream recovery processes and their advantage in purity and recovery of polymer are listed in Table 6.

Table 6 PHAs isolation methods and their advantages
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Future trends

In the early stages, PHAs production had been limited due to high costs which restricted their production. Researchers, therefore, had always been looking for low cost substrates to reduce overall production cost. In the recent years, large number of cheaper carbon sources such as waste frying oil, vegetable oil, whey and organic substrate etc., have been investigated to minimize the production cost. A mutant strain of P. aeruginosa produced high yields of PHAs using cheap carbon sources. The strain showed good synthesis of PHAs in preliminary investigation. The future development in PHAs production would be based on two factors; (a) lower production cost with higher yield (b) wider applications in different fields. Synthetic biology and genetic engineering techniques are expected to produce higher yield PHAs strain with high growing density under optimizing fermentation conditions in short period of time. The purification techniques with controllable lysis can be used to accumulate large size granules and reduce the cost of production. Continuous fermentation with MMC can be used to minimize the production cost operated in two SCSTR and three SCSTR configurations using cheap substrates to obtain higher yield. Different functional groups are attached to the side chain of basic structure of PHAs to give them different properties. The molecular evolution technique should be used to produce the expected structure product of short chain length or medium chain length PHAs. Downstream processing is needed to obtain the desired purity of product for specific application in the mechanical sector and many other fields of science and engineering. The mutant bacterial strain P. aeruginosa was used with soybean oil always produces mcl-PHA like P(3-HB). P. aeruginosa synthase has a good ability to produce mcl-PHA monomers efficiently utilizing vegetable oil as growth substrate to produce PHA with low production of by-products. Moreover, there is need to work on the production of desirable congeners of PHAs under controlled fermentation and to elucidate the physico-chemical and biological properties of each individual congener.

Conclusions

The present review exemplifies a number of different bioreactors (fed batch, continuous fed batch and multiple stages continuous stirring reactors) systems and bacterial strains with large number of feeding regimes to produce PHAs of different molecular mass. It became clear that a number of different combinations of bacterial strain and substrates demand different fermentation schemes, bioreactor facilities and feeding regimes. The cultivation time and process design have to be accommodated to the physiological kinetics of the system to optimize the final cell biomass productivity and product quality. As a concluding result of recent studies or experiments, it could be expected that future PHAs production would be combined with continuous stirring method. Robust extremophilic bacterial strain would be used to produce PHAs with minimum energy input. The utilization of cheap carbon enriched feed stocks should be applied to minimize substrate cost and develop sustainable DsRP method for recovery and purification of PHAs. Only the combination of these different techniques would allow for economic and sustainable production of PHAs biopolyesters. A downstream recovery process allows us to obtain PHAs with high purity to be used in for medical application.