Current trends in medium-chain-length polyhydroxyalkanoates: Microbial production, purification, and characterization

Abstract

Polyhydroxyalkanoates (PHAs) have gained interest recently due to their biodegradability and versatility. In particular, the chemical compositions of medium-chain-length (mcl)-PHAs are highly diverse, comprising different monomers containing 6-14 carbon atoms. This review summarizes different feedstocks and fermentation strategies to enhance mcl-PHA production and briefly discusses the downstream processing. This review also provides comprehensive details on analytical tools for determining the composition and properties of mcl-PHA. Moreover, this study provides novel information by statistically analyzing the data collected from several reports on mcl-PHA to determine the optimal fermentation parameters (specific growth rate, PHA productivity, and PHA yield from various structurally related and unrelated substrates), mcl-PHA composition, molecular weight (MW), and thermal and mechanical properties, in addition to other relevant statistical values. The analysis revealed that the median PHA productivity observed in the fed-batch feeding strategy was 0.4 g L^-1 h^-1, which is eight times higher than that obtained from batch feeding (0.05 g L^-1 h^-1). Furthermore, 3-hydroxyoctanoate and -decanoate were the primary monomers incorporated into mcl-PHA. The investigation also determined the median glass transition temperature (-43°C) and melting temperature (47°C), which indicated that mcl-PHA is a flexible amorphous polymer at room temperature with a median MW of 104 kDa. However, information on the monomer composition or heterogeneity and the associated physical and mechanical data of mcl-PHAs is inadequate. Based on their mechanical values, the mcl-PHAs can be classified as semi-crystalline polymers (median crystallinity 23%) with rubber-like properties and a median elongation at break of 385%. However, due to the limited mechanical data available for mcl-PHAs with known monomer composition, identifying suitable processing tools and applications to develop mcl-PHAs further is challenging.

Keywords: biopolymer; downstream processing; medium‐chain‐length; polyhydroxyalkanoate; thermal and mechanical properties.

FIGURE 1

Overview of publications and citations per year from the WoS databases (©Clarivate 2023). Search terminology “medium‐chain‐length” and “polyhydroxyalkanoate*” in 1990−2022. The bar graph represents publications per year, and the line represents the number of citations on these articles.

FIGURE 2

Classification of PHAs based on the chain length and diversity of the monomers, mcl = medium‐chain‐length, scl = short chain length (adapted from [6]).

FIGURE 3

Metabolic processing of structurally related or unrelated substrates to mcl‐PHA (adapted from [9, 10]). ACP, acyl carrier protein; ACS, acyl CoA‐synthase.

FIGURE 4

Flow scheme focusing on different unit operations of mcl‐PHA downstream processing. The most relevant purification step, solvent extraction, is highlighted.

FIGURE 5

mcl‐PHA structure and diversity of its monomers.

FIGURE 6

¹H‐ (A) and ¹³C‐NMR spectra (B) of mcl‐PHA mainly comprise 3HD. The numbers in the spectra assign the specific peaks of the C atoms themselves or the protons linked to them with the structure shown in Figure 5.

FIGURE 7

Schematic of a stress–strain curve with estimated median values for Young's modulus and elongation at break.