Polyhydroxyalkanoates (PHAs) are one of the most interesting biopolymers due to their mechanical properties and biodegradability. Their chemomechanical properties have been extensively discussed [
1], but the same attention has not been dedicated to biodegradability aspects. The marketed product derived from pure strain; its biodegradation has been related to those microorganisms that could secrete extracellular PHA depolymerase even though other factors such as the environmental condition and/or PHA properties (e.g., composition, crystallinity, additives) could affect the PHA degradation [
2]. As an example, the effect of chemical structure on PHA biodegradability has been investigated by Weng et al. [
3]: according to ISO 14855-1, the controlled composting conditions showed that higher 3-hydroxyvalerate (3HV) content corresponded to higher biodegradability. In the form of composites (together with waste wood sawdust fibers), the ASTM D5338-98 and ISO 20200-2004 standard methods revealed that the presence of fibers favored the physical disintegration of PHA, increasing the biodegradation rate of the polymeric matrix [
4]. To the best knowledge of the authors, there are no biodegradability data in the literature for PHA synthetized by mixed microbial culture (MMC). The approach of MMC technology consists in the exploitation of organic wastes as feedstock [
5,
6]. In addition, MMC technology involves the use of waste-activated sludge (always available in municipal wastewater treatment plants, WWTPs) as inoculum that needs to be enriched in microorganisms with high polymer accumulation capacity. PHA-storing organisms can grow by applying aerobic dynamic feeding conditions (ADF) through the well-known feast-famine regime [
7]. In the last decade, MMC-PHA production using different types of waste has been the topic of several applied researches; moreover, recent investigation has been conducted at pilot scale to demonstrate the technical feasibility of the technology [
8,
9]. In particular, PHA production from the organic fraction of municipal solid waste (OFMSW) has been assessed in order to quantify the process PHA productivity (g PHA/L day) potentially achievable and final polymer quality, also in consideration of a possible integration in the existing wastewater WWTP [
10‐
12]. These are fundamental aspects for MMC-derived PHA commercialization and relative market scenario, which do not exist yet. In the view of a larger-scale MMC-based process, PHA recovery and, more in general, the whole downstream processing from PHA-rich biomass stabilization to PHA extraction are still the bottlenecks of the entire production chain. A recent review summarizes the sustainable approaches and latest developments for the downstream processing [
13], emphasizing the environmental unsuitability of chlorinated solvents. With particular attention for the MMC PHA-rich biomass, several non-chlorinated solvents have been firstly exploited [
14] together with a biomass stabilization protocol, where a temperature range of 120–160 °C was adopted in order to preserve the polymer quality. The latter might be also improved by optimization of time, temperature, and concentrations of reagents in the purification steps as a function of PHA composition [
15]. Moreover, PHA content in the biomass at the end of the accumulation step has to be preserved to maximize the efficiency and the productivity of the process. In addition, recovery methods need to be flexible towards conditions that may be readily tuned to a range of polymer types [
14]. For application and realization of bioplastic products, it is necessary to obtain a polymer with specific chemical, physical, and mechanical properties. The mechanical properties of PHA are good when the average molecular weight (
Mw) is higher than 400 kDa, and specifically thermoplastic applications may require
Mw higher than 600 kDa. Molecular weights typically range between 200 and 3000 kDa, and the distribution can be affected by the method of accumulating PHA in the biomass, the method for PHA recovering, and PHA processing into end-user products [
15]. The few studies on chemomechanical and thermal characterization of MMC-PHA have been reviewed by Laycock and coworkers [
15]. It has been noted that there is often a higher melting temperature (
Tm) and a lower enthalpy of melting for MMC-PHA than those of pure culture-derived PHA [
16]. This is probably due to the formation of block copolymers or polymer blends which in turn is a consequence of the feeding strategies adopted in MMC systems. The pulse feeding causes frequent variability in substrate concentrations that leads to a block formation depending on the substrate availability (e.g., 3-hydroxyvalerate synthesis in the presence of propionate) [
17]. Thermogravimetric analysis on MMC-PHA samples showed a similar thermal stability (with decomposition temperatures of about 270 °C) to the commercial poly-3-hydroxybutirate [P(3HB)] and poly-3-hydroxybutyrate-co-3-hydroxyvalerate [P(3HB-co-3HV) (10 mol% HV)] [
18]. Besides, the crystallization rate slows down as 3HV content increases [
1]. Arcos-Hernández et al. have conducted a study in which PHA produced with different 3HV content were characterized, evaluating the possible effects on polymer properties [
19]. In terms of mechanical properties, it has been demonstrated that the MMC-PHA Young’s modulus (779–2893 MPa) increased significantly when the 3HV content was lower than 40 mol%; furthermore, the elongation to break of all but one of the samples was in the range of 3–6% which is similar to that expected for P(3HB) (brittle and stiff) [
19]. In general, the progressive incorporation of 3HV in PHA from pure culture, mainly referred to commercial polymer, brings about a constant increase of
εb from about 2.5 mol% at 3HB equal to 100 mol%, to 44 mol% at 3HB equal to 60 mol%. The elongation to break of samples obtained from MMC remains constant to about 3HB 65 mol% (
εb ≈ 3%) and shows a small increase from 54 to 28 mol% 3HB (ε
b ≈ 5%) [
20].