Valorization of Brewery Waste through Polyhydroxyalkanoates Production Supported by a Metabolic Specialized Microbiome

Abstract

Raw brewers' spent grain (BSG), a by-product of beer production and produced at a large scale, presents a composition that has been shown to have potential as feedstock for several biological processes, such as polyhydroxyalkanoates (PHAs) production. Although the high interest in the PHA production from waste, the bioconversion of BSG into PHA using microbial mixed cultures (MMC) has not yet been explored. This study explored the feasibility to produce PHA from BSG through the enrichment of a mixed microbial culture in PHA-storing organisms. The increase in organic loading rate (OLR) was shown to have only a slight influence on the process performance, although a high selectivity in PHA-storing microorganisms accumulation was reached. The culture was enriched on various PHA-storing microorganisms, such as bacteria belonging to the Meganema, Carnobacterium, Leucobacter, and Paracocccus genera. The enrichment process led to specialization of the microbiome, but the high diversity in PHA-storing microorganisms could have contributed to the process stability and efficiency, allowing for achieving a maximum PHA content of 35.2 ± 5.5 wt.% (VSS basis) and a yield of 0.61 ± 0.09 Cmmol_PHA/Cmmol_VFA in the accumulation assays. Overall, the production of PHA from fermented BSG is a feasible process confirming the valorization potential of the feedstock through the production of added-value products.

Keywords: biopolymers; circular economy; microbial community; waste valorization.

Figure 1

Three-stage PHA production set-up: this study focused on MMC enrichment (SBR) and PHA production (batch reactor) (highlighted by the dashed line); the production of the fBSG was published by Teixeira et al. [12].

Figure 2

Nile blue images obtained from the observation of the reactor biomass samples of the selected culture collected under the highest OLR applied. The white granules correspond to PHA granules.

Figure 3

Typical profile of the accumulation assays performed with the culture selected at OLR 48.6 Cmmol/(L·d). The six pulse periods are limited by the vertical dashed lines.

Figure 4

Principal component analysis of the 16S rRNA gene sequencing data at the OTU level from days 0, 21, 49, and 63, where different OLRs were applied. PC1 and PC2 explain 57.6% and 26.9% of the total variance, respectively.

Figure 5

Taxa relative abundance at class (A) and family (B) levels on the different reactor operational days. Minor classes and minor families include classes and families for which the relative abundances are lower than 1% for all of the sampling days.

Figure 6

Heatmap of the core microbiome taxa with relative abundance >0.1% across reactor operation. The genus or the lowest assigned taxonomic level of each taxon is presented in the heat map.

Figure 7

Distribution of the taxa, at a genus level, with a relative abundance higher than 1% on day 63, and their dynamics on the previous days.