Biotechnological exploitation of expired fruit juices for combined biohydrogen and polyhydroxyalkanoates production
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Date
2025-10
Authors
Γκαμπράνα, Μυρτώ
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Abstract
The increasing demand for renewable energy and biodegradable materials, combined with the urgent need to reduce food waste, creates an urgent need for developing sustainable strategies for converting organic residues into valuable products within the framework of a circular economy. In this context, expired fruit juices (EFJ) represent a carbon-rich and readily biodegradable feedstock, rendering them highly suitable substrates for microbial bioconversion. The present thesis sets out to evaluate and experimentally substantiate the potential of EFJ to be repurposed through a two-stage biotechnological process for the sequential production of biohydrogen and polyhydroxyalkanoates (PHAs). This integrated approach aims to bridge academic research with practical applications, supporting environmentally conscious innovations in waste management and resource recovery.
In the first stage, EFJ were subjected to dark fermentation (DF) in an up-flow packed bed column reactor (UFCR) operated at mesophilic conditions (35 °C) and a hydraulic retention time (HRT) of 24 h. The influence of different organic loading rates (OLR 20, 30, and 40 g sugars/L·d) on hydrogen production and metabolite distribution was systematically investigated. Results showed that moderate loading rates provided the most balanced performance, with OLR30 achieving the highest hydrogen production (0.67 ± 0.04 mol H2/ mol glucose and 10.83 ± 0.41 m³ H₂/m³ EFJ) and a VFA profile favorable for downstream polymer biosynthesis. In contrast, higher OLRs led to substrate accumulation, metabolic stress, and increased lactate formation, reducing hydrogen output and highlighting the importance of carefully controlling substrate availability in dark fermentation systems.
In the second stage, the dark fermentation effluents (DFEs) were tested as substrates polyhydroxyalkanoates (PHAs) biosynthesis under nitrogen-limited batch conditions using Zoogloea-enriched mixed microbial cultures. The experiments revealed a two-phase microbial response: an initial growth phase followed by a polymer storage phase. Among the tested effluents, DFE1 (from OLR20) enabled the highest PHA yield (49.90 ± 1.11 g PHAs / 100g DCW and 20.69 ± 1.01 kg PHAs/m³ EFJ), owing to its well-balanced VFA composition dominated by acetate, butyrate, and caproate, and near-complete sugar depletion. Effluents with higher lactate content, such as DFE3, were less suitable for efficient polymer accumulation. Copolymers containing hydroxybutyrate (HB) and hydroxyvalerate (HV) were obtained, with monomer composition linked to the VFA distribution of each effluent.
The diploma thesis demonstrates that EFJ can be effectively valorized in sequential bioprocess, producing renewable energy in the form of hydrogen while simultaneously generating biodegradable polymers. This integrated approach highlights the potential of waste-derived feedstocks for sustainable biorefineries, offering a practical strategy to reduce food waste, recover energy, and create high-value bioproducts.
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Keywords
Dark fermentation, Biohydrogen, Polyhydroxyalkanoates (PHAs), Food waste, Circular economy