The wine industry generates approximately 20 million tonnes of winery by-products (WBPs) annually, representing a significant environmental challenge whilst simultaneously offering substantial opportunities for valorisation. Dias-Costa et al. (2025) provide a comprehensive review of phenolic compound extraction from WBPs, critically evaluating conventional methodologies in comparison with emerging unconventional approaches.
The analysis reveals that whilst traditional methods such as solid-liquid extraction remain widely utilised, innovative approaches including ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and pressurised solvent extraction (PSE) demonstrate superior efficiency, reduced environmental impact, and enhanced bioactive compound recovery.
These findings have significant implications for the development of sustainable extraction protocols and the commercial exploitation of grape pomace, grape stems, wine lees, and vine pruning woods as sources of high-value antioxidants for food, cosmetic, and pharmaceutical applications.
Introduction
The global wine production industry faces mounting pressure to address the substantial volumes of organic waste generated during vinification processes. Approximately 30% of the total volume of vinified grapes comprises winery by-products, totalling nearly 20 million tonnes annually, with 50% attributed to the European Union alone (Dias-Costa et al., 2023). These by-products include grape stems, grape pomace (seeds, skins, and pulps), wine lees, and vine pruning woods, materials that have traditionally been regarded as waste requiring disposal through landfilling, landfarming, or composting (Teixeira et al., 2014).
However, contemporary research has revealed that these WBPs represent a valuable reservoir of natural antioxidants, particularly polyphenolic compounds including hydroxybenzoic acids, hydroxycinnamic acids, stilbenes, flavonols, flavan-3-ols, flavones, flavanones, flavanonols, proanthocyanidins, and anthocyanins (Goufo et al., 2020; Fernandes et al., 2025). The phenolic content of these materials varies significantly depending upon grape variety, geographical origin, climatic conditions, and processing methods, necessitating optimised extraction protocols to maximise bioactive compound recovery whilst maintaining sustainability principles.
The extraction of phenolic compounds from plant matrices represents a complex challenge requiring careful consideration of numerous parameters including solvent selection, temperature, extraction time, particle size, and solvent-to-solid ratios. Traditional conventional extraction methods have dominated the field for decades, relying primarily on organic solvents and extended processing times. However, growing environmental concerns and the pursuit of enhanced efficiency have driven the development of unconventional extraction techniques that promise improved yields, reduced environmental impact, and superior preservation of thermolabile compounds.
Methodology
Conventional extraction methodologies
Conventional solid-liquid extraction (SLE) encompasses a range of established techniques characterised by straightforward maceration processes conducted at atmospheric pressure within temperature ranges from ambient conditions to the boiling point of the extraction solvent under reflux conditions (Lama-Muñoz & Contreras, 2022). These methods include Soxhlet extraction, liquid-liquid extraction (LLE), and various maceration techniques.
The selection of appropriate extraction solvents represents a critical consideration in conventional methodologies. Traditionally, organic solvents such as methanol, ethyl acetate, and acetone have been employed due to their outstanding extraction capabilities for phenolic compounds. However, concerns regarding their inherent health hazards, environmental impact, and regulatory restrictions have prompted increased interest in eco-friendly alternatives including ethanol, water, and their mixtures (Alara et al., 2021).
Extraction parameters significantly influence the outcomes of conventional methods. These include the polarity of extraction solvents, the number and positioning of hydroxyl groups in target compounds, molecular size, extraction temperature and time, particle size, interactions with other food components, pH, and light exposure effects. The solvent-to-solid ratio typically ranges from 1:10 to 1:125 (w/v), with extraction times varying from 1 hour to 72 hours depending upon the specific methodology employed.
Unconventional extraction methodologies
Unconventional extraction methods represent a paradigm shift towards green chemistry principles and enhanced process efficiency. These techniques harness various forms of energy to facilitate the transfer of phenolic compounds into the extraction solvent, thereby providing numerous advantages over conventional approaches including reduced solvent consumption, decreased energy requirements, and shortened processing times.
Ultrasound-assisted extraction (UAE) utilises the principle of acoustic cavitation, involving the generation of mechanical energy through cycles of compression and rarefaction transmitted via ultrasonic waves. This process generates nanobubbles that, upon collapse, rupture plant cell walls and enable enhanced solvent penetration, resulting in improved bioactive compound transfer rates (Irakli et al., 2023).
Microwave-assisted extraction (MAE) employs microwave energy to disrupt hydrogen bonds of polar molecules within the extraction system, rapidly rotating them via ion conduction or dipole-dipole rotation. This technique is particularly recommended for extracting short-chain polyphenols such as flavonoids and phenolic acids from plant materials, offering advantages including simplicity, low consumption of both solvent and energy, and short extraction times,.
Pressurised solvent extraction (PSE), also termed Pressurised Liquid Extraction (PLE) or Accelerated Solvent Extraction (ASE), employs pressurised extraction chambers (10-15 MPa) to maintain solvents in liquid state at temperatures exceeding their boiling points (up to 200°C) under elevated pressures. This methodology enhances extraction efficiency through improved mass transfer and compound solubility.
Additional unconventional techniques include Enzyme Assisted Extraction (EAE), utilising cellulases, hemicelluloses, and pectinases to catalyse polysaccharide hydrolysis in cell walls; High Hydrostatic Pressure (HHP), applying pressures of 100-1000 MPa to damage cell membranes; High Voltage Electrical Discharges (HVED), employing electrical breakdown phenomena; Pulsed Electric Fields (PEF), creating temporary or permanent pores in cell membranes; and Surfactant Mediated Extraction (SME), utilising surfactants to enhance solubilisation of target compounds.
Results and discussion
Comparative efficacy of extraction methods
The comprehensive analysis of extraction methodologies reveals significant differences in efficiency, environmental impact, and compound selectivity between conventional and unconventional approaches. Grape pomace emerges as the most extensively studied WBP, with numerous investigations comparing different extraction methods, whilst grape stems remain relatively underexplored, particularly regarding unconventional extraction applications.
Studies examining white grape varieties demonstrate consistent patterns in phenolic compound recovery. Conventional extraction using methanol:water mixtures (70:30, v/v) with solvent ratios of 1:125 (w/v) represents the most frequently employed methodology across varieties. However, investigations utilising response surface methodology (RSM) for optimisation reveal that ethanol:water combinations often provide superior results whilst offering enhanced sustainability profiles.
Red grape varieties exhibit similar extraction patterns, though with generally higher phenolic compound concentrations compared to white varieties. The presence of anthocyanins in red grape materials necessitates modified extraction protocols, often requiring acidified solvents or controlled pH conditions to maintain compound stability.
Unconventional method performance
Ultrasound-assisted extraction demonstrates particular promise for grape stem processing, with studies reporting optimal conditions of ethanol:water (50:50, v/v) at extraction temperatures of 5-65°C, ultrasound amplitudes of 30-70%, and processing times of 5-15 minutes. These conditions achieve phenolic compound yields comparable to or exceeding conventional methods whilst significantly reducing solvent consumption and extraction times.
Microwave-assisted extraction shows exceptional efficacy for grape pomace and wine lees processing. Optimal conditions typically involve ethanol:water mixtures (48:52 to 80:20, v/v) with irradiation powers of 140-300 W and extraction times of 5-17 minutes. Studies report yield increases of up to 57% for polyphenols and 85% for anthocyanins compared to conventional maceration methods (Álvarez et al., 2017).
Pressurised solvent extraction demonstrates superior performance for vine pruning woods, achieving higher flavonoid content compared to both conventional extraction and other unconventional methods. Optimal conditions involve ethanol:water mixtures at elevated temperatures (120-180°C) and pressures (1500 psi), with extraction times reduced to 1-11 minutes.
Environmental and economic considerations
The adoption of unconventional extraction methodologies offers substantial environmental benefits through reduced solvent consumption, decreased energy requirements, and minimised processing times. Life cycle assessments indicate that methods such as UAE and MAE can reduce environmental impact by 30-50% compared to conventional approaches whilst maintaining or improving extraction yields.
Economic analyses reveal that whilst unconventional methods require higher initial capital investment for specialised equipment, operational costs are significantly reduced through decreased solvent usage, energy consumption, and processing times. Return on investment (ROI) calculations suggest payback periods of 2-4 years for commercial-scale operations, with profit margins enhanced by improved product quality and market positioning.
Challenges and limitations
Despite the promising results, several challenges persist in the implementation of unconventional extraction methods. Equipment costs remain prohibitive for smaller operations, whilst technical expertise requirements may limit adoption rates. Scale-up considerations present additional challenges, as optimal conditions determined at laboratory scale may require modification for industrial applications.
Compound degradation represents a significant concern, particularly for thermolabile phenolic compounds sensitive to elevated temperatures or prolonged exposure to ultrasonic radiation. Process optimisation requires careful balance between extraction efficiency and compound preservation, necessitating comprehensive understanding of degradation kinetics and stability parameters.
Regulatory considerations present additional challenges, particularly for food and pharmaceutical applications where solvent residues and processing aids must comply with strict safety standards. Green solvents such as ethanol and water offer advantages in this regard, and our FARE (Food and Agriculture Requirements) unit helps innovative companies tackle these regulatory hurdles and bring products to market faster.
Major outcomes and implications
Commercial viability
The research demonstrates clear commercial viability for WBP valorisation through optimised phenolic compound extraction. Market analysis indicates growing demand for natural antioxidants in food, cosmetic, and pharmaceutical industries, with grape-derived compounds commanding premium prices due to their established safety profiles and bioactivity.
Product development opportunities extend beyond simple extract production to include encapsulated compounds, standardised preparations, and functional ingredients for specific applications. Intellectual property considerations suggest substantial opportunities for patent protection of novel extraction protocols and product formulations.
Sustainability impact
The implementation of unconventional extraction methods aligns with circular economy principles and sustainable development goals (SDG23). Waste stream utilisation transforms environmental liabilities into value-added products, whilst reduced resource consumption minimises ecological footprints.
Carbon footprint analyses indicate potential reductions of 40-60% compared to conventional processing, primarily through decreased energy consumption and solvent requirements. Water usage reductions of 30-50% are achievable through solvent recycling and closed-loop systems.
Future research directions
Emerging technologies including supercritical fluid extraction, ionic liquid-based extraction, and deep eutectic solvents present opportunities for further improvements in extraction efficiency and environmental performance. Hybrid approaches combining multiple unconventional methods show particular promise for maximising compound recovery whilst minimising processing costs.
Biorefinery concepts integrating phenolic compound extraction with bioethanol production, biogas generation, and organic acid recovery offer opportunities for comprehensive WBP utilisation. Systems integration approaches may enable cost-effective processing through shared infrastructure and energy integration.
Conclusions
This comprehensive analysis of polyphenol extraction from winery by-products reveals the substantial potential of unconventional extraction methodologies to revolutionise the upcycling of wine industry waste streams. Ultrasound-assisted extraction, microwave-assisted extraction, and pressurised solvent extraction demonstrate superior performance compared to conventional methods across multiple metrics including extraction efficiency, environmental impact, energy consumption, and processing time.
The commercial viability of WBP valorisation is clearly established, with market opportunities spanning food, cosmetic, and pharmaceutical industries. Grape pomace emerges as the most promising feedstock due to its abundance and high phenolic content, though grape stems, wine lees, and vine pruning woods offer complementary opportunities for comprehensive by-product utilisation.
Environmental benefits associated with unconventional extraction methods align with sustainability objectives and circular economy principles, whilst economic analyses demonstrate favourable return on investment profiles for commercial implementations. However, challenges including equipment costs, technical expertise requirements, and scale-up considerations necessitate continued research and development efforts.
Future research priorities should focus on process optimisation, equipment design, product standardisation, and regulatory compliance to facilitate widespread adoption of these promising technologies. Integrated biorefinery approaches combining multiple valorisation strategies offer the greatest potential for maximising economic value whilst minimising environmental impact.
The transition from waste disposal to value creation through optimised phenolic compound extraction represents a paradigmatic shift that can simultaneously address environmental challenges and create economic opportunities for the wine industry. The successful implementation of these technologies will require collaboration between researchers, industry stakeholders, and regulatory bodies to overcome existing barriers and realise the full potential of winery by-product valorisation.
Dario Dongo
Cover art copyright © 2025 Dario Dongo (AI-assisted creation)
References
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Dario Dongo, lawyer and journalist, PhD in international food law, founder of WIISE (FARE - GIFT - Food Times) and Égalité.
