The global wine industry, despite facing a declining trend in production and consumption, remains a significant generator of organic waste. In 2023 alone, 237.3 million hectoliters of wine were produced worldwide, with the European Union contributing 144.5 million hectoliters. This massive production volume results in substantial quantities of by-products, including grape stalks, wine lees, winery waste-activated sludge, and the primary residual biomass: grape pomace.
Grape pomace, consisting of skins, seeds, and occasionally stems left after grape pressing, accounts for 15 to 25% of the total processed grape weight. Based on 2023 production figures, this translates to approximately 4.7 to 7.9 million tonnes of grape pomace generated globally each year. For decades, this material presented both technical and financial challenges for disposal, with untreated residues posing serious environmental risks due to their high organic content and potential for pollution.
Fresh grape pomace is a complex, heterogeneous bio-waste with a moisture content ranging from 50% to 72%. Its composition includes approximately 425 kg of grape skins, 225 kg of grape seeds, and 249 kg of grape stalks per tonne, along with significant amounts of lignin (16.8-24.2%), pectic substances (~20%), soluble sugars (1.5-6.2%), proteins (2.7-3.8%), phenolic compounds (5-10%), and minerals (1.8-4.6%).
The dealcoholization process transforms this raw pomace into a more stable, versatile material. For fermented pomace with low residual sugar (typically from red wine production), steam stripping distillation recovers ethanol, resulting in a hydroalcoholic solution that undergoes redistillation to reach 52–80% alcohol by volume. For pomace rich in unfermented sugars (from white or fortified wine production), a countercurrent leaching process precedes fermentation and distillation.
After dealcoholization, the resulting material undergoes partial dehydration by pressing and drying in rotary furnaces to a moisture content of approximately 5%. This stabilization allows for sieving to separate grape skins from seeds, with the seeds often used for oil extraction, animal feed, or as biomass fuel. The remaining seedless material, known as spent seedless grape pomace (SSGP), forms the basis for innovative material applications.
The processing of grape pomace into SSGP results in significant physicochemical transformations. Compared to fresh pomace, SSGP exhibits drastically reduced moisture (3-7%) and complete removal of soluble sugars. Phenolic compounds are substantially reduced from 5–10% to 0.5–0.6%, while lignin content increases sharply to 27-56% due to the concentration effect of removing moisture and soluble components.
Protein levels remain relatively consistent at 2.8-3.0%, while mineral content broadens to 1.5-10%, potentially exceeding the upper limits found in fresh pomace. Notably, pectic substances, prominent in fresh pomace at ~20%, are not reported in SSGP, suggesting their removal or significant alteration during processing. These changes—particularly the high lignin content and reduced moisture—contribute to SSGP's suitability as a material feedstock, providing structural integrity while maintaining biodegradable properties.
Effective utilization of dealcoholized grape pomace (DGP) and SSGP requires specialized pretreatment to unlock their material potential.
Mechanical pretreatments form the foundation, beginning with drying to prevent microbial spoilage and concentrate components. Methods such as convective air drying, freeze-drying, and microwave-assisted drying each offer trade-offs in energy consumption and preservation of thermolabile compounds. Milling or grinding follows, reducing particle size to increase surface area, directly enhancing extraction efficiency and fiber accessibility.
Chemical pretreatments primarily involve solvent extraction, with ethanol-water mixtures favored for their Generally Recognized as Safe (GRAS) status and broad extraction capabilities. Emerging green solvents, such as deep eutectic solvents (DES), show promise for their environmental benefits and efficacy. For fiber modification, mild acid or alkaline treatments can hydrolyze hemicellulose or delignify the material, improving cellulose accessibility while requiring careful control to preserve valuable components.
Enzymatic pretreatments provide targeted deconstruction of plant cell walls using cellulases, hemicellulases, pectinases, and tannases. These enzymes catalyze hydrolysis of specific polysaccharides, enhancing release of polyphenols and increasing soluble dietary fiber fractions. Enzyme-assisted extraction often yields higher polyphenol concentrations under gentler conditions, preserving bioactivity while improving functional properties of dietary fibers such as water-holding capacity.
Dealcoholized grape pomace has demonstrated significant potential as an enhancer or filler in bio-based materials, with studies consistently reporting improvements in thermal stability, heat resistance, and overall performance. When incorporated into poly(3-hydroxybutyrate) (PHB), a biodegradable polymer with limited mechanical performance, grape pomace extract (GPE) and lignocellulosic biomass from pomace improved thermal resistance, reduced degradation during processing, and preserved molecular weight. The lignocellulosic components functioned as heterogeneous nucleating agents, promoting crystallization and mitigating physical aging.
In poly(butylene succinate) (BioPBS) composites, DGP additions of up to 50 wt% significantly improved mechanical properties. A 57:40:3 BioPBS/GP/MA-g-BioPBS blend achieved 28.4% higher flexural strength and 59% higher impact strength compared to neat BioPBS, with a 14.3% increase in heat distortion temperature. Scanning electron microscopy confirmed improved interfacial adhesion in compatibilized samples, while thermogravimetric analysis verified thermal stability under processing conditions.
Bioactive laminated films incorporating GPE have shown particular promise for sustainable food packaging. Bacterial cellulose and chitosan films enriched with GPE as an antioxidant and glycerol as a plasticizer demonstrated enhanced strength and flexibility, with tensile strength values ranging from 16.92 to 32.71 MPa and up to a fourfold increase in elongation compared to controls. When used as separators for Havarti cheese, these films reduced lipid oxidation by 67.3% over 60 days while maintaining antioxidant activity.
Incorporating DGP into biodegradable plastics represents a strategic approach to improving sustainability and functionality. A notable application is the development of biodegradable vine net clips using DGP and PLA, addressing the estimated 16.8 million plastic clips used annually in New Zealand vineyards that often accumulate as microplastic waste. The resulting material maintains sufficient mechanical strength for vineyard use while exhibiting enhanced biodegradation compared to PLA alone.
In commercial biodegradable polymer blends like Mater-Bi (a mixture of PLA and poly(butylene adipate-co-butylene terephthalate)), additions of 10% and 20% DGP enhanced antioxidant properties and modified rheological behavior, particularly increasing complex viscosity at low frequencies. These biocomposites showed modest improvements in elastic modulus and crystallinity, with soil degradation tests indicating accelerated weight loss at higher DGP content. NMR analysis revealed compositional changes in the polymer matrix during degradation, including increased terephthalic acid content, confirming the material's environmental compatibility.
Other DGP-derived components find applications in biodegradable plastics: grape stalks incorporated into cassava starch-based foams result in fully biodegradable materials within seven weeks, suitable for packaging low-moisture foods; grape seed flour enhances strength and antioxidant activity in biodegradable spoons; and grape seed lignin improves gas barrier properties, antioxidant activity, and biodegradability in PHB/PHA blend films, with non-toxic degradation products.
Incorporating DGP into bio-based materials delivers multiple environmental benefits by transforming agricultural waste into valuable products. This approach reduces reliance on fossil-based plastics, lowers greenhouse gas emissions, and supports development of biodegradable alternatives. By diverting grape pomace from landfills or incineration, the wine industry addresses seasonal waste management challenges while creating functional materials.
Life cycle assessments confirm that DGP-based biocomposites reduce global warming potential, fossil resource use, and terrestrial acidification compared to both sugarcane-based and fossil-based polyethylene, particularly when utilizing clean energy in processing. These bio-based polymers demonstrate a lower environmental footprint through renewability, biodegradability, and reduced plastic pollution.
Within the circular economy framework, DGP valorization reduces biomass waste and greenhouse gas emissions, extends material life cycles, and improves resource efficiency across the agri-food sector. Despite slight reductions in specific bioactive compounds during dealcoholization, SSGP retains sufficient composition to support diverse applications, reducing reliance on synthetic additives and non-renewable resources. The alcohol removal process enhances safety and regulatory compliance, particularly beneficial for food and nutraceutical applications.
From a bioeconomy perspective, DGP valorization supports a regenerative model with efficient biomass repurposing across sectors. Combined with green extraction technologies like supercritical CO₂ or subcritical water extraction, or integration into biopolymer systems based on PLA, poly(butylene succinate), or starch from agri-food by-products, DGP enables sustainable recovery of high-value compounds with low environmental impact.
Recent research has focused on extracting nanocellulose from grape pomace, leveraging agro-industrial waste as a low-cost, sustainable source. Cellulose nanocrystals (CNCs) have been successfully extracted using deep eutectic solvents (DES) composed of lactic acid and choline chloride in a 2:1 molar ratio. This eco-friendly method involves pretreating crushed grape pomace to remove impurities, lignin, and hemicellulose, followed by hydrolysis with DES. The resulting rod-like CNCs exhibit an average length of 241.5 ± 45.3 nm and diameter of 22.0 ± 3.9 nm, with high crystallinity (95.2%) and surface carboxylic acid groups from esterification with lactic acid, contributing to good thermal stability and excellent water dispersibility.
Wine industry wastes also show potential as nutrient sources for bacterial cellulose (BC) production, replacing costly commercial media. White grape pomace has proven more suitable than red pomace, yielding BC with significantly higher production, five times higher water-holding capacity, and greater flexibility compared to BC produced in standard media. While red pomace BC exhibits brittleness suitable for porous particle applications, white pomace BC shows broader potential in textiles, biomedicine, wound dressings, and active ingredient carriers.
Thermochemical conversion technologies offer another pathway, with gasification and pyrolysis processing grape pomace into producer gas, biochar, and activated carbon. These products find applications in energy production, soil amendment, and adsorption processes, further expanding the material's utility while preventing open burning of waste residues.
The valorization of winemaking by-products offers significant economic, social, and environmental advantages, often exceeding financial returns from wine production itself. Rising waste disposal costs in the EU have underscored the need for viable recycling solutions, with waste valorization reducing management costs, sustaining jobs, and enhancing productivity through high-value by-products like anthocyanins for food, pharmaceuticals, and cosmetics.
A case study from Crete, Greece, demonstrates the potential for successful commercialization. A start-up focusing on polyphenol production from pomace, supplied by seven local wineries, projects annual production of 656.25 tons of polyphenols with first-year revenue of €1,968,750, increasing to over €2 million within five years. Despite initial infrastructure costs exceeding €1.9 million and annual operational costs over €1.58 million, key financial indicators confirm investment sustainability, supported by growing global demand for natural compounds and sustainable products.
Market potential assessments for red grape pomace in Ontario and British Columbia estimate a combined theoretical market value of approximately CAD 504 million for supplements and grape seed oil, though actual values would be lower after accounting for processing costs. Current primary uses remain composting, but significant opportunities exist in functional foods, supplements, cosmetics, and biomedical applications, with innovative uses in the meat industry—facilitated by nano- and microencapsulation technologies—showing particular promise.
Despite its potential, DGP valorization faces significant challenges, beginning with compositional variability stemming from grape variety, viticultural practices, climatic conditions, winemaking processes, and dealcoholization methods. This variability leads to inconsistent outcomes in polyphenol profiles and fiber characteristics, requiring customized processing techniques rather than one-size-fits-all approaches.
Scaling up valorization processes presents practical challenges, particularly for distilleries not equipped for sophisticated downstream processing required for high-value products. Investment in specialized equipment, personnel, and quality control systems is substantial, while lacking certifications like Good Manufacturing Practice (GMP) limits access to high-value markets. Economic viability remains a hurdle, with energy-intensive drying, extraction, and purification processes requiring the final products to compete against established alternatives.
Regulatory and standardization issues further complicate commercialization, with limited specific regulations or quality standards for novel SSGP-derived ingredients. Navigating approval processes for food additives, novel foods, or pharmaceutical ingredients demands significant effort and investment, deterring smaller enterprises.
Future research priorities include application-specific performance testing, optimization of processing parameters, robust economic feasibility analysis, and standardization protocols for collection, drying, and storage. Addressing supply chain variability, energy demands, regulatory barriers, and market awareness gaps will be critical for industrial uptake. With coordinated efforts among researchers, industry partners, and policymakers, DGP-based materials can significantly contribute to a circular bio-based economy, particularly in wine-producing regions where this resource remains abundant and underutilized.
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This article is for research use only and cannot be used for any clinical purposes.