White wine lees, the sediment that settles at the bottom of fermentation tanks or aging barrels, are often overlooked as a winemaking byproduct. However, these nutrient-dense materials hold tremendous potential for valorisation – the process of extracting valuable compounds and converting them into high-value products. One innovative approach to lees valorisation is the use of subcritical water extraction, a green and efficient technology that can unlock the hidden treasures within this winemaking waste.
Characteristics of White Wine Lees
White wine lees are a complex mixture comprising primarily of dead yeast cells, residual grape solids, tartrates, and other organic and inorganic compounds that precipitate during the winemaking process. The exact composition can vary depending on factors like grape variety, vinification methods, and lees management practices. Nonetheless, these lees are known to be rich in beneficial compounds, including polyphenols, glutathione, and sulfur-containing compounds, all of which possess potent antioxidant properties.
Composition and Properties of White Wine Lees
Quantitative analysis has shown that white wine lees can contain total polyphenol content (TPC) ranging from 2 to 16 mg gallic acid equivalents per gram of dry matter (mg GAE/g DM), glutathione (GSH) levels up to 1.5 mg/g DM, and total sulfhydryl groups (TSH) as high as 5 μmol/g DM. These bioactive compounds contribute to the lees’ impressive antioxidant activity, which can reach values of 5-30 mg Trolox equivalents per gram of dry matter (mg TE/g DM) when measured by common assays like DPPH and FRAP.
Valorisation Potential of White Wine Lees
Recognizing the untapped potential of white wine lees, researchers have explored various strategies to valorise this winemaking byproduct. Conventional approaches have involved using organic solvents like methanol, ethanol, or acetonitrile to extract the desired compounds. However, these methods can be environmentally unfriendly and may leave undesirable solvent residues in the extracts.
Subcritical Water Treatment
Principles of Subcritical Water Technology
An emerging and more sustainable alternative is the use of subcritical water extraction (SWE), also known as pressurized hot water extraction. In this technique, water is heated and pressurized to temperatures typically between 100°C and 374°C, while maintaining it in a liquid state. Under these subcritical conditions, water exhibits unique properties, such as decreased dielectric constant and increased solvating power, allowing it to effectively extract a wide range of polar and non-polar compounds.
Optimization of Subcritical Water Conditions
Researchers have employed Doehlert experimental design and response surface methodology (RSM) to systematically optimize the SWE conditions for extracting antioxidant compounds from white wine lees. The key parameters investigated include extraction temperature, time, and stirring speed. The results showed that temperature was the most influential factor, with the optimal conditions being 240°C, 15 minutes, and 550 RPM, respectively. These optimal conditions yielded lees extracts with significantly higher antioxidant activity, up to 29.2 mg TE/g DM, compared to the 12.8 mg TE/g DM obtained using conventional solvents.
Applications of Subcritical Water in Lees Valorisation
The advantages of SWE for lees valorisation are manifold. Firstly, it is a “green” and environmentally friendly process that avoids the use of toxic organic solvents. Secondly, the high-temperature extraction can effectively disrupt the cellular structures of the lees, facilitating the release of valuable intracellular compounds. Lastly, the SWE process can be tuned to selectively target and recover specific bioactive compounds, such as polyphenols, glutathione, and sulfur-containing antioxidants, making it a versatile tool for the oenological and industrial applications of white wine lees.
Extraction and Purification of Valuable Compounds
Extraction of Bioactive Compounds from Lees
The SWE process has demonstrated its effectiveness in extracting a wide range of antioxidant compounds from white wine lees. The extracts obtained under optimized conditions were found to have significantly higher concentrations of total polyphenols, glutathione, and total sulfhydryl groups compared to the original lees. These bioactive compounds are known to play crucial roles in wine stability, organoleptic properties, and human health.
Purification and Characterization of Extracted Compounds
Further purification and characterization of the SWE extracts can provide valuable insights into the specific phenolic profiles and antioxidant mechanisms associated with the recovered compounds. Advanced analytical techniques, such as high-performance liquid chromatography (HPLC) and mass spectrometry, can be employed to identify and quantify the individual polyphenolic constituents, including flavonoids, phenolic acids, and stilbenes.
Potential Applications of Recovered Compounds
The valorisation of white wine lees through SWE can unlock a wide range of potential applications for the recovered bioactive compounds. These extracts could be utilized as natural antioxidants in the food, beverage, and cosmetic industries, or as nutritional supplements in the nutraceutical and pharmaceutical sectors. Additionally, the sulfur-containing compounds may find applications in the oenological domain, where they can contribute to wine aroma, mouthfeel, and ageing potential.
Sustainability and Environmental Aspects
Waste Reduction and Resource Efficiency
The valorisation of white wine lees through SWE aligns with the principles of the circular economy, as it transforms a winemaking byproduct into valuable high-added-value products. By extracting the bioactive compounds and minimizing waste, this approach contributes to the overall sustainability and resource efficiency of the wine industry.
Environmental Impact of Lees Valorisation Processes
Compared to conventional solvent-based extraction methods, the SWE process has a lower environmental impact due to the use of water as the primary solvent. This “green” approach avoids the generation of toxic waste streams and reduces the carbon footprint associated with the production and disposal of organic solvents.
Circular Economy Approaches
The integration of SWE-based lees valorisation into the winemaking process exemplifies the transition towards a more circular economy in the wine industry. By recovering and repurposing the valuable compounds from white wine lees, this strategy not only minimizes waste but also creates new revenue streams and fosters sustainable innovation.
The valorisation of white wine lees through optimized subcritical water extraction holds great promise for the wine industry. By unlocking the hidden treasures within this winemaking byproduct, this green and efficient technology can contribute to the development of innovative oenological and industrial applications, while also promoting sustainability and environmental stewardship. As the wine industry continues to evolve, the valorisation of white wine lees can be a crucial step towards a more circular and resource-efficient future.