UC Davis

Wine grape pomace, composed of skins, seeds, stems, and pulp, is the major byproduct of the winemaking industry. While this byproduct can be diverted towards other agricultural uses, such as compost or animal feed, the environmental burden and poor animal digestibility of the material hinders the sustainability and effectiveness of these low-value mitigation strategies. Since wine grape pomace contains valuable bioactive compounds, especially phenolics, there remains an opportunity to valorize this material to benefit human health with the development of innovative food, beverage, cosmetic, and supplement applications. The extraction of wine grape pomace phenolics typically relies on the use of hazardous solvents like ethanol or methanol, which requires subsequent downstream processing steps for use in food-grade products. To present an eco-friendly alternative, this thesis focuses on the design of green extraction methods that improve phenolic extractability and support the in vitro antioxidant activity of grape pomace extracts while using water as the only solvent under optimized extraction conditions.

In Chapter 1, an overview of the recent advances and research gaps in green extraction technologies is discussed with a focus on the large-scale feasibility of these methods to support the potential for commercial adoption. In addition, the compositions of various winemaking byproduct fractions (pomace, leaves, lees, vinasse, and wastewater) are highlighted to showcase the diversity of components available for the development of value-added products.

In Chapter 2, the aqueous extraction process (AEP) is explored as an environmentally-friendly strategy for enabling the release of phenolics from the grape cell-matrix without the use of conventional solvents. A series of experiments, using a central composite rotatable design paired with a kinetic study, was used to identify the concurrent effects of pH, solids-to-liquid ratio, temperature, and time on the total phenolic content (TPC) of the grape pomace extracts. To reduce water consumption without compromising phenolic extractability, a two-stage countercurrent method was employed, which recirculates the extraction processing fractions and increases the concentration gradient to drive phenolic diffusion. This technique provides an economically-viable and environmentally-friendly alternative to conventional solvent extraction methods.

The findings from Chapter 2 (i.e., the role of alkaline conditions in phenolic extraction) guided the design of enzyme- and microwave-assisted extractions as outlined in Chapter 3. Enzyme specificity and alkaline conditions enable the degradation of ester- and ether-linkages between phenolics, structural proteins, and cell-wall carbohydrates within the grape cell-matrix, while microwave radiation causes rapid temperature and pressure changes that can rupture the cell. Multiple stepwise screening experiments were performed to select the optimal conditions for maximizing TPC while reducing total enzyme concentration, water usage, and extraction time. The structural changes to the grape cell-matrix, as illustrated by scanning electron microscopy, represent the successful cellular disintegration caused by the synergistic effects of enzymatic hydrolysis, microwave heating, and intracellular pressure on promoting phenolic extractability.

The series of studies herein provides new approaches to extracting phenolics from wine grape pomace. Notably, the starting material used in these studies represents a non-conventional winemaking process using red wine grapes (Vitis vinifera L. cv. Cabernet Sauvignon) to produce white wine. The use of red wine grape pomace collected prior to fermentation ultimately affected the phenolic composition of the grape pomace extracts, as detailed in Chapters 2 and 3 with the abundance of flavonol glycosides, which represent high residual sugars in the matrix. This finding emphasizes the influence of upstream winemaking conditions on the downstream phenolic profiles and potential biological properties of the extracts. In addition, this study offers support for the use of alkaline treatments for the extraction of phenolics from wine grape pomace, which is not yet widely reported due to the use of acidic conditions in conventional ethanol and methanol extractions. These insights, along with further work on scaling-up the microwave and enzyme-assisted extraction process, illuminate the possibilities for green extraction methods to revitalize agricultural byproducts while boosting human health, supporting a circular economy, and promoting environmental sustainability.