|Fig. 1: Grapes on a vine. (Source: Wikimedia Commons)|
Many fruit-based beverages do not incorporate the entire fruit in the final product. This holds true in the wine industry as well. Although processing differs between red and white wines, the two are similar in that winemakers go through great lengths to ensure that the wine we drink is solid-free. In doing so, this means that the fermentable juice must inevitably be separated from the grape's skin. Considering the large scale of the global wine industry, this means that the leftover, pressed grape skin, known as pomace (which also includes seeds and pulp), is a significant lignocellulosic waste streams the wine industry produces 122,000 tons of it a year in California alone.  This pomace is traditionally disposed of, used as livestock feed or fertilizer. However, recent research has pointed to another potential value-added use: as a feedstock in biofuel production.
Bioethanol are subsets of the broader biofuel category, which is an increasingly popular source of sustainable energy that can be produced from a variety of organic material including agricultural, food (including wine), and municipal wastes. In all cases, the biomass waste must undergo microbial fermentation in order to convert it to usable fuel. Grape pomace is a promising and relatively unexplored feedstock that could provide wineries with a readily available source of fuel for large scale, energy intensive winemaking operations with a nearby biofuel generation plant.
Currently, there is not a large scale implementation of this technology and laboratory-scale research is fairly limited. Early studies raised some doubts about grape pomace's feasibility as a pomace pretreatment generated inhibitory compounds that were toxic to the yeast. The first attempt to solve this issue was with detoxification strategies. However, this raised further questions of feasibility. The technical challenges and costs of detoxification were viewed unfavorably, and lead engineers to consider other options. The most promising option involved the incorporation of genetically modified microbes that would be resistant to the pre-treatment toxins. However, this led to issues in scalability, as successful lab-scale microbial fermentation does not guarantee success at the industrial scale. Researchers finally settled on investigating bacterial strains with native tolerance that were somehow overlooked in the genetic engineering process. In a study conducted by researchers at the University of Padova, various strains of Saccharomyces cerevisiae were tested for productivity.  After laboratory-scale testing, it was determined that this type of yeast's resistance to sulfuric inhibition and tolerance to temperature made the yeast a strong candidate for bioethanol production. These researchers found that the highest ethanol yield from tested strains at fermentation temperatures of 40°C was 91% of the theoretical maximum yield of .51 g ethanol/ g glucose. 
Just like any other type of agricultural product, grapes also have a harvest season. This means that potential bioethanol generation from grape pomace is a seasonal process. However, this is not necessarily a downside. The most energy intensive part of winemaking occurs during the harvest season, when the generation of grape pomace is at its highest rate. While it is true that there is still energy expenditure in wine storage and transportation, these processes still occur during the main grape processing period and thus creates the highest period of demand for a winery. The addition of bioethanol generation during this peak power demand could significantly reduce cost and grid strain for winery operations. This is especially true because of the tendency for large, unsightly manufacturing plants to be distant from populated areas and thus, a well-established power grid. The bioreactors could provide locally generated power without the need to modify distant power infrastructure to handle surges in power use for only a few months out of the year.
A Brazilian study found that found that there was 29.2 g of glucose per 100 g grape pomace in the samples they analyzed.  If we use this figure as a basis for calculation, with the Favaro study's high yield of 91%, it should be possible to attain roughly 26 g of ethanol per 100 g grape pomace.  Using the fact that roughly 28.9 kJ can be obtained from 1 g of ethanol, this means that 100 g grape pomace can produce roughly 8.7 kJ if we assume a biogas turbine efficiency of 30%.  If all 122 000 aforementioned tons of dry California grape pomace were converted to bioethane, the wine industry could generate roughly 2.6 GWh of energy. 
This number is fairly low compared to the 400 GWh consumption figures estimated by the Lawrence Berkely National Laboratory.  In most cases however, it is simply not worth the efforts of scattered, individual wineries to install their own bioreactors. Smaller wineries simply lack the volume, capacity, knowledge and capital to have a worthwhile biofuel production facility onsite. Most of these wineries simply use waste pomace as a supplemental fertilizer for their grow operations. Between the smaller scale of these wineries and the overall lack of applied engineering in the wine industry, it is safe to say that these wineries are not necessarily as concerned with energy efficiency as their larger scale competitors. However, large scale winery operations with ready access to large amounts of grape pomace are the best candidates for biofuel production. These wineries, which can produce over 100 million gallons of wine, consume more energy and are more efficiency oriented than their smaller, boutique wine counterparts and are thus better candidates to consider grape pomace biofuel production.
© Thomas Logan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 Y. Zheng et al., "Ensilage and Bioconversion of Grape Pomace into Fuel Ethanol," J. Agr. Food Chem. 60, 11128 (2012).
 L. Favaro et al., "Exploring Grape Marc as Trove for New Thermotolerant and Inhibitor-Tolerant Saccharomyces cerevisiae Strains For Second-Generation Bioethanol Production," Biotechnol. Biofuels 6, 168 (2013).
 E. C. Sousa et al., "Chemical Composition and Bioactive Compounds of Grape Pomace (Vitis vinifera L.) Benitaka Variety, Grown in the Semiarid Region of Northeast Brazil," Food Sci. Technol. 34, 135 (2014).
 V. Smil, Energy in Nature and Society: General Energetics of Complex Systems, (MIT Press, 2008).
 "Best Winery Guidebook: Benchmarking and Energy and Water Savings Tool for the Wine Industry," California Energy Commission, CEC-500-2005-167, November 2005.