Slowing the sugar rush to yield better grapes

Climate change poses many challenges to grape growers. Grapes will ripen faster in warmer climates. This can lead to poor aroma and colour development. Researchers from the University of Adelaide published a study in the Journal of Agricultural and Food Chemistry that showed it was possible to enhance the flavor potential of Cabernet Sauvignon grapes through slowing down their ripening processes using strategies such as crop load manipulation and irrigation control. Pietro Previtali, a University of Adelaide School of Agriculture, Food and Wine, was the lead author. He stated: "Advanced maturity due to warmer temperatures are a key problem for grape growers worldwide and particularly in warm and dry regions like Australia and California." It leads to faster sugar accumulation, which allows grapes to reach the desired sugar levels even when their concentrations of aroma compounds and colour are lower than their maximum. "Growers must make a compromise between harvesting sugar when it is ready, but not the desired flavours, and prolonging grape maturation to achieve an optimal combination of colour, mouthfeel, and aroma compounds. "The problem with prolonged maturation is that grapes shrivel and yields drop, which has a negative impact on profitability and leads to higher sugar levels, which can lead to high-alcohol wine." While earlier research had shown that techniques like thinning vines or intense irrigation late in the season could alter wine composition, this new study focused on how these techniques affect the development and expression of aroma compounds within the grapes. Researchers grew Cabernet Sauvignon grapes in a commercial vineyard in California's San Joaquin Valley. The grapes were harvested during the ripening period and either thinned or irrigated later in the growing season. These grapes were compared to grapes from the same block, where neither technique was used. Researchers discovered that reducing ripening speeded up sugar accumulation. This led to a decrease of green aroma compounds, which are undesirable in winemaking. It also resulted in an increase in fruity aromas, colour, and mouthfeel compounds, all associated with red wine quality. They also discovered that grape quality traits are not dependent on one strategy. "Rather, groups were responsive to different factors including crop load, irrigation and ripening rates, in some cases even an interaction of these," said Mr Previtali. The researchers used the available strategies to obtain the longest possible delay to study the relationship between sugar accumulation, flavour development. A reduction of crop load by 35% and 50 percent late season irrigation resulted in a delay of three week. Associate Professor Christopher Ford from the University of Adelaide, Project leader and coauthor, said that while this is a useful experimental tool, it may not be practical because of high costs and availability of irrigation. This is especially true as water becomes more scarce. "Tailoring these strategies seems like the best way to get the desired levels of aroma compounds, color and mouthfeel in wine." Researchers believe that future vineyard trials will be necessary in order to fully understand the effects of year-to–year variation and to gain a better understanding of how to combine crop load with late-season irrigation to delay sugar accumulation. ### The study was done within the ARC Training Centre for Innovative Wine Production, based at the University of Adelaide. Research partners included E. & J. Gallo Winery and the Training Centre.

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