Managing phenolics in the vineyard
Phenolics within the grape play an important role in a wine’s sensorial properties, such as color, astringency, mouthfeel, and bitterness. For example, increasing the size of tannin molecules will increase astringency, or a higher level of epicatechin-3-O-gallate (ECG) will lead to more chalky dryness and a lower coarseness. Thus, vineyard management practices play a key role in final wine outcome due to their impact on phenolics.
With a changing environment and climate on California’s North Coast, studying the effect of increasing temperature and light exposure on phenology and flavonoid profiles will be key. Phenology drives the moment in time when the initial pool of phenolics is set (typically before veraison) and how quickly afterwards phenolics evolve (typically after veraison).
Light and temperature will affect the biosynthesis of phenolics and how they modify their shapes and sizes.
Kaan Kurtural from UC Davis Department of Enology and Viticulture, presented his research on vineyard management effect on the phenolic compounds formed and sustained within the grape at the 2016 Napa Vintage Report. His study, which was conducted at the Oakville Experiment Station in Napa, looked at the effect of five different shade nets (blue, pearl, Aluminet, red, black) on anthocyanin content and the physical condition of berries in a bi-lateral cordon, relaxed shoot-positioned Cabernet Sauvignon under two deficit irrigation strategies.
Polyethylene cloths used as nets in Dr. Kurtural’s study (Source: Kann Kurtural’s presentation at the 2016 Napa Vintage Report)
The sustained deficit strategy involves keeping irrigation at 65% of ETc to maintain leaf water potential at -1.2 MPa from fruit set to harvest. The regulated deficit strategy was largely similar to the sustained strategy, except for the period between fruit set and veraison when the researchers applied 25% of ETc to maintain leaf water potential at -1.4 MPa.
In the afternoon, berry temperature can go all the way up to 48oC during the sugar loading phase. According to Mori et al. (2007) and Sweetman et al. (2014), extreme temperatures can cause organic acids and anthocyanin degradation, and thus lower fruit quality. Dr. Kurtural’s study revealed that at its most effective, the shade netting reduced peak berry temperature by 3.5 Celsius, but temperature alone did not noticeably improve berry composition. However, the berries that were uncovered (the control) showed a higher percentage of sunburned berries than those under netting.
One main discovery was that the Aluminet treatment net combined with sustained irrigation treatment resulted in a 25% reduction in yield. The reason was that the Aluminet reflects light out but it also reflects a lot of light back in, resulting in berry issue death. According to Dr. Kurtural, regulated deficit irrigation also increased total sugar across the board (regardless of net treatment). As for anthocyanin content, blue netting increased hydroxylation of anthocyanin under regulated deficit irrigation.
The researchers also look at the mean degree of polymerization which affects the level of bitterness. They found several combinations of colored net and deficit irrigation treatments including blue/red net + sustained deficit and blue net + regulated deficit can lead to lower bitterness level.
The study illustrates the need to carefully coordinate shade management and irrigation strategies. As climate and light exposure continue to change within the season and from one season to the next, it’s critical that vineyard managers continue to monitor and correlate management practice variations and outcome variables to determine the best set of strategies for their vineyards.