 IVES 9 IVES Conference Series 9 Inhibition of reductive characters in wine by cu-organic acids: predicting the duration of protection

Inhibition of reductive characters in wine by cu-organic acids: predicting the duration of protection

Abstract

Cu organic acid complexes efficiently bind hydrogen sulfide in wine and therefore prevent its accumulation and subsequent reductive off-flavour [1]. This fraction of Cu can also bind methanethiol, the other main contributor to reductive wine characters, albeit with less efficient binding than for hydrogen sulfide [1]. However, during bottle aging of wine, the concentration of Cu organic acid complexes gradually decline and the sulfide-bound form of Cu increases. The point at which the Cu organic acid concentration is depleted signifies a potential for reductive aroma development to occur. The aim of this study is ascertain how long Cu organic acid complexes in wine can offer a protection against the reductive aroma compounds. High (0.6 mg/L), medium (0.3 mg/L) and no (0 mg/L) Cu(II) additions were made to Pinot Grigio and Chardonnay wines at bottling, and the bottled wines were then stored at 14 °C in darkness. Analysis was performed on the wines at 0, 2, 4, 8 and 12-14 months after bottling. Throughout bottle ageing process, the concentrations of three different Cu fractions, attributed to Cu organic acid complexes, Cu thiol complexes and sulfide-bound Cu, were monitored by stripping potentiometry and colorimetric methods. The free and salt-releasable concentration of hydrogen sulfide and methanethiol were determined by gas chromatography with chemiluminescene detection. Sensorial analysis was also performed on the wines after 12 months. During the first 2-months of bottle aging of all wines, the Cu-organic acid concentrations initially remained stable or increased, as total packed oxygen was consumed. Afterwards, during the low oxygen aging phase of the wines, the Cu-organic acid concentration declined exponentially with a concomitant increase in sulfide-bound Cu. These changes in sulfide-bound Cu were matched by an increase the salt-releasable hydrogen sulfide concentrations of the wines during aging. Free concentrations of hydrogen sulfide and methanethiol were only found to accumulate in wines without any Cu-organic acid present at bottling. For the Pinot Grigio without Cu-organic acid present at bottling (i.e., the no Cu addition treatment), the free methanethiol concentrations in the wine were above the aroma threshold and this wine was assessed as reductive. Alternatively, for the Pinot Grigio with Cu-organic acid complexes at bottling, only concentrations of free methanethiol below the aroma threshold were measured and the wine was not reductive. The decay in Cu-organic acid complexes, in the low oxygen aging phase of the wines, were found to follow first order kinetics that were similar for the Chardonnay and the Pinot Grigio. These results enable determination of the time-frame that Cu-organic acids can offer white wine protection against the potential accumulation of reductive aroma compounds.

DOI:

Publication date: September 14, 2021

Issue: Macrowine 2021

Type: Article

Authors

Xinyi Zhang

National Wine and Grape Industry Centre, Charles Sturt University, Wagga Wagga, Australia,Nikolaos KONTOUDAKIS (Laboratory of Enology and Alcoholic Drinks, Department of Food Science and Human Nutrition, Agricultural University of Athens) John W. BLACKMAN (National Wine and Grape Industry Centre, Charles Sturt University) Andrew C. CLARK (National Wine and Grape Industry Centre, Charles Sturt University)

Contact the author

Keywords

cu organic acid complexes, hydrogen sulfide accumulation, white wine bottle ageing

Citation

Phenolic composition of Tempranillo Blanco grapes changes after foliar application of urea

Our research aimed to determine the effect and efficiency of foliar application of urea on the phenolic composition of Tempranillo Blanco grapes. The field experiment was carried out in 2019 and 2020 seasons and the plot was located in D.O.Ca Rioja (North of Spain). The vineyard was Vitis vinifera L. Tempranillo Blanco and grafted on Richter-110 rootstock. The treatments were control (C), whose plants were sprayed with water and three doses of urea: plants were sprayed with urea 3 kg N/ha (U3), 6 kg N/ha (U6) and 9 kg N/ha (U9). The applications were performed in two phenological stages, pre-veraison (Pre) and veraison (Ver). Also, each of the treatments was repeated one week later. Control and treatments were performed in triplicate and arranged in a randomised block design. Grapes were harvested at optimum ripening stage. High-performance liquid chromatography was used to analyse the phenolic composition of the grapes. Finally, the results obtained from the analytical determinations – flavonols, flavanols and non-flavonoid (hydroxybenzoic acids, hydroxycinnamic acids and stilbenes) – were studied statistically by analysis of variance. The results showed that, in 2019, U6-Pre and U9-Pre treatments increased the hydroxybenzoic acid content in grapes, and also all foliar treatments applied at Pre enhanced the stilbene concentration. Moreover, U3-Ver was the only treatment that rose flavonol and stilbene contents in the Tempranillo Blanco grapes. In 2020, all treatments applied at Pre enhanced the flavonol concentration in grapes. Furthermore, U3-Pre and U9-Pre treatments increased stilbene content in grapes. Nevertheless, the hydroxybenzoic acid content was improved by U6-Ver and U9-Ver and besides, hydroxycinnamic acid concentration in grapes was increased by all treatments applied at Ver. In conclusion, the lower and highest dose of urea (U3 and U9), applied at pre-veraison, were the best treatments to improve the Tempranillo Blanco grape phenolic composition.

Effects of graft quality on growth and grapevine-water relations

Climate change is challenging viticulture worldwide compromising its sustainability due to warmer temperatures and the increased frequency of extreme events. Grafting Vitis vinifera L.

Updating the Winkler index: An analysis of Cabernet sauvignon in Napa Valley’s varied and changing climate

This study aims to create an updated, agile viticultural climate index (similar to the Winkler Index) by performing in-depth analyses of current and historical data from industry partners in several major winegrowing regions. The Winkler Index was developed in the early twentieth century based on analysis of various grape-growing regions in California. The index uses heat accumulation (i.e. Growing Degree Days) throughout the growing season to determine which grape varieties are best suited to each region. As viticultural regions are increasingly subject to the complexity and uncertainty of a changing climate, a more rigorous, agile model is needed to aid grape growers in determining which cultivars to plant where. For the first phase of this study, 21 industry partners throughout Napa Valley shared historical phenology, harvest, viticultural practice, and weather data related to their Cabernet sauvignon vineyard blocks. To complement this data, berry samples were collected throughout the 2021 growing season from 50 vineyard blocks located throughout 16 American Viticultural Areas that were then analyzed for basic berry chemistry and phenolics. These blocks have been mapped using a Geographic Information System (GIS), enabling analysis of altitude, vineyard row orientation, slope, and remotely sensed climate data. Sampling sites were also chosen based on their proximity to a weather station. By analyzing historical data from industry partners and data specifically collected for this study, it is possible to identify key parameters for further analysis. Initial results indicate extreme variability at a high spatial resolution not currently accounted for in modern viticultural climate indices and suggest that viticultural practices play a major role. Using the structure of data collection and analyses developed for the first phase, this project will soon be expanded to other wine regions globally, while continuing data collection in Napa Valley.

Second pruning as a strategy to delay maturation in cv. ‘Touriga nacional’ in the Portuguese Douro region

The advance in maturation of wine grapes is an important climate change risk related effect that could affect warm regions like Portuguese Douro Wine Region. Indeed, the climate analysis over the past years registered a decrease in the precipitation, significant higher average temperatures, and a more frequent occurrence of extreme weather events, including heat waves. In these conditions the length from anthesis until maturation is shortened and the uncoupling of technical and phenolic maturity results in berries with higher sugar concentration (and lower acidity), but lower anthocyanins, tannins, and total phenolic concentration, which produce unbalanced wines.
In this work, an innovative strategy of crop forcing, based on forcing vine regrowth after a second pruning of green shoots, was tested, aimed at delaying ripening until the temperature becomes lower and, therefore, preventing acidity loss and increasing anthocyanin-to-sugar ratio. The experiments were conducted in 2019 and 2020 in a commercial vineyard of ‘Touriga Nacional’ located in the Douro Region. Crop forcing was conducted 15 (CF1) to 30 (CF2) days after fruit set. Vines pruned with conventional methods were used as control (CF0). Results confirmed that fruit ripening was shifted from the hot season (August/September), until a cooler period (October through early-November). At harvest, grapevine berries from CF1 and CF2 presented lower pH and higher acidity, than control, with no significant differences in colour intensity and phenolic levels composition. Sugar content was lower in CF2-treated vines in both seasons. However, in CF-treated vines the number and size of clusters were significantly lower (up to 88% reduction) than in control plants. A metabolomics analysis of mature berries from CF-treated vines and control is underway. Crop forcing was indeed effective in producing a more balance berry composition but severely reduced grapevine yield,

Postveraison shoot trimming in Tannat and Merlot: preliminary results on yield components, plant balance and berry composition

There is currently a trend towards the production of wines with low alcohol content. To achieve this, grapes with low sugar content must be used. There are techniques at the vineyard level that can delay ripening and avoid excessive sugar accumulation without, a priori, affecting the final polyphenol content. Postveraison shoot trimming (PVST) is experimentally evaluated for these purposes, but its impact under Uruguayan climatic conditions with high interannual variability is not known. The aim of this work is to assess the PVST in Tannat and Merlot cultivars and their impact on yield components, plant balance and berry primary composition. In this study, two commercial vineyards of 10 years old Tannat and Merlot (grafted on SO4) at Canelones Department were selected. During the 2020-201 growing season, grapevines were submitted to PVST when grapes reached 15º Brix. In a randomized block, trimmed (T) and control (C) plants were evaluated with three repetitions each cultivar. Evaluation of the evolution of primary berry composition during ripening, measurement of yield components and plant balance were performed. For both cultivars, PVST did not affect yield components. Merlot reached 5.4 kg per plant and Tannat 7.1 kg, with not statistical significance between treatments. However, statistical differences were observed in terms of plant balance. In Merlot Ravaz Index reached a difference of 5.3 (12.0 in T and 6.7 in C) meanwhile Tannat reached 3.5 of statistical difference (13.7 in T and 10.2 in C). The tendency to imbalance for the treated plants had an impact on the final grape composition. Merlot grapes showed statistical difference in final total acidity (0.3 g of difference between treatments) while treatments impact final sugar content on Tannat grapes (10.0 g of difference between treatments). Further studies are needed to assess the impact of different canopy management techniques in our conditions.