Managing the impact of vintage advancement and compression

Abstract

Recent vintages in Australia have been characterised by rapidly maturing fruit and compression of the ripening window. This means that varieties that used to ripen over four to six weeks in the 1990s are now maturing over a much shorter time period. This places significant pressure on harvest logistics, with growers struggling to find sufficient harvesters and wineries being forced to delay harvest due to a lack of fermenter capacity (Petrie and Sadras 2016). Delays in harvest may result in yield loss due to berry dehydration, elevated grape sugar and wine ethanol concentration, and other negative impacts on fruit composition and wine style. On the other hand, some producers may perceive the increase in the concentration of grape compounds (especially grape and wine phenolics) associated with delayed harvest, together with other changes in wine volatile profile and sensory properties, as a potential improvement for wine quality attributes. This is of particular importance where the issue of higher wine ethanol can be overcome by using pre-ferment water addition.

A key focus of this project related to regulatory changes that allowed the addition of water to must in Australia under specific conditions, and the importance of this change in relation to the effects of vintage compression. The impacts of water addition on the chemical and sensory properties of Shiraz and Chardonnay wines (Australia’s principal varieties) were investigated, with wines made following dilution compared against wines made from earlier harvested grapes. Of particular interest was the question whether losses in fruit volume associated with more extreme delays in harvest (e.g. fruit shrivel) might be overcome by water addition without detriment to wine quality. The results demonstrated that water addition lowered tannin and colour in Shiraz wines independently of the mode of addition (direct addition vs juice replacement), showing that maintaining the solids to juice volume ratio did not necessarily increase the extraction and/or retention of phenolics in the wine. Nevertheless, wine produced with dilution consistently had higher levels of tannin and colour than wines of the same ethanol concentration produced from less ripe grapes from earlier harvest dates. In the Chardonnay wines, differences in chemical composition and sensory properties were relatively small. The fruit harvested at 15.5°Baume, irrespective of dilution, separated from the wines made from earlier harvested grapes according to the chemical analysis. From the sensory analysis the main attribute affected by dilution was wine ‘hotness’, which related to the lower ethanol concentration.

The observation that grapes are reaching a sugar concentration suitable for winemaking at an earlier date is well established in Australia and throughout the world. Yet there has been less effort spent characterising the impact of climate change on other berry maturity parameters such as the titratable acidity (TA) and pH. From analysis of a commercial dataset, an increase in pH and decrease in TA were observed with a similar magnitude to that seen for sugar concentration. Interestingly, the ratio between grape pH or TA and sugar concentration remained consistent across the 25-year period investigated.

Increases in grape sugar concentration can be driven by either the import of sugar from the vine, or by berry dehydration. Dehydration also results in a loss in yield, with a significant impact on vineyard profitability. A meta-data analysis of studies on grape ripening across Australian growing regions for Shiraz and Cabernet Sauvignon was completed. The results suggest that Shiraz ceases sugar accumulation at 13°Baume and Cabernet Sauvignon at 14.5°Baume. Beyond these values, additional increases in grape sugar concentration are likely to be the result of fruit dehydration.

Taken together, the improved understanding of the processes that affect grape ripening and fruit maturity and the development of winemaking processes to manage fruit that has to be harvested with high sugar concentrations will support the industry’s efforts to maintain wine quality under warmer conditions.

Summary

Recent vintages have been characterised by rapidly maturing fruit and compression of the ripening window. This means that varieties that used to ripen over four to six weeks in the 1990s are now maturing over a much shorter time period. This places significant pressure on harvest logistics, with growers struggling to find sufficient harvesters and wineries being forced to delay harvest due to a lack of fermenter capacity. As one countermeasure, recent changes in Australian legislation allow the addition of water to must in Australia under specific conditions.

To compare benefits and risks from water addition to grape must, direct water addition and juice substitution approaches were trialled with Shiraz must from grapes sourced from the Barossa Valley. In this case it was found that water addition lowered tannin and colour in wines independently of the mode of addition, showing that maintaining the ratio of solids to juice volume did not necessarily increase the retention of phenolics in the wine. Nevertheless, wine produced with must dilution consistently had higher levels of tannin and colour than wines of the same ethanol concentration produced from grapes harvested earlier at reduced ripeness. Sensory analysis of the wines showed that the treatments were chiefly defined by differences in ‘dark fruit’ aroma/flavour, hotness, viscosity and opacity. The wines prepared from 15.5°Baume fruit were rated higher in these attributes than wines with lower ethanol, with the earliest harvest (13.5°Baume) rated lowest. The wines made from the must with water additions maintained many of the fuller-bodied and richer flavours that are typical of the styles produced in the Barossa and other warmer climate regions, and were also associated with the wines made from the undiluted must at the latest harvest time. The wines made from the fruit that was harvested earlier were generally of a lighter style with less colour and tannin and more ‘red fruit’ aromas. The mode of water addition (i.e. if juice was run off prior to the addition of the water or water was added directly to must) had a relatively small impact on wine phenolics, but introduced larger changes in wine sensory properties. As a note of caution, it was observed that higher quantities of water addition introduced off-odours in some wines. A more moderate addition of water using a ‘run-off and replace’ technique produced wines more similar to the undiluted control wine and without off-odours.

To understand the effects of water addition on white wine composition and quality, a further study was conducted using Chardonnay grapes sourced from the Currency Creek region. Chardonnay was harvested at different ripeness stages and diluted to 13.0°Baume using direct addition of rainwater to the must. Wine made from earlier harvested grapes at 13.0°Baume was included for comparison. In the Chardonnay wines, differences in chemical composition were minor, with the 15.5°Baume treatments, irrespective of dilution, separated from the wines made from grapes harvested at earlier dates. Sensory assessment showed that differences between the wines were small, with the main attribute affected by dilution being wine ‘hotness’. ‘Hotness’ was similar between the wines made from earlier harvests, but not surprisingly, the 15.5°Baume control wine was rated highest for ‘hotness’. However, it was interesting to note that the diluted 15.5°Baume treatment had the lowest perceived hotness out of all the treatments, despite having a similar ethanol concentration to the wines made from earlier harvest grapes. These preliminary results show that water addition may be used according to the FSANZ regulations to reduce wine ethanol in Shiraz and Chardonnay, without significant negative impacts, with the cautionary observation that higher levels of water addition in Shiraz might introduce off-odours.

In a subsequent season, a follow-up study on Shiraz from the same Barossa vineyard was performed to understand the outcomes of a more extreme extended ‘hang time’ of grapes on the vine, as it related to berry shrivel, water addition and phenolic outcomes. In that study, it was found that marked seasonal variation existed, with grape and wine phenolics being far higher in the second season than the first, independent of the harvest date. An important outcome from this second season of research was that while increases in fruit and wine phenolics were previously found up to 15.5 °Baume, no further significant increases were seen as a result of delaying harvest, even when shrivel increased from 8% to 30%. As for the first season of the study on Shiraz, the quantity of water addition did not affect phenolic outcomes in the wine.

Another key focus of the project was a meta-analysis of data sourced across a range of existing research datasets with particular reference to the interpretation of sugar accumulation data in relation to changes in berry weight, in order to separate the impact of sugar importation from increased concentration through berry dehydration. A systematic search of the literature identified 36 Australian-based studies that assessed the ripening of grape berries and had records of both berry weight and sugar concentration within one season. Boundary layer regression was used to empirically determine the maximum sugar concentration where the sugar content per berry stopped increasing, under Australian conditions. This was found to be 13Baume for Shiraz grapes, and for Cabernet Sauvignon it was closer to 14.5Baume. These results were particularly relevant in light of the findings for Shiraz described above, since other parameters relevant to quality (e.g. phenolic compounds) may continue to increase as ‘hang time’ is extended, but the importance of these changes should be considered with the recognition that further increases in sugar concentration in the fruit are most likely due to dehydration of the berries, and not genuine accumulation.

While the advancement in grape maturity over time in response to warmer conditions has been well categorised, changes in other key parameters that affect maturity assessments (titratable acidity (TA) and pH) have not been investigated. To address this, a commercial dataset was analysed to investigate trends in pH and TA over 25 consecutive vintages. The date when nominated values for TA (9 g/L) and pH (3.4) were reached followed a trend as observed for total soluble solids during the time period studied. The calculated average TA and pH of the fruit, when it had an average TSS of 12 Baume, was remarkably consistent across the 25 years of data. For Shiraz grown in the Barossa the TA and pH averaged 8.9 g/L and 3.3 respectively and for Cabernet Sauvignon grown in Coonawarra the TA and pH averaged 7.5 g/L and 3.3, respectively. Similar responses were seen for other cultivar and regional combinations

Acknowledgements:

This work was supported by Australia’s grapegrowers and winemakers through their investment body Wine Australia with matching funds from the Australian Government. The AWRI is a member of the Wine Innovation Cluster in Adelaide, SA.

The authors thank SARDI and specifically Dr Victor Sadras and Dr Marcos Bonada for their support in developing the data analysis methods for evaluating the effects of faster ripening on grape maturity and for the supply of Shiraz grapes for the dilution trial. Treasury Wine Estates (Dr Catherine Kidman, Dr Anthony Robinson and Dr Vanessa Stockdale) is thanked for the access to the commercial dataset.

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