Sugar levels a guide to future (and desired) wine style

A recently completed project at Charles Sturt University has found potential for a novel way to predict wine style based on sugar accumulation in the berry.

Professor Alain Deloire brought the project idea with him when he arrived to lead the National Wine and Grape Industry Centre (NWGIC) in early 2013, and the work was largely completed late last year, about the time he left to return to his native France.

‘Quite a lot has been done in France in this area, so it was rewarding to be able to demonstrate that the concept is applicable and valuable in Australian conditions’, said the chief investigator Dr Leigh Schmidtke.

‘The ability to determine an optimum harvest time for specific wine styles is one of the main challenges of vineyard and winemaking decisions.’ 

The concept – now proven – is that you can consistently produce a wine that is fresh, mature or somewhere in between by waiting a specific number of days following the cessation of sugar accumulation level in the grape berries.

‘If you look at the sugar profile in the berry it will increase over a period of ripening then plateau, but not increase or decrease’, Dr Schmidtke said. ‘From that point, we can then say that 10 or 20 days forward we can predict that this will be the style of wine. The actual length of time is variable according to cultivars.’

The key term here is ‘sugar accumulation’, which is not quite the same as ‘sugar content’ as measured by Brix or Baumé. These could be described as indirect measures; they look at ripeness or overall sugar content in a bunch of grapes, rather than the amount of sugar in each berry.

To get an accurate picture of sugar accumulation, you need to also measure the mass of the grapes then convert the traditional sugar content figure into an amount of sugar per gram of berry weight.

It’s not a difficult task but it can take a bit more time than measuring Brix or Baumé, which may impact on immediate take-up by the sector. Dr Schmidtke, who is currently Acting Director of the NWGIC, says the next goal should be to try to develop new tools that could streamline the measurement process targeting the development of grape flavour compounds identified as key markers for wine styles.

The researchers did identify some marker compounds in both red and white grapes that correlate to specific wine styles, and also found a link in white wines between how the colour of the berry skin changes and the progression of the aroma compounds. That’s led to a new project where researchers are trying to use smart phone technologies to monitor skin colour development.

The recently completed project was carried out with wine sector partners in Griffith, Orange, Gundagai, Barossa Valley and McLaren Vale. The main series of experiments using Shiraz and Cabernet Sauvignon cultivars was undertaken across three successive vintages.

Read the final report here.