Taking steps towards lower-alcohol wines

Cristian Varela’s research, funded by AGWA, is still a work in progress, but it has recorded two impressive milestones.

The first – in which he and colleagues at the AWRI used a non-Saccharomyces yeast strain to reduce the alcohol level of Shiraz by 1.6% – made news in such publications as Scientific American.

Recently the team was able to decrease alcohol by 1.8% in both Shiraz and Chardonnay, while also dealing with an issue to do with off-flavours. Now they just have to replicate that outside the laboratory.

Dr Varela, a senior research scientist, has been working with colleagues in Australia, Chile and Spain to study the ability of non-conventional yeasts to naturally produce lower-alcohol wines that are still full in flavour.

The first stage was to determine which of around 50 strains had the most potential. Metschnikowia pulcherrima stood out, particularly in sequential inoculation with the commonly used wine yeast Saccharomyces cerevisiae (S. cerevisiae).

That approach was needed to give M. pulcherrima the best chance to have an impact. Many non-Saccharomyces strains fail to finish fermentation because they struggle to compete with S. cerevisiae, which is both fast and strong.

The results were the 1.6 per cent ethanol reduction in Shiraz and a 0.9 per cent ethanol reduction in Chardonnay (though this was accompanied by an unwanted increase in ethyl acetate). A key difference between the Shiraz and Chardonnay ferments was that the Chardonnay juice had previously been treated to remove all other microbes, but the Shiraz had not.

'We then decided to dig a little bit deeper to try to find the difference between these two', Dr Varela said. 'To do that we looked at population dynamics – at what strains were present in those ferments.'

The next step was to compare a range of Shiraz ferments including a wild ferment and one that was inoculated with M. pulcherrima. The key discovery was that when the M.pulcherrima died away it wasn’t S. cerevisiae that took over, but S. uvarum.

When all three of these yeasts were later used together on juices that had been treated to remove other microbes, the ethanol level in both Shiraz and Chardonnay was reduced by 1.8 per cent, and analysis suggested that the Chardonnay was no longer suffering issues with ethyl acetate.

The researchers have now begun pilot-scale trials at the Hickinbotham Roseworthy Wine Science Laboratory on the Waite Campus in Adelaide.

'Now we need to work with bigger volumes and with naturally-occurring yeast and bacteria coming from the grapes in the mix, to see if the two non-conventional strains are still able to deliver the goods', Dr Varela said.

'Sometimes things that work in the lab don’t work in the real world, if only because of the larger volumes. Temperature could be an issue or wild yeasts might have a negative impact. If the non-conventional yeast strains aren’t able to grow we won’t see the reduction in ethanol.

'If there is a reduction in ethanol then we have to look at sensory profile of those wines to make sure the ethanol reduction hasn’t negatively affected flavour. If there isn’t a drop in ethanol, that will make us think again – what can we do to make the growth of those two strains easier?

'The ultimate aim is to translate this into something that winemakers can use to make lower alcohol, full-flavoured wines.'