Metals in wine is a complicated story
Copper in wine used to be bad, now it’s good – but it’s important to put it in at just the right time.
Research at Charles Sturt University’s (CSU’s) National Wine and Grape Industry Centre (NWIGC) has discovered that adding a little copper to white wine prior to bottling ‘just in case’ is usually ineffective and can actually be counterproductive. This follows up research undertaken by Dr Liz Waters and her team at the AWRI before she moved to Wine Australia.
The whole issue has come to light since the sector’s wholesale adoption of the screwcap. The big positive of the screwcap – that it keeps oxygen out – is also a minor negative because, if wine has no access to oxygen, certain sulfur compounds can start to accumulate over time, creating unpleasant odours.
The common response is to add copper (as a salt), which attaches to the sulfur compounds and converts them into a form that doesn’t smell. However, research and anecdotal evidence have started to show that some finished white wines have increased copper concentrations as well as the reductive characteristics caused by the very compounds the copper is supposed to eliminate.
‘That suggested there was a bit of a problem’, said Dr Andrew Clark, with just a touch of scientific understatement.
A Senior Lecturer in Wine Chemistry at CSU, Dr Clark worked with colleagues at the NWIGC (Ms Paris Grant-Preece), the University of Melbourne (Prof Geoffrey R Scollary) and Yalumba (Ms Natalie Cleghorn), to try to get to the bottom of the issue.
Their project, which won the award for best oenology paper (sponsored by Wine Australia) at this year’s ASVO awards, presented the bad news that copper added late in the process cannot simply be filtered out, and actually remains active in the wine.
‘This was a big surprise’, Dr Clark said. ‘We found that if we add copper to finished wine, even if it does attach itself to sulfides, the particles generated are too small to filter out. So at that stage you aren’t going to remove the material.
‘Also they are not inert and locked away as had been assumed. Copper in the form of copper sulfide is reversible and can still participate in other reactions in the wine, which isn’t desirable. The copper added to solve a problem can end up increasing the problem.’
There is good news, however. A subsequent project found that copper present in white wines, when there are still proteins in the wine, can be efficiently removed by the bentonite fining process used to remove protein. ‘It’s not guaranteed but it’s a much safer bet’, Dr Clark said.
More research is needed to determine the best approach with red wines, for which bentonite fining is not part of the production process.
Dr Clark and colleagues started with white wines because they had already looked at how copper can cause other reactions in white wines, including oxidation. A comprehensive review of all the recent chemistry knowledge around copper in wine appears in the current edition of the ASVO’s Australian Journal of Grape & Wine Research.
‘It is quite interesting that 80 years ago the concentration of copper in wine was a big problem because they used lots of brass equipment and to get rid of it they were adding sulfides such as hydrogen sulfide’, Dr Clark said. ‘They were doing the opposite of what we do today.’
Dr Clark is also leading another Wine Australia-funded collaborative project between NWGIC and the AWRI looking at the impact of both copper and iron in wine.
‘Metals are quite efficient catalysts of reactions – they can speed them up, even at small concentrations – and the consequences of those reactions can sometimes have a big effect on wine colour and sensory aspects’, he said.
‘Our new project is about measuring different forms of metal ions. Traditionally we have just measured total concentration and related that to some of these spoilage problems. Now we are measuring the different specific forms and seeing if that gives better indicators of spoilage.’
The ultimate aim of the three-year project, which is due for completion in mid-2017, is to develop analytical tools for winemakers to evaluate the potential of their wines to be spoilt.