Researchers have developed a colorimetric tool with the potential to minimise white wine spoilage during production caused by excess copper.
Its creators say the test will allow winemakers to assess the amount of copper in their white wine after fermentation, during a copper fining trial, after copper addition, after bentonite fining and/or after membrane filtration.
Copper and iron are known to cause oxidation and reduction reactions (‘redox’ reactions) in wine that can influence its flavour. However, prior to this project, little was known about the key wine components binding the metals, or the impact of such binding on the rates of oxidative and/or reductive processes in wine.
A research team from the National Wine and Grape Industry Centre/Charles Sturt University and the Australian Wine Research Institute, led by Andrew Clark carried out a Wine Australia funded research project to better understand how the presence of different forms of metals (metal ‘species’) can influence ageing of bottled wine.
Once we understood which metal species were the active ones and what levels they exist in wine, then a simple tool to measure the important ones could be developed.
The team assessed numerous way of measuring copper and iron species in wine, and then surveyed a wide range of Australian wines.
They found that copper primarily exists in a sulfide-bound form. Dr Clark said the form of copper in wine was only significantly important to wine oxidation when ascorbic acid was present in the wine.
‘The sulfide-free form catalysed oxygen consumption by the wine much faster than the sulfide-bound form.’
‘The sulfide-free form of copper could also readily sequester sulfide from precursors under reductive conditions. Because most wines have sufficient sulfide precursors, most of the copper ends up in the sulfide-bound form during bottle ageing’, Dr Clark said.
National Wine and Grape Industry Centre researchers Dr Andrew Clark (right) and Dr Nikolaos Kontoudakis performing the colorimetric analysis of copper in wine.
These sulfide-bound form of copper do not appear to precipitate, as has been widely assumed. The team found that the particle size of the sulfide-bound copper was sufficiently small to preclude removal by filtration on the basis of size. Membrane filters did have some limited capacity to adsorb some sulfide-bound copper when used on a small scale, but it is likely that pre-bottling filtration does not significantly impact on the levels of sulfide-bound forms of copper.
Unlike copper, iron-binding in wine was not dominated by a single binding agent and the forms of iron had minimal influence on oxidation in the presence of ascorbic acid.
Using the data, the team developed a colorimetric tool to test for total copper concentration in white wine.
The test uses an addition of silver to wine in order to effectively release copper from sulfide and allow its determination by a colorimetric reagent. The process is rapid and only requires a spectrophotometer with the ability to hold a larger than normal sized cuvette (a container for holding liquids samples in a spectrophotometer).
Dr Clark said the test would be of great benefit to winemakers.
‘Until now, it has been not possible for winemakers to measure copper concentration of their wine without sending samples away for analysis. This is despite the fact that all wines will contain copper, and the concentration may depend on the amount present on the grapes prior to fermentation.’
He said while yeast are able to remove a large portion of copper, studies have shown in Chardonnay that higher concentrations of copper in the juice have led to wines of increased copper concentration.
‘The colorimetric test will allow winemakers to more easily assess the amount of copper in their white wine throughout the production process.’
Dr Clark said the technique was being further refined to allow measurement of total copper in red wine.
‘We are also working on the measurement of sulfide-free copper in white wine and subsequent determination of the sulfide-bound copper concentration in white wine.’
The final report from the project can be downloaded here. A step-by-step guide to the test is available on the NWGIC website here (PDF).