About half of all post-bottling wine instabilities identified by the Australian Wine Research Institute (AWRI) helpdesk are due to ineffective heat or cold stabilisation. Geoff Cowey, Adrian Coulter and Eric Wilkes have provided their advice for getting it right.
Most wineries remove unstable proteins from white wine before bottling by fining with bentonite. Often winemakers use the same dose each year, yet the amount of protein present in grapes is affected by ripeness and disease and so will differ from year to year. A bentonite trial should be carried out each year to calculate the minimum amount required to reduce protein levels and infer protein stability.
Errors can occur when performing a bentonite fining trial. Often people will perform trials in the lab using a different type or concentration of bentonite, or bentonite made up with a different type of water than that used in the cellar. Sometimes a sample of juice or wine may be taken that isn’t representative of the whole tank. These all affect the bentonite dosage required and may result in over- or under-fining of the wine. Cellar conditions should be matched exactly during a lab trial.
If wines are fined with protein-based additives such as egg or milk after bentonite fining, or if grape juice concentrate is added to sweeten the wine before bottling, then there is a chance that extra protein will be added to the wine, potentially making it unstable. If the acidity of a wine is adjusted, or copper is added before bottling, this can also affect stability. Thus, the heat stability should be checked after all additions have occurred in case a final bentonite fining is required.
Interpretation of the heat test is done using turbidity measurement. The standard stability test involves heating 0.45 µm filtered samples at 80°C for two hours (minimum) and up to six hours. A stable wine has a difference in turbidity of <2 NTU between a heated and a non-heated sample, although some labs like AWRI Commercial Services often use a tighter difference of <0.5 NTU. Heated samples should be left to cool to 20°C before measurement. A minimum of two hours cooling is required for the heated proteins to aggregate and precipitate so chilling on ice quickly to lower temperature is not recommended.
A common error with cold stabilisation is that the wine is allowed to warm up before racking/filtering off the precipitated potassium bitartrate (KHT) crystals. This allows some KHT to re-dissolve into the wine and the wine is thus only stable to the temperature it was filtered at.
As with heat stabilisation, if any additions, acid adjustment or blending occurs after cold stabilisation then the stability will be affected and the wine will need to be rechecked for cold stability. As a wine matures it can also become cold unstable due to the precipitation of other wine compounds, such as polyphenolics, which can then affect the wine matrix and stability. Ideally the final blend going to bottling should be checked just before being bottled.
Sparkling wine producers stabilise their base wines to prevent KHT precipitation in the final sparkling wine after secondary fermentation, using a modified version of the cold stability test. Realising that secondary bottle fermentation will add 1 to 1.5 per cent alcohol, which will affect the wine matrix and thus cold stability, they fortify a small quantity of the cuvee and perform a cold stability test on the fortified sample.
Some winemakers may use electrodialysis to remove potassium and tartrate ions. Crystallisation inhibitors – such as metatartaric acid, mannoproteins or carboxymethylcellulose (CMC) – can also be used to prevent crystal formation. These are rarely used without some form of prior cold stabilisation, and are often used as an extra guarantee of stability for higher risk wines.
For more information or assistance, contact the AWRI helpdesk on (08) 8313 6600 or email@example.com.