Wine can often contain unstable proteins that will cause a haze to develop if the wine is subjected to cold or hot temperatures, or over time. These must be removed before bottling.

Only white wines suffer from protein instability as the phenolics in red wine precipitate out any proteins early on in the winemaking process.

The most common test used for unstable proteins is to immerse a 10 ml sample of wine (in a test-tube) in a hot water-bath for a period of time. then leaving it to cool. Unstable protein can be seen as a haze, which later collects as a ‘milky’ deposit at the bottom of the tube. Alternatively, a turbidity-meter can be used to measure the cloudiness of the tube and compare it with an unheated control. The temperature and time will depend upon country of production and likely market. UK wine is often done for 15 minutes at 70°C, while Australian wine shipped across the equator is done for 6 hours at 80°C.

The most common treatment for wines with unstable proteins uses bentonite: a refined clay discussed in the previous session on fining agents. However, bentonite will strip the wine of colour and flavour, and so it is important to add as little as possible to the wine. In order to do this, a bentonite trial must be carried out:

1. Fill 10 x 10 ml test tubes with clear wine
2. Add increasing doses of bentonite to each tube (0 – 1 g/l). Note that bentonite often needs swelling in water for 24 hours prior to addition to wine.
3. Shake well at cellar temperature
4. Centrifuge or filter the wine to remove the bentonite.
5. Carry out protein stability tests on all the tubes
6. Note the minimum concentration of bentonite that is required to prevent any deposit from forming, and add this to the wine in question, ensuring that it is well mixed into the wine as quickly as possible.
7. After bentonite addition to the actual tank, re-test the wine to ensure the addition has been successful

The bentonite is then filtered off the wine or allowed to settle.