Tartrate crystals that occur naturally in chilled unstabilised wine, create an unacceptable perception among consumers due to the visual impact of the crystals. Tartrate stabilisation of packaged wine is therefore essential
|so that consumers are not unnecessarily upset by the possible occurrence of crystals in wine.
The concentration of potassium bitartrate (cream of tartar) in unstabilised wines always exceeds that which is practically soluble, especially if the wine is chilled. Higher alcohol content and lower temperatures promote the natural precipitation of the crystals. However, it may take a fair amount of time for the oversaturated tartrates to be precipitated from the wine and consequently cellars prefer to accelerate the natural process.
The traditional method of tartrate stabilisation involves promoting the precipitationof cream of tartar by storing the wine for a certain period at a temperature just above the freezing point of the wine. This creates an oversaturated wine solution and the excess cream of tartar will be precipitated as crystals. The precipitation process does not take place, however, unless certain favourable conditions are created. The precipitation process entails two phases, i.e. firstly the formation of a crystal nucleus that initiates and promotes the precipitation process, and secondly the formation of sufficient crystal around the nucleus to induce precipitation. By simply relying on reduced temperature to enable these processes to take place, it may take too long and therefore it is often promoted by the addition of fine tartaric acid crystals that are used as a nucleus for the crystal growth process. However, the cold stabilisation of wines to prevent tartaric instability has several disadvantages:
The energy cost is high.
Considerable infrastructure is required and the maintenance thereof is expensive.
It is time-consuming and requires a great deal of manpower.
The chemical composition of the wine is altered seeing that potassium bitartrate (cream of tartar) is removed from the wine.
Despite the above disadvantages tartrate stabilisation of wine is essential from a consumer point of view. Any alternative method of tartrate stabilisation should therefore address the processes of tartrate precipitation, i.e. the formation of crystal nuclei and crystal growth.
It has been noted that wines which mature on the lees have greater tartaric stability. This is the result of the mannoproteins released during yeast autolysis. Research on model wine solutions has proved that mannoproteins inhibit the crystallisation of potassium bitartrate. These react as a protective colloid which covers the surface of the crystal nucleus used for the crystallisation and the crystallisation process is therefore unable to proceed. As a result Laffort Oenologie has developed a purified mannoprotein product, known as Mannostab, for tartrate stabilisation of wine. The product is soluble and has no colour, flavour or taste. Use thereof has already been sanctioned in the European Union and Argentina and is currently also being investigated for permissibility in Australia and the USA. (In South Africa it is not yet permissible.) Mannostab must be added to the wine the day before bottling at a dosage of 200 to 250 mg/l and good mixing is obviously important.
The use of mannoproteins should not be confused with meta tartaric acid. The latter is a polymer of tartaric acid that is able to create temporary tartrate stability in wines. Although it functions in a similar way to the mannoproteins, the meta tartaric acid eventually hydrolises and long term tartrate stability is not obtained. (Bowyer and Moine-Ledoux, 2007)
Bowyer, P.K & V. Moine-Ledoux. 2007. Mannostab: the award-winning new potassium bitartrate stabilisation product.
Austr & New Zealand Grapegrower & Winemaker. June 2007: 57-62