In Processing

“Diamonds are forever,
They are all I need to please me,
They can stimulate and tease me…”

Shirley Bassey’s lyrics from the famous 1971 James Bond Movie, Diamonds are Forever. In the movie of course, he enjoys a very fine sherry. Interestingly enough, also a wine which was associated with the occurrence of “wine diamonds”, or tartrate crystals as a function of the presence of potassium, particularly in the case of sherry, calcium tartrates as a function of fermentation in traditional cement tanks or treatments with calcium sulphate to lower the pH.

Nowadays, as a result of many technologies and additives, the occurrence of this type of instability (regarded as a wine fault, mostly by consumers who may associate it with glass crystals) can be prevented.

The traditional way of course to rid the product of these crystals is to keep a still wine at -4oC for a week, and a fortified wine at -7oC. This would give the tartrate crystals enough time to crystallize and settle and the clear wine can be racked off the sediment. The downside of this technique though is the time it takes, energy required and other variables that may induce poor nucleation. Variation of “traditional cold stabilization” therefor exists and is known as contact stabilization. The addition of potassium bi-tartrate crystals is to increase the rate of crystallization and to decrease the energy required after which the wine is separated from the crystals.

During the 1950’s though, Down Under came up with even a more innovative way of preventing wine diamonds, called ion-exchange. Used both for acidification and lowering the pH, treatment of wine leads to a decrease in other cations like K+and Ca2+. The process entails a cation-exchange resin to be charged with hydrogen ions and passing wine through the resin where the hydrogen ions are released and K+ and Ca2+ are taken up by the resin.

Electrodyalysis (ED) became a sudden buzz word the past few years. ED is a powerful technique for selectively removing excess potassium and bi-tartrate ions that cause instability. The principle of ED is based on the flow of wine through a stack of alternating cation- and anion-selective membranes, in parallel with “brine” under an electrically charged field. Thinking back to science classes in school, K+, Ca2+ and Na+ will be attracted to the negatively charged cathode and conversely the HTa (bi-tartrate ions) to the anode.

Off course we live in the Fast Food era, and there is basically a quick fix for most challenges. Such a fix is MTA (Meta-Tartaric Acid) and CMC’s (Carboxy Methyl Cellulose).  MTA acts as a nucleation inhibitorby sheathing the crystals of potassium bi-tartrate and calcium tartrate, thus preventing crystal growth. This unfortunately is not a permanent reaction, as it slowly rehydrates to tartaric acid over time, especially if the wine is exposed to higher temperatures. CMC is used in food science as a thickener and to stabilize emulsions in various products like ice cream. It is also a constituent of many non-food products, such as K-Y Jelly, toothpaste, laxatives, diet pills, water-based paints, detergents, textile sizing and various paper products. Although in some products it has the ability to lower freeze points and subsequent crystal forming, e.g. white wine, the results on some rose wines and particularly red wines, are not favourable.

Mannoproteins colloids in wine with a carbohydrate part of 70% and 30% part protein, occurs in wine naturally as a result of autolysis of yeasts. It was found not only to reduce a white wine’s requirement for bentonite treatment to enhance heat stability, but also increase a wine’s tartrate stability. The instability of the phenolic matter in red wines due to many variables may compromise potassium bi-tartrate stability over time. Under these conditions, the anti-nucleation effect of mannoproteins can be reduced.

So, as they say, “different strokes for different blokes”. With all additives and actions you have to take full cognizance of potential upsides and downsides… or else I suppose, diamonds may well be, forever.


Bertus Fourie is a winemaker, turned Enology lecturer and creator of the Barista coffee Pinotage.

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