- Potassium bitartrate (KHTa) is formed in wine, through the reaction between the bitartrate ion (HTa-), from tartaric acid (H2Ta), and the potassium ion (K+) found in grapes, especially grape skins.
K+ + HTa- <==> KHTa
- Synonyms for KHTa (potassium bitartrate) are potassium salt of tartaric acid, tartrates, argols, tartres (French), Weinstein (German - "wine stones")
- The Potassium bitartrate (KHTa) precipitates out as crystals when the solution is supersaturated with KHTa.
- The increased alcohol levels produced during fermentation"
This increases the potential tartrate deposits.
- increases the concentration of KHTa, through increased K+ (potassium ion) extraction from (red) grape skins
- and decreases the solubility of KHTa.
- These tartrate crystals are quite apparent at the bottom and sides of fermentation vessels and can occasionally be found in bottles of red wine as red stained crystal deposits or attached to the bottom of the cork.
- With time, old bottles of Sauternes can develop thin sheets of fine KHTa (potassium bitartrate) crystals called butterfly or angel's wings by the French.
- It is because of the thermodynamics involved, that it requires a long time for the solution to change from a supersaturated solution to a reasonably stable, just saturated solution, through the deposition of KHTa crystals. Even then, a reduction in temperature can see further crystal formation.
- The modern practice of cooling bulk white wine down to -5 to -10 degree C, to reduce the solubility of KHTa, and feeding the wine with very fine KHTa crystals, to provide a substrate for crystal growth, sees most of the excess KHTa crystalise out, which is then removed, leaving a stable wine in regard to KHTa.
- Because of the extra time red wines spend in oak and exposed to colder winter temperatures, sees most of the tartrates deposit out in the oak barrels, leaving bottles of red wine less susceptible to KHTa deposits and what little deposit occurs with time has a greater consumer acceptance than it has with clear white wines.
Potassium bitartrate formation
- Put simply, one of the laws of thermodynamics states that the universe is heading and will spontaneously head or be driven towards the lowest state of energy.
- It requires energy from the universe to maintain order in a similar way that it requires energy to tidy a room.
It is simply a matter of odds. .
The odds of your room, through random activity, resulting in a ordered, tidy room is quite unlikely, rather the reverse is far more likely.
- Therefore the odds of an ordered crystal formation of KHTa is an unlikely event requiring an input of energy.
- The sum of the individual energy states of K+ and HTA- ions are higher than the energy state of a KHTa molecule.
- To satisfy the drive towards a lower energy state means that there is a drive towards KHTa formation.
- The drive towards KHTa formation and the resistance towards an orderly KHTa crystal formation are delicately balanced, although balanced in favour of KHTa formation (in a supersaturated solution).
It just requires time and favourable circumstances.
- Favourable circumstances include collision between a number of K+ and HTA- ions with the correct orientation to bond with the correct formation to provide the required template for crystal formation and growth.
- Solid impurities in the wine and irregularities of container walls act as nucleating points for crystal formation.
That is why they form crystal growth patterns matching container wall abrasions, deposit out and grow on the underside of the cork.
- The addition of finely ground KHTa crystals to a supersaturated solution of KHTa tips this energy balance well over to the formation side and will deposit the bulk of supersaturated solution within 24hours at very low temperatures..