Spring season in the wine industry generally means one thing – bottling time. Bottling may be a year-round operation for larger wineries, but most small to medium facilities generally dedicate a portion of spring into early summer for bottling the fruity whites and rosés from the just-finished harvest, and the reds and aged whites from the previous year’s efforts.
Bottling also means a ramp up in the downstream processing that comes with winemaking after the fermentations are complete. Blending, fining, filtration, and any last treatments or adjustments are made to your wines so you can put the best product in the bottle for sale.
One of the common treatments performed, but often poorly understood, in the winemaking process is cold stability treatments. A number of wineries will simply perform this treatment in the belief that their wines require it. However, a bit of bench testing in the lab can often show wines already possess the physical stability that the treatment is meant to provide. The ability to glean that knowledge can bypass what is often a costly labor and resource process in winemaking, and also spare extra handling of wines to better preserve wine quality.
The Physical Process of Cold Instability in Wines
Cold instability is the penchant for your wines to develop acid salt crystal precipitates from supersaturated metal cations and acid anions. Theoretically this can be any combination of common metal cations in wine (K+, Ca2+, Na+, Mg2+, etc.) with any common acid anion (bitartrate, tartrate, malate, bimalate, citrate, etc.). Far and away though, the combination of most concern is the formation of potassium bitartrate (KHT), as the concentrations of that cation/anion pairing are most likely to reach supersaturation points versus any other. The resulting precipitate is often confused by some consumers as looking like glass shards in the bottom of the bottle.
The precipitation most often occurs when the wines are chilled, as the drop in temperature reduces the overall solubility of KHT in the wine. Whether crystallization happens is dependent on the mechanics of crystal formation, which is a whole chapter unto itself in a chemistry textbook. Two factors often contribute though: any agitation in the wines, and whether there is a nucleation seed point for crystals to form and grow on (such as the porous, uneven surfaces on the underside of a wine cork).
The common cold stability treatment method for most wineries is to force a wine into those three conditions just mentioned by chilling the wine to near freezing, seeding it with excess KHT, and then stirring to agitate the wine and expose the liquid to as much surface area of the acid crystals as possible.
Bench Testing Cold Stability in Wine
It’s possible to test for the cold stability of your wine in the wine lab though to determine if performing a cold stability treatment is even necessary. The most common methods used are to hand-bottle a couple samples of the wine and chill it to near freezing to see if any crystals form, to evaluate the Concentration Product values of KHT at a given temperature and alcohol in your wine, or to test for conductivity in the wine.
Alternative Cold Stability Testing in Wines
The freeze test is a crude cold stability treatment that looks at one of the factors associated with promoting crystal formation in wine – a reduction in temperature. The largest benefit is that it mimics the conditions your wine will be in once you’ve bottled the product. Also, it’s a very simple and inexpensive test to run. The main drawback is that it is often inconclusive as passing a freeze test for a few days does not guarantee that as the wines age over time in the bottle that changing conditions won’t promote crystal formation at some point. Plus, the test does not account for the other two drivers of crystal formation which are seeding and agitation.
Concentration protocols are based off the work of several researchers looking at the general solubility of potassium-based acid salts in juices and fermented beverages by examining the concentrations of potassium and the various speciations of tartaric acid across a range of temperatures and alcohol concentrations (Berg and Akiyoshi, De Soto and Yamada, Leske et al.). While the calculations are all based off of empirical physical properties observed in solubility, the main drawback is that wines often have complexing colloidal factors that are able to stabilize KHT at levels above what are normal saturation points in model solution, and there is no way to predict those stabilizing factors, or lack of them, in any given wine. As a result, this method has fallen out of favor as testing standard.
Conductivity Testing in Wine for Cold Stability
The recommended method for testing KHT stability in wines is by measuring the wine’s conductivity under conditions that promote crystal precipitation. K+ ions are the main source of conductance in juice or wine. The loss of K+ ions in crystallization and precipitation will then give a measurable drop in electrical conductivity if measured in wines.
Testing involves the use of a conductivity meter capable of reading through the range of 100 µS/cm to 10 mS/cm with an accuracy of 0.5% or better. A benchtop scale of a seeding treatment is performed by chilling a wine to near freezing temperatures, then taking a baseline conductivity measurement while stirring the sample. An excess of KHT is then added and conductivity is observed over a 20 or 30 minute timeframe.
Any gain of conductivity means that K+ ions are dissolving into solution and the wine is below saturation levels and stable from KHT precipitation. No change in conductivity means the wine is stable under current temperature conditions, but my become unstable at lower temperatures. A drop in conductivity means the wine is precipitating KHT. There is some subjectivity in evaluating the last condition, as a drop does not necessarily mean your wine is unstable. The industry standard for declaring instability is generally defined as a drop of 5% in conductivity over the time of the test. Many people prefer a more stringent 3% standard, and there are many cautious winemakers who declare any reduction in conductivity as a wine being unstable for KHT precipitation.
*What KHT can look like in a glass.
Solutions for Cold Stability Testing in Wine from Hanna Instruments
Performing conductivity testing for the average wine lab is fortunately a fairly simple and inexpensive test. Many pH meters come capable of measuring electrical conductivity as well with the simple addition of a conductivity probe.
The edge meter from Hanna Instruments is an ideal solution for your wine lab EC needs. Our base HI2020W edge unit come ready for pH measurements, and only requires the addition of our HI763100 EC probe to add EC testing to your wine lab repertoire.
For a small investment in a simple test, you can take control of your cold stability needs and determine whether your wines require treatment or not, potentially saving large amounts of time, energy, materials, and money. Not to mention improving the overall quality of your wines.
For more information on testing, contact us using one of the channels below!
Berg, H.W., Akiyoshi, M. 1971. The utility of potassium bitartrate concentration product values in wine processing. J. Enol. Vitic. 22(3): 127-134.
De Soto, R.T., Yamada, H. 1963. Relationship of solubility products to long range tartrate stability. J. Enol. 14: 43-51.
Leske, P.A., Bruer, N.G.C., Coulter, A.D. 1996. Potassium tartrate – how stable is stable? Stockley, C.S., Sas, A.N., Johnstone, R.S., Lee, T.H. (eds) Proceedings of the ninth Australian wine industry technical conference; 16-19 July 1995. Adelaide, SA: Winetitles: 39-45.