To: Virginia Vintners

From: Bruce Zoecklein

Subject: Bitartrate (Cold Stability) and Estimating Cold Stability

There is no universal definition for cold stability, which is a relative term defined differently by different producers. Two common methods used internationally for evaluating cold stability are freeze tests and electrical conductivity.

Freeze Tests

These procedures rely on the formation of KHT crystals in a suspect wine, held at reduced temperature for a specified time period. There is a wide variety of times and temperatures for this analysis. Often, a sample is frozen and then thawed, to determine the development of bitartrate crystals and whether those crystals resolubilize. The absence of crystal formation or resolubilization indicates KHT stability.

As water in the sample freezes, there is an increase in the relative concentration of all species in the sample, including alcohol, thus enhancing nucleation and crystallization. It is difficult to accurately relate crystal formation in this concetrated wine sample with KHT instability.

Preliminary laboratory treatment of samples may call for filtration. However, filtration removes crystal nuclei which may affect test results and, therefore, should not occur.

The freeze test is essentially a measure of precipitation rate, that is, the formation of nuclei (nucleation), secondary crystal growth, and subsequent precipitation. Unless one provides seed crystals, precipitation over the relatively short period of the test is, in fact, a measure of the wine’s ability to form nuclei and precipitate. Thus, the freeze test is of limited value in the prediction of bitartrate stability in wine. Despite the deficiencies, however, it is probably the most widely used technique for attempting to predict cold instability. Frequently, a companion sample is held at refrigerator temperature, and examined for protein-tannate or chill haze.

Conductivity Test

A relatively-accurate determination of KHT stability may be achieved by seeding a juice or wine sample with finely ground KHT powder. The oversaturated portion of tartaric acid and potassium deposits on the added seed. Estimation of KHT stability is then based on the reduction in electriacal conductivity of the juice or wine from the beginning to end of the test.

Changes of less than 5% in electrical conductance during the test period may be considered stable, although some define stability in more strict terms (e.g., 3% change or lower). Samples passing the test are stable only at (or above) the test temperature. In practice, most winemakers select 0°C (32°F) as stabilization temperature for whites and 4-5°C (39-41°F) for reds.

The conductivity test provides a value that is unique for the product tested. Complexing factors, that may be present and possibly affect KHT crystal formation, are taken into account.

Correction of Bitartrate Instability

KHT stability has been traditionally accomplished by chilling (with or without seed). In conventional cold stabilization (chill-proofing), wines are chilled to a temperature designed to decrease KHT solubility to a level that, optimally, results in precipitation of the salt. Important variables affecting precipitation of KHT include (1) the concentration of reactants, specifically tartaric acid and K+; (2) availability of nuclei for cystal growth; and (3) the solubility of potassium bitartartrate formed. A formula for estimating stabilization temperature was given in the previous Enology Notes.

Conventional cold stabilization may last for several weeks or more. Dessert wines with higher alcohol and sugar levels require lower temperatures and longer holding times. Due to the energy costs involved, alternatives have been developed to accelerate the cold stabilization process.

Seeding

Seeding (contact seeding) techniques are used as a means of reducing time and improving efficiency. Addition of an excess of finely powdered KHT creates a supersaturated solution. The enormous surface area of powdered KHT reduces or eliminates the energy-consuming nuclei induction phase, and allows for immediate crystal growth.

Several processing considerations are important in achieving stability using seeding, including quantity of KHT, crystal size, time, and temperature.

Because of the involvement of complexing agents in precipitation equilbria, the amount of KHT seed used must produce a supersaturated solution. Optimal levels of seed may be up to 4 g/L. However, stability may be achieved using lower concentrations. Reductions in the quantity of seed used prolongs the contact time needed for stabilization. Seed particle size should be about 40 µm. Because the reaction rate depends on available surface area, use of larger particles will slow the rate of crystal growth and extend processing time.

Seeding should be conducted in tanks where adequate mixing can be accomplished. The density of KHT is greater than juice and wine. Crystal growth is dependent on available interactive surface area. Therefore constant agitation is essental.

Bitartrate seeding can occur at any temperature. However, treatment temperature should be identical to desired stability temperature. For example, many winemakers seed white wines at 0°C (32°) and reds, when stabilized, at 4-5°C (39-41°F). If stabilization is correctly performed, wines held at or above these temperatures will be stable with respect to KHT precipitation.

During the first hour of contact seeding, there is a rapid reduction in tartaric acid and potassium. This reduction slows after the first hour and then levles off at the end of 3 hours in most wines. Using 40 µm KHT seed allows stabilization to occur in only 90 minutes. Reduction in the quantity of seed used prolongs the stabilization period.

Table. Influence of potassium bitartrate seed (40µM) level on wine components at 0°C.

Source: Blouin et al. 1982.

Filtration of the wine after contact seeding is essential to prevent resolubilization of KHT crystals. This step must be performed at the same temperature as the stabilization procedure.

Subscription to Enology Notes. All past Enology Notes and Vintner's Corner newsjournals are posted on the Enology-Grape Chemistry Group's web site at: www.fst.vt.edu/zoecklein/index.html. Enology Notes are different in content from this subscription-based Vintner's Corner newsjoural. To be added to the Enology Notes list serve send an email message to bzoeckle@vt.edu with the word "ADD" or "REMOVE" in the subject line.

Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head, Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech, Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293
E-mail: bzoeckle@vt.edu