Measurement of cold stability of wine

During the primary fermentation of wine, the increasing alcohol content reduces the solubility of potassium bitartrate (KHT) present in the juice, resulting in the supersaturation of the young wine with respect to KHT. As the solubility of KHT is temperature-dependent, excess KHT can then be precipitated out of solution with time and on exposure of the wine to cold temperature. Therefore, a test for cold stability gives an indication of the likelihood that precipitation of KHT will occur after bottling.

This document gives a brief summary of the procedures and equipment requirements for some commonly used techniques for determination of cold stability in wines.

Refrigeration

Description: This test involves holding a sample of filtered wine at -4C for three days and then inspecting for any sign of crystalline tartrate precipitation. If crystalline deposits are observed to redissolve upon warming to ambient temperature the wine is generally interpreted as being cold-stable, but the presence of any persistent crystalline precipitate is taken to indicate that the wine is cold-unstable. The results should be expressed as a ‘pass’ if there is no permanent crystalline deposit after refrigeration, or ‘fail’ if crystals are present. Note that some phenolic material, i.e. amorphous pigments and tannins, might form a transient precipitate from some red wines; allowing the sample to warm up to ambient temperature usually results in their resolubilisation, which differentiates these precipitates from potassium bitartrate crystals.

Equipment: Water bath (-4C), membrane filtration apparatus
Reagents: None
Services: Wash up area, power
Space required: Bench space for water bath

Freeze/thaw

Description: This method is a variant on the refrigeration technique, but is believed by some industry practitioners to provide less precise results (Wilkes, personal communication, 2004). A sample of filtered wine is frozen for a set period of time (e.g. overnight) and then thawed prior to inspecting for any sign of crystalline tartrate precipitation. If crystalline deposits are observed to redissolve upon warming to ambient temperature, the wine is generally interpreted as being cold-stable, but the presence of any persistent crystalline precipitate is taken to indicate that the wine is cold-unstable. It should be noted that many industry practitioners feel that this test is too harsh and therefore have found that freezing until a slurry is formed and then thawing is more appropriate (Wilkes, personal communication, 2004).

Equipment: Freezer (domestic is suitable), membrane filtration apparatus
Reagents: None
Services: Wash up area, power
Space required: Minimal bench space

Conductivity (contact process)

Description: By observing the change in conductivity of a stirred sample of filtered wine held at a given temperature (0C for white and 5C for red wines; Zoecklein et al. 1989) upon addition of potassium bitartrate seed crystals (10 g/L), the tartrate stability of a wine at or above that temperature can be determined. If the change in conductivity of the sample is 5% or more (due to precipitation of KHT and a resulting decrease in conductivity), the sample is considered to be unstable with respect to KHT. It should be noted that some industry practitioners vary the temperature at which this test is conducted (e.g. within the range of -4 to 5C) to reflect what they believe is more relevant to their wine styles (Wilkes, personal communication, 2004). It must be emphasised that the chosen test temperature should be validated to improve confidence in the results.

Equipment: Conductivity meter (100-1000 mS/cm), water bath, magnetic stirrer, membrane filtration apparatus, thermometer
Reagents: Potassium bitartrate (cream of tartar)
Services: Wash up area
Space required: Bench space for water bath and conductivity meter

Concentration product

Description: The stability of wine with respect to KHT precipitation can be predicted on the basis of the concentration product (CP). This is calculated from the concentrations of potassium, tartaric acid (total tartrate), pH and alcohol in the wine, using the following formula:
CPKHT = ((Potassium, mol/L) x (Tartaric acid, mol/L) x (% bitartrate ion))/100
= ((Potassium, mg/L)/39.1 x 1000) x ((Tartaric acid, g/L)/150.1) x ((% bitartrate ion)/100)

The % bitartrate ion, i.e. the percent of tartaric acid present in the bitartrate form, can be obtained from the pH and alcoholic strength of the wine, using the tables derived by Berg and Keefer (1958). The CP can also be calculated from potassium, tartaric acid, pH and alcoholic strength using a ‘Miller Dial’ sliding scale KHT stability calculator. The CPKHT value thus obtained is compared with historical data for the appropriate wine type; some alternative criteria for establishing tartrate stability have been proposed:

  • DeSoto and Yamada (1963) suggested safe CPKHT values at 0C for nine different wine types; if the CPKHT of a sample exceeds the recommended safe level for that wine type, then the sample can be considered to be cold-unstable.
  • Berg and Akiyoshi (1971) suggest a maximum CPKHT value of 9.4 x 10-5 for white wines, and a maximum CPKHT value of 17.6 x 10-5 for red wines; wines with a CPKHT value greater than these maxima can be considered to be unstable.
  • Leske et al. (1996) suggest a maximum CPKHT value of 8.0 x 10-5 for white wines, and a maximum CPKHT value of 18.0 x 10-5 for red wines; wines with a CPKHT value greater than these maxima can be considered to be unstable.

Equipment: Analytical capability for the determinations of potassium, tartaric acid, pH, and alcohol
Reagents: As per individual determinations
Services: As per individual determinations
Space required: As per individual determinations

References and further reading

  • Amerine, M.A.; Ough, C.S. (1980) Methods for analysis of musts and wines. New York Wiley-Interscience.
  • Berg, H.W. (1960) Stabilisation studies on Spanish sherry and on factors influencing KHT precipitation. Amer. J. Enol. Vitic. 11: 123-128.
  • Berg, H.W.; Akiyoshi, M. (1971) The utility of potassium bitartrate concentration product values in wine processing. Amer. J. Enol. Vitic. 22(3): 127-134.
  • Berg, H.W.; Keefer, R.M. (1958) Analytical determination of tartrate stability in wine. 1. Potassium bitartrate. Amer. J. Enol. 9: 180-183.
  • Bertrand, G.L.; Carroll, W.R.; Foltyn, E.M. (1978) Tartrate stability of wines. 1. Potassium complexes with pigments, sulphate and tartrate ions. Amer. J. Enol. Vitic. 29(1): 25-29.
  • De Soto, R.T.; Yamada, H. (1963) Relationship of solubility products to long range tartrate stability. Amer. J. Enol. 14: 43-51.
  • Ewart, A.J.W. (1984) A study of cold stability of Australian white table wines. Aust. Grapegrower Winemaker 244: 104-107.
  • Iland, P.; Ewart, A.; Sitters, J.; Markides, A.; Bruer, N. (2000) Techniques for chemical analysis and quality monitoring during winemaking. Campbelltown, SA Patrick Iland Wine Promotions.
  • 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. Adelaide, SA: Winetitles; 1996: 39-45.
  • Pilone, B.F., Berg, H.W. (1965) Some factors affecting tartrate stability in wine. Amer. J. Enol. Vitic. 16(4): 195-211.
  • Rankine, B.C. (1998) Making good wine: a manual of winemaking practice for Australia and New Zealand. South Melbourne, Sun Books (Macmillan Australia).
  • Wilkes, E. (2004) Technical Laboratory Manager, Beringer Blass Wine Estates, Nuriootpa SA, personal communication.
  • Zoecklein, B.W.; Fugelsang, K.C.; Gump, B.H.; Nury, F.S. (1989) Production Wine Analysis. AVI Van Nostrand Reinhold. New York: 291-305.
  • Zoecklein, B.W.; Fugelsang, K.C.; Gump, B.H.; Nury, F.S. (1995) Wine analysis and production. New York Chapman & Hall.