Heat instability of wine is generally due to precipitation of unstable grape proteins. Most frequently, this is of interest in white table wines. Most wineries remove unstable proteins from white wine before bottling by fining with bentonite. The bentonite reduces the amount of protein in the wine to levels where protein will not precipitate over time, but several factors can affect this stability and it is important to test heat stability prior to bottling. Several tests are available to check stability and a comparison of some of these is given by Toland et al. (1996). This page gives a brief summary of the procedures and equipment for some commonly used tests for heat stability of wines.
Description: A filtered wine sample is heated to 80C for two hours (minimum) and up to six hours and then, after cooling, examined by eye against a light source for the presence of a haze in comparison with an untreated control sample of the filtered wine. A minimum of two hours cooling is required for the heated proteins to aggregate and precipitate, so chilling on ice quickly to lower temperature is not recommended. Narrowing the focus of the light beam so as to give a cylindrical shaft of light of approximately 3 mm in diameter helps to illuminate suspended particles responsible for haze. A turbidimeter can be used for more objective comparison of the turbidity in the two samples. In this case, wines that exhibit a turbidity increase of greater than a given criterion for Nephelometer Turbidity Units (NTU) after heating, as compared with the unheated control, can be considered to have failed the heat stability test. Some laboratories use a criterion of 0.5 NTU, but other practitioners in industry have indicated that a more reasonable criterion is 2.0 NTU (Wilkes, personal communication 2004). Addition of 0.5 g/L of tannic acid makes the test more stringent in simulating cork contact (Rankine 1989). Further information about the heat test can be found here.
Equipment: Containers for wine samples (e.g. 25 mL screw capped glass tubes), water bath or heating mantle, light source, membrane filtration apparatus, turbidimeter (optional)
Reagents: none required
Services: Wash-up area
Space required: Minimal bench space
Description: This chemical test is rapid, does not require heating of the test samples and is considered to be more stringent than the heat test (Rankine 1989). The test uses phosphomolybdic acid reagent (5% phosphomolybdic acid, 5% sodium sulfate, 0.25 g/L glucose in 15% stock sulfuric acid in deionised water) to denature and precipitate proteins through the formation of cross-linkages with the molybdenum ion (Zoecklein et al. 1999).
Add 1 mL of BentoTEST® reagent to 10 mL of wine and examine the solution by eye for the presence of haze. As in the case of the heat test, a turbidimeter can be used for more objective assessment of the turbidity in the sample.
Equipment: Test tubes, turbidimeter (optional)
Reagents: BentoTEST® reagentServices: Wash-up area
Space required: Minimal bench space
- 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:80-81.
- Pocock, K.F.; Rankine, B.C. 1973. Heat test for detecting protein instability in wine. Aust. Wine Brew. Spirit Rev. 91: 42-43.
- Rankine, B.C. 1989. Making good wine. Melbourne, Vic: Macmillan Pty Ltd: 313.
- Rankine, B.C.; Pocock, K.F. 1976. Wine stability tests for quality control. Aust. Wine Brew. Spirit Rev. 96(12): 24-25.
- Toland, T.M.; Fugelsang, K.C.; Muller, C.J. 1996. Methods for estimating protein instability in white wines: a comparison. Am.. J. Enol. Vitic. 47(1): 111-112.
- Zoecklein, B.W.; Fugelsang, K.C.; Gump, B.H.; Nury, F.S. 1999. Production Wine Analysis. New York: AVI Van Nostrand Reinhold: 469-473.
- Wilkes, E. 2004. Group Manager – Commercial Services, AWRI, personal communication.