How to Peform Heat Stability Testing at Your Winery
Nick Smith, Enologist, University of Minnesota
Publisher’s note: This article is the fourth installment in a four-part series for winemakers by Nick Smith with the Enology Laboratory at the University of Minnesota. In this article on heat stability, the basic test procedure for heat stability is described in greater detail.
To read the previous articles, please click on any of the links below:
Hazy wines have minimal customer appeal. In the developing Midwest wine industry, hazy wines happen more frequently than they should. One area of hazy wine has been covered, protein instability. Scientific explanations for the cause have been discussed previously along with guidelines for preventing heat instability. The following section will provide a basic outline for conducting a heat stability test
Equipment Needed:
- Heating Apparatus
- Incubator, water bath
- Graduated Cylinders
- 100 mL or larger
- Beaker
- Stir Plate
- 200-1000 mL pipettor
- Bentonite
- Filtration
- Bench top filter with vacuum flask and vacuum pump, syringe filter, and or centrifuge
- Glass vials with caps
Procedure:
Prepare 5% w/v bentonite solution
Measure out 5 g of bentonite. In a beaker add 95 mL’s of warm (~30C) water and a stir bar. Place the beaker on a stir plate and mix at medium to high speed. While the water is mixing, slowly add the bentonite. Adding the bentonite too quickly will result in clumping and incomplete dispersal. Continue mixing at medium speed until the bentonite is well mixed and there are no clumps. Add water, if necessary to achieve 100 mL’s of total solution.
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Prepare wine trials
Fill each graduated cylinder with wine to the 100 mL marking. Pipette bentonite solution into each beaker at the corresponding addition rate. Make sure bentonite solution is well mixed before pipetting. The treatment amount will vary with each wine. Experience will dictate the appropriate trial range and you may have to run the test multiple times to find the correct range. A starting range might be 0.25 g/L to 1.0 g/L at 0.25 g/L intervals.
Using this treatment range, the trial would appear as follows:
|
Treatment |
|||||
|
Control |
0.25 g/L |
0.50 g/L |
0.75 g/L |
1.0 g/L |
|
|
mLs wine |
100 |
100 |
100 |
100 |
100 |
|
mL’s bentonite solution |
0 |
0.5 |
1.0 |
1.5 |
2.0 |
Cap or place Parafilm over the opening and invert each cylinder several times to mix the bentonite.
Settle
Allow the samples to settle overnight. The reaction is quick and the samples can be centrifuged shortly after addition. If the lab lacks a centrifuge, allow the samples to settle before filtering to reduce filter fouling.
Filter
Centrifuge 50 mL of each sample. Decant each sample off of the sediment and filter through a 0.45 um membrane filter. If a centrifuge is not available, or the 0.45 um membrane filter fouls quickly, use a glass fiber prefilter ahead of the membrane filter, or use a larger pore membrane first. Place 25 to 35 mL of the filtered sample into a clean vial. Cap lightly. Filter the samples to a similar extent as the wine will be filtered for bottling.
Heat
Visually inspect and record initial observations. If using a turbiditimeter, take and record the initial NTU’s for each treatment and the control.
For hot box tests, place samples in an incubator set to 50C for 24 or 48 hours. For water bath tests, place samples in water bath at 80C for 6 hours. Ensure water level reaches the level of the wine. Do not fill the sample vials completely full. Leave the caps slightly loosened.
Read and Interpret Results
Allow the samples to cool.
After cooling, inspect samples visually for haze and sediment. If sediment and haze is present only in the control, repeat the test using a lower addition range. Similarly, if the highest treatment sample is very hazy, retest using a higher addition range.
Document any haze found in the samples. Chill and check for haze daily for up to a week.
Visual Interpretation
As mentioned in previous articles on heat stability, the level of heat stability is determined by the winery or winemaker. Wineries needing high levels of stability can choose an addition rate corresponding to the trial that leaves little to no visual haze. Wineries wanting some stability, but not wanting to over fine, can choose a rate that achieves sufficient results without necessarily reaching completely clean test results.
Turbidity Meter
Visual interpretation might leave too much guesswork for some winemakers. For more precise testing, compare each sample’s final and initial NTU reading. If the reading increases by 2 or more NTU’s, the wine is often considered unstable. Different wineries will have different values for stability.
If you do not have a copy of the following book, I highly suggest acquiring one:
Chemical Analysis of Grapes and Wine: Techniques and Concepts by Patrick Iland, Nick Bruer, Greg Edwards, Sue Weeks, and Eric Wilkes
Available at UC Davis Bookstore
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