Heat Stability Test
The heat stability test (HS test) for wines is part of bunch of stability tests required for finished wines prior to bottling. HS test is showing if are proteins in wine or not. The proteins in finish wine are not welcome because they can create protein haze in the bottle down the road. To avoid this you have to remove these proteins by fining the wine with bentonite. You don’t want your wine to get cloudy in bottle after is exposed to high temperatures.
When wine is shipped overseas, depends on seasons, it is exposed to extreme temperatures in regular containers or trucks. The best way is to ship wines in temperature control containers/trailers, but still have to stabilize your wine prior to bottling. No matter what.
Heat stability test (HS test) it is part of 5 generic stability tests, such as: cold stability test, heat stability test (protein stability test), microbiological stability test, oxidation stability test, color stability test. The microbiological, oxidation, color stability tests are more complicated and not many wineries doing these last tests, usually by good winemaking practices the wines get stabled. For example to avoid any microbiological faults in bottled wine you assure you do a sterile filtration before bottling by using membrane sterile filters. But about bottling we’ll talk in another post. For oxidation stability, there are some tests to find out how much oxygen it is in wine or how much oxygen wine needs, but usually this is regulated by amount of free SO2 in wine.
But, let’s stay for now with heat stability test. According with some articles research such as this article, measuring the protein stability in wines is very important and it is a key for quality control.
Stability predictive assays usually involve 1) inducing a haze, by heat or other methods, followed by 2)measuring the induced haze through a variety of methods available. In theevent that the wine were to be declared “protein unstable”, the winemaker would “clean it up”, or reduce its protein content, normally through bentonite fining.
They describe 3 different methods for HS test and also for measuring the induced haze in wine: 1) Nephelometry, 2) Spectrophotometry, 3) Visually. If you have already Spectrophotometer in your winery lab (for Megazyme kits and red color analysis) should be enough for doing HT test.
I use nephelometer or turbiditymeter to do my HS test. So, today we’ll talk about the HS test using the turbidytimeter.
The Heat stability test using a turbiditymeter.
This test can be done prior to bentonite fining and also can be done on lab trials to determinate the exact dosage of bentonite to stabilize the wine.
The method: the wine sample is filtered, then you read initial NTU (Nephelometric Turbidity Units), followed by heating the wine sample for 4 hours @ 60*C 2 hours @80*C. After the heating hours you wait to naturally cool down the sample , it takes about 30-45 minutes. And you read again the final NTU.
If the difference between initial and final NTU values is higher than 2, the wine is not heat stable. The conclusion: wine need bentonite fining.
To determine the right dosage of bentonite the easiest way is to do lab trials with different dosages and second day redo the HS test on those trials.
Here are steps in pictures:
1. Filter the wine sample first – use 0.45μm membranes.
2. Read the initial NTU on turbiditymeter
3. Heat the sample for exactly 4 hours @ 60*C
You can use an oven with temp control or a lab bath with temp control.
4. Wait about 45 mins to cool down the wine samples and read again NTU
5. Conclusion on HT test results.
If the difference between initial NTU and final NTU is < 2 , wine is heat stable.
For example: if you read initial NTU = 0.55, and after 2 hours @ 80*C you read NTU=3.23, the difference will be 2.68 – wine is not heat stable or may be short term heat stable.
The next step is to find the right dosage of bentonite addition for stabilizing the wine. For this, you have to do lab trials with wine from the same tank with different dosage of bentonite (for example in the previous case you can do 3 lab trials with 0.15 g/L, 0.2 g/L , 0.3g/L and 0.4 g/L bentonite).
If the difference between initial NTU and final NTU is higher (like 20, for example) you have to go with higher dosage on bentonite for lab trials.