September 6, 2014

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Bench Trials – Why you should get comfortable using the Metric System

We’re getting to the point in the year where wineries are thinking about some ways in which they may fine-tune their wines before putting them in the bottle. Perhaps you will be blending different batches together, or doing some fining to soften some of the wines. Maybe you realized that your deacidification wasn’t quite enough or perhaps that wine that you initially wanted to leave about 10 g/L of sugar in ended up fermenting to dryness, no matter how hard you tried to kill off the remaining yeast. Now that it is stable, you want to add back some sweetness. However, in all these cases the final result irreversible so it is important to get an idea of what the final outcome may be before you do it to an entire tank or barrel of wine.

Here are some important tools you will need:

  • a scale that will accurately measure to 0.1 g
  • a 50 or 100 mL graduated cylinder
  • a volumetric flask of 50 or 100 mL
  • a micropipette (ideal) with tips, or a pipette that will accurately measure 0.5 – 1.0 mL
  • a friend to help you taste your trials

Now let’s see why the metric system makes bench trials so much easier.

Let’s say that your wine has too much Hydrogen sulfide (H2S), that reduction aroma produced during fermentation that smells a bit like rotten egg. Only small amounts of copper sulfate (CuSO4) are usually sufficient to remove the offending odor. There is a legal limit of how much copper you may add, and it is very small – only 6 ppm (mg/L). Because excess copper additions can lead to other problems such as haze formation, and copper casse, as well as the fact that any remaining copper in the wine can’t exceed 0.5 ppm by law, you want to use the smallest amount possible to remove the offending odor (here you will find a standard procedure for copper addition bench trials along with others).

To carry out the bench trial, measure out 100 mL of the stinky wine into 4 clean glasses. To make sure the glasses don’t smell like soap, it is a good idea to rinse the glasses with wine before starting. One glass is your control, the next three we will add copper sulfate at the following rates: 5 ppm, 7 ppm, 9 ppm.*

In order to make our addition easier, we will make up a 1% w/v solution of copper sulfate (CuSO4) by measuring 1.0 g into our 100 mL volumetric flask, and filling to the volume graduation mark with distilled water (see photo).

Now the calculation is easy (in the metric system):

Since our solution has 1000 mg CuSO4 in 100 mL of water, each mL of our stock solution contains 10 mg of CuSO4.

1000 mg / 100 mL = 10 mg/L

We want to add CuSO4 to our wine at a rate of 5.0 mg/L (5.0 mg/1000mL). That means that each mL of wine will have 0.005 mg of Copper Sulfate. So, our 100 mL sample of wine will have 0.5 mg of CuSO4.

5 mg / 1000 mL = 0.005 mg/L      0.005 mg/L x 100 mL = 0.5 mg

Now, knowing that 1.0 mL of our stock solution has 10 mg of CuSO4, we can calculate how much of the solution we should add to our wine sample by dividing 0.5mg by 10. We therefore need to add 0.05 mL (50 µL) of the solution to our 100 mL wine sample.

5 mg / (1 mL / 10 mg) = 0.05 mL = 50 µL

This is where a micropipette comes in handy. It allows you to add very small and precise amounts, and you aren’t causing error by diluting your sample. Of course, if you don’t have one, you would have to simply dilute your stock solution by a factor of 10 (1% to 0.1%).

All of those calculations were done in my head. Try doing that working in the Imperial System! It even is easier when it comes time to actually use these trials to make the final addition in your winery. Let’s say you did your 3 trials of Copper Sulfate additon, and you and your tasting buddy found the 5 ppm addition to be the best (ie: the least amount required to be effective in removing the offensive odor). Now, if you know the volume of your tank in liters, the amount of copper sulfate to add to the tank is also a calculation you can do in your head (no need to convert back to the imperial system). If you had 1500 Liters of wine, and want to add 5 mg/L;

5 mg/L x 1500 L = 7500 mg = 7.5 g

Simple.  Try using the metric system in your winery. I’ll bet you never go back! And, if you insist on using some sort of combination of both systems in your winery (like bottling and commerce in the metric, and winemaking in Imperial), I refer you here for further guidance.

*For simplicity, we won’t calculate how that translates to the actual amount of copper that you are adding. Copper Sulfate is most often purchased as a hydrated salt – Copper Sulfate Pentahydrate (CuSO4·5H2O). It looks like a blue crystalline powder. When adding it to wine, you have to remember that only about 25% of the compound is actually copper. So, if you add 5 ppm of (CuSO4·5H2O), you are actually adding 1.25 ppm of actual copper. I also am not endorsing these trial values as the value you should use for your trials. You may find that only 2 or 3 ppm of copper sulfate is sufficient to help rid the wine of H2S. The numbers I’m using are for example only.