Potencial Alcohol in Grape Must
credits: Vinotecarium Pixabay
Winemaking: How to measure how much potential alcohol is in my grape must?
I say “potential alcohol” because grape must (before fermentation) has 0% alcohol. To measure how much potential alcohol does grape must (“grape juice”) have, we need to measure how much sugar it has. Confused? Let me tell you why:
SUGAR Vs. ALCOHOL
Wines alcohol is a byproduct. Yeasts (in our case saccharomyces cerevisiae) ferment sugars. It is one way to generate energy for survival. As result from this pathway, alcohol is formed. Rule of thumb (depends on a few factors), each 17g(grams of sugar)/1L(liter of wine) will turn into 1% v/v Alcohol. This can vary depending on presence of oxygen (promoting aerobic fermentation), yeast strand efficiency, etc.
Ho can I measure how much sugar is in Wine must?
Well, since wine must is mostly water and sugar (literally!), we can measure its density and ignore everything below density below 1g/dm3. Why? Water has 1g/dm3 of density (depending on temperature and purity). So, if we measure our must density, and its density is 1,085g/dm3, we know that sugar is elevating that number above 1g/dm3. After that, we use comparison.
With density and temperature of grape must, we can look up a conversion table (someone has tested this for us) to know (more or less) how much sugar is in it. Most tables will convert directly to potential alcohol. Sugar can be measured in different scales (Brix, Baumé, etc) but all you need to know is that every scale will have a table to convert its value to %potential alcohol.
Winemaking: How to use a densimeter?
First thing first, a densimeter will allow you to now a liquids density by floating on it. I think it has something to do with Arquimedes and a bathtub (seriously). And you will need some sort of jar or cup. It needs to be able to be filled with wine and accommodate the densimeter in it without touching the walls. In any winery during vinification, you will most likely see someone going around with five things: pen, paper, plastic test tube, thermometer and a densimeter. It is usually the intern. Don’t take these words lightly, even though it is easy, you need to do it right. During wine fermentation, we need to know the wine density for many reasons. We can know that by:
· Filling the test tube with fermenting wine.
· Measuring its temperature with the thermometer.
· Putting the densimeter in the wine (like the thermometer) and spinning it with your fingers (not necessary but try it and you will figure it out).
To read its density, you should use the scale on this device (some will have many scales) but I like to use density. Where the wines upper limit marks in this scale there is our value.
“Do’s” and “Don’ts” of using a densimeter
If it’s your first-time making wine and you’ve just crushed your grapes and you’re anxious to know your grape must density to find out how much sugar it has, you are right! You should be anxious and do it as soon as possible, because after it starts fermenting it will be much harder to figure your wine potential alcohol. If it is a small container (let’s say: 500 Liter), you can stir it a bit and take a sample to fill your test tube. Strain it lightly, just to make sure that no grape seeds or skin gets into your test tube (won’t affect the result but will make it easier). Read that value and don’t forget about it. I can tell you some secrets to increase the potential alcohol of your grape must. During fermentation it’s a little bit trickier. Your sample will be bubbling as carbon dioxide (Carbon dioxide - another fermentation byproduct) gets released which can make it difficult to read your densimeters scale (spin your densimeter with your thumb and index finger).
Theoretically you can wait until it stabilizes but remember that as carbon dioxide is release it will push your densimeter up (hence the spinning), so don’t get stressed if readings aren’t constant. It’s of extreme importance to read your “wine” (fermenting must) density every day during fermentation to be ready in case of a stuck fermentation (“Stuck fermentation – what is and what to do?”). As your must ferments, its density will decrease to near 1g/dm3 (1000 g/L - same thing), and as this happens, your densimeter will get lower into your test tube. The reason I’m pointing this phenomenon is simple, it will break if you just drop it into your test tube (sounds stupid and obvious but you will remember this).
Conclusion
Alcohol is a byproduct of yeasts fermentation of sugar in absence of oxygen. There is 0% alcohol in grape must. We can predict how much % of alcohol will be in our wine by the end of its fermentation (when most sugar has been converted into alcohol and other byproducts). To find it out we need two instruments: thermometer and densimeter. By reading both the temperature and density we can use a table to convert those values into %potential alcohol. It is useful to keep reading your wine density throughout the fermentation process to find any issues early on.
These are tools you will leave with and learn to enjoy. If mastered, they can tell you many secrets.
