I've never seen this topic covered on any homebrew forum, though no doubt it has been on occasion. ABV (Alcohol By Volume) calculators generally focus on the original and final gravity of beer during primary fermentation, without taking the secondary fermentation into consideration. I'm using my old school chemistry knowledge to calculate the effect of the secondary fermentation on alcoholic strength.
The equation C6H12O6 (one molecule of glucose sugar) > 2C2H5OH (two molecules of ethyl alcohol) + 2CO2 (two molecules of carbon dioxide) is well known. Supposing you put 1 teaspoon of priming sugar (about 4g) into a 500ml bottle of beer. The molecular weight of glucose is 180g and of ethyl alcohol is 46g. Therefore, 180g of glucose (1 mole) would produce 92g of alcohol (2 moles) if fully fermented. So 4g of sugar would produce 4/180 x 96 = 2.13g alcohol.
2.13g of alcohol = approx 2.13/0.79 ml = 2.70ml alcohol, since the density of ethyl alcohol is approx 0.79. So the extra ABV produced in the bottle by the priming sugar would be 2.7/500 x 100 % = 0.54%. So, when you filled the bottle, you thought you had 4.00% ABV but now will have 4.54% ABV when the secondary fermentation is complete. That's a big difference. A half teaspoon of 2g sugar would produce an extra 0.27% ABV; also significant.
Another point is that sometimes the initial fermentation is not quite complete. How many people take the trouble to measure the final gravity of the finished beer, after secondary fermentation, to check what the value is? I have had a beer which appeared to have completed its first fermentation and was down to 1.010 SG (Specific Gravity). It started to re-ferment in the bottle (the temperature in my storeroom was higher with a heatwave) and tasted stronger and stronger as I used up the batch. I checked the gravity and found it had gone down to 1.004 in the bottles. That's a huge difference, and meant the actual strength had gone up from 3.7% to nearly 5%.
Maybe drink for thought? Very worthwhile to check the final strength after secondary fermentation and compare it to the ABV according to the primary fermentation. Don't forget to get rid of all the bubbles in the beer before using a hydrometer. A refractometer would waste a lot less beer.
The equation C6H12O6 (one molecule of glucose sugar) > 2C2H5OH (two molecules of ethyl alcohol) + 2CO2 (two molecules of carbon dioxide) is well known. Supposing you put 1 teaspoon of priming sugar (about 4g) into a 500ml bottle of beer. The molecular weight of glucose is 180g and of ethyl alcohol is 46g. Therefore, 180g of glucose (1 mole) would produce 92g of alcohol (2 moles) if fully fermented. So 4g of sugar would produce 4/180 x 96 = 2.13g alcohol.
2.13g of alcohol = approx 2.13/0.79 ml = 2.70ml alcohol, since the density of ethyl alcohol is approx 0.79. So the extra ABV produced in the bottle by the priming sugar would be 2.7/500 x 100 % = 0.54%. So, when you filled the bottle, you thought you had 4.00% ABV but now will have 4.54% ABV when the secondary fermentation is complete. That's a big difference. A half teaspoon of 2g sugar would produce an extra 0.27% ABV; also significant.
Another point is that sometimes the initial fermentation is not quite complete. How many people take the trouble to measure the final gravity of the finished beer, after secondary fermentation, to check what the value is? I have had a beer which appeared to have completed its first fermentation and was down to 1.010 SG (Specific Gravity). It started to re-ferment in the bottle (the temperature in my storeroom was higher with a heatwave) and tasted stronger and stronger as I used up the batch. I checked the gravity and found it had gone down to 1.004 in the bottles. That's a huge difference, and meant the actual strength had gone up from 3.7% to nearly 5%.
Maybe drink for thought? Very worthwhile to check the final strength after secondary fermentation and compare it to the ABV according to the primary fermentation. Don't forget to get rid of all the bubbles in the beer before using a hydrometer. A refractometer would waste a lot less beer.
Last edited:
