Expert: John Snyder - 2/18/2010

Question
QUESTION: Hi. I am just about ready to bottle my second batch of homebrew beer (a stout kit from an online retailer). The kit says I should have a starting gravity over 1.05 and mine was 1.052. The kit also says my finish gravity should be below 1.021. The gravity was checked at 7 days in the fermenter and was 1.026. I stirred the beer (with a sanitized spoon) and re checked the beer at 10 days and the gravity remained at 1.026. I assume I am ready to bottle. The kit gave an estimated ABV of 4.9%, however, using some online calculators it seems my beer will only have an ABV of 3.5%. Using these same calculators if my FG was indeed 1.021 as the kit suggests it would still only be about 3.8%. Why the discrepancy between the online calculators and the kit documentation? I have done my best to make sure my hydrometer readings are at about 65deg F. Thanks in advance for your time.

ANSWER: On  the assumption,that your readings were accurate,and your  procedures were done according to instructions:

1--the malt extract was too old.(1 year from date,on can)Malt extract over time,loses enzymes,that are needed to convert the malt extract starches to sugar,for fermentation.
2--the yeast was not viable enough,resulting in a deficient fermentation

As for the differences in online calculators:

1-- alcohol calculations were not based on lab tests,but on rule of thumb,such as "points"and factors relating to ABW,and ABV.
2--Interpretation of Plato,from S.G.


Note: hydrometer reading accuracy is extremely important.If you so desire,i will give pointers on using and calibrating a hydrometer



---------- FOLLOW-UP ----------

QUESTION: Thanks for your quick reply.
1. I used a liquid extract kit and I made sure it was fresh when I recieved it.
2. I am confident that my hydrometer readings are correct but will happily accept any advice you can provide.

I guess my real question is how do the homebrew kit manufacturers calculate alcohol content, because regardless of whether my readings were correct or not, using their anticipated OG and FG and the online calculators, there is a major difference in the alcohol content reported on their kit and what is reported by the calculator. Is this a legitimate question to ask the homebrew kit manufacturer?

Thanks again in advance for your time and patience.

Answer
Mfgrs.make malt extract by mashing grains,using traditional processes to produce wort(a sweet sugary liquid).The wort is then concentrated from a O.G. of around 1.080to a thick syrup with gravity of between 1.40 and 1.450.

Following kit instructions,based on brew type,the extract is diluted to the desired S.G.,followed by fermentation to convert the sugar to alcohol(ethanol)

Beer alcohol is then calculated,as follows:

1--Pure ethanol has an S.G. of 0.79 Kg/L
2--amount of Co2 produced for every gram of ethanol produced during fermentation is 1.05

Example:if O.G. is 1.06 and the TG(terminal gravity)1.02.this means 0.04Kg/L of CO2 has bubbled off.The alcohol content left behind is therefore:

1.05x(1.06-1.02) =0.42kg/l.

To calculate the % of alcohol:
0.042/1.02=0.041,or 4.1% by weight

To calculate by volume :4.1%/0.79=5.2%

0.79 is the density of alcohol(pure) as stated above

If the mfgrs.specs are accurate,and all procedures are correctly followed,then one should arrive at the desired alcohol.

Not arriving at the desired final S.G.would most likely be due to old extract,or poor yeast viability.Malt extract should be kept sealed in an air tight container,in the refrigerator.

I will put together,a hydrometer tips list,tomorrow.

         John