Thoughts on Gravity and pH

 Posted by on March 19, 2012
Mar 192012
 
drawing of a medieval brewer in a warehouse

Specific gravity, that magic number that allows us to easily figure how alcoholic our brews are, or are likely to be; how sweet they may be, and something about their anticipated mouthfeel…lots to think about, with gravity.  I am making the assumption that a swift, efficient fermentation is best for your brew – healthy yeast feremnts with fewer off-flavors and undesirable esters.  My notes here are about meads.  While the numbers would translate about the same way for beer wort as for mead must, I haven’t paid really close attention to my wort pH the way I have with musts, and I don’t want to make any claims I can’t back up from personal experience.

Some rules of thumb:  In a 5 gallon batch o’ brew, 1 pound of honey will raise your specific gravity by 8 points or 2 degrees Plato/Balling.  1 gallon of honey in 5 gallons total volume will get you a specific gravity of 1.0i4 or 22.5 degrees.  A pound of honey produces about 1% alcohol by weight.  Mead parameters are very much like wines; wine commonly has 1-3% residual sugars, but dessert wines can have 20% or more.

Remember that some yeasts struggle in a gravity over 1.140, so make sure to match your yeast to your honey concentration.  Your must will probably finish ~0.100 lower than it starts, if all goes well with fermentation.  A rough rule of thumb for relative mead sweetness (though don’t use this gauge for competition, the judges’ taste buds may not register the same way as your hydrometer):

Dry: gravity 0.990-1.006

Medium 1.006-1.015

Sweet 1.012 – 1.020

Dessert 1.020+

Keep the must acidity greater than 3.5 for optimum yeast health and fermentation – if your must is too acidic, you can add calcium carbonate 1/2 tsp at a time, stirring well.  More often, I find must isn’t acidic enough.  A lot of mead brewers add a little lemon juice to get their meads, particularly traditional ones, acidic enough to ferment thoroughly.  Optimum yeast health is generally achieved at a pH of around 3.5-3.8, which is relatively acidic – on a 14 point scale, vinegar registers around 2.2  and tomatoes 4.5  (human blood is about 7.4).  Not sure how to tell how acidic your must is?  Most homebrew stores have little packets or tubes of litmus paper strips that you can dip in the liquid, then compare to a color chart.  Shades of elementary school science class…

If your mead is not acidic enough, you can balance the acid at bottling time.  Many very experienced brewers prefer to adjust then, rather than add acid when they first compile their must, when they would have to predict how it will ferment out.  Dissolve 1/2 teaspoon of acid blend into 1/4 cup of water, stir it in thoroughly, taste and repeat as needed.  Acid blend is a combination of malic, citric, and/or tartaric acid, and gives a pretty nice balance.  Of course, there is also grape tannin powder, and as I said earlier some folks use lemon juice.  When adjusting flavor at this stage, I tend to prefer the balance of acids for better flavor.

Some common problems include:

Fermentation can’t get started: initial gravity too high for the yeast

Prolonged, slow fermentation: too little nutrient for the yeast

Fermentation slows drastically after 2-5 days: too low pH (too acidic, go for that calcium carbonate)

Fermentation stops completely before all sugars are attenuated (stuck fermentation): try aeration and a small dose of nutrients.  If that doesn’t work, switch to a yeast strain with a higher alcohol tolerance (Lalvin EC-1118 and Red Star Pasteur Champagne yeasts are my go-tos for this).  Must stratifies, so stir it up and make sure the honey is evenly distributed in solution!

 

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