Strawblog: Day 2: Chaptalization, or the boring math part.

7 July 2009
By Tom Mansell

When we last left our berries they were cold-soaking overnight in the fridge after crush and pectinase treatment.

Hydrometer, zoomed-in

Hydrometer, zoomed-in

The next day it was time to take some measurements. I squeezed the pulp in a hop bag to get straight juice out into a graduated cylinder. I suppose you don’t need a graduated cylinder but the amount you measure has to be deep enough to fit a hydrometer. The hydrometer, pictured right, correlates density of the mixture to height, which is related to the amount of stuff dissolved in there. The density can be converted to Brix (named after Herr Brix, reputed to have invented the hydrometer and the formula which converts specific gravity (i.e., density) to percent sugar), which is a measure of soluble solids per 100g of solution, or more simply, percent sugar. A reading of 20 degrees Brix corresponds to 20 grams of dissolved solids per 100 g of solution.

(NB: Brix is not necessarily a measurement of sugar, however, in a given {must}, 90-95% of dissolved solids are fermentable sugars. Others include non-fermentable sugars like arabinose and xylose, organic acids, polyphenols, etc. )

Hydrometry on the juice, with graduated cylinder for extra science-y goodness.

Hydrometry on the juice, with graduated cylinder for extra science-y goodness.

The strawberries came in at 7 Brix, which is 70g/kg and the must is about 2.88 kg (estimated from the density), so we’ve got about 202 g of sugar. If I were to ferment this as is, I’d have the alcoholic equivalent of strawberry light beer. We’re making wine here, not Arbor Mist. Let’s up the ante a little, shall we? More sugar means more alcohol.

Let’s say I want to chaptalize (i.e., add sugar to the must, named after Monsieur Chaptal, who discovered the relationship between sugar and alcohol in wine) up to 20 Brix. How much do I have to add? ALGEBRA!

202 + x (g sugar)
——————— = 0.2 (g/g); x= 467.5 g sugar
2880 + x (g total)

Unlike Alton Brown, I don’t have a scale in the kitchen, so I have to work with volume. According to sugartech.com , the density of bagged white sugar (sucrose) in 700g/L = 165 g/cup. So to get up to 20 we need 2 5/6 cups. Let’s make it an even 3.

From strawberry juice to strawberry syrup.  I think I underestimated the volume.

From strawberry juice to strawberry syrup. I think I underestimated the volume.

I added 3 cups sugar. If this were Beaujolais, I’d be locked up. Just as a check, I took the Brix measurement again. Whoops, the final Brix was 26! I guess I underestimated my volume of must. Apparently I did, because when I filled up the growlers I had about 3 quarts instead of the gallon that I thought I had. I didn’t want too much headspace on the wine so I topped them up with some cherry juice (no preservatives…) I had on hand. After this addition, the final Brix reading was 23. Depending on what formula you use (and the yeast), that should lead to an ABV of 12-13%.

Science!
During fermentation, sugars such as glucose and fructose are converted into carbon dioxide and ethanol in the following reaction:
C6H12O6 –> 2 C2H5OH + 2 CO2
This means the theoretical yield for ethanol on a per mass basis is about 0.51 * sugar, or ~62% on a “by volume” basis. This doesn’t really happen and often the Brix to ABV conversion is more like 0.55. Why? Some of that sugar goes into yeast biomass, some of it becomes glycerol, and some alcohol is carried away by CO2… More on CO2 next time, when we prepare to start fermentation…

PS I know what you’re thinking… I added sucrose, which is C12H22O11 ! That doesn’t fit into the formula! Luckily for me, yeast produce an enzyme known as invertase, which converts sucrose into its constituent monosaccharides, fructose and glucose, both of which are C6H12O6 and both of which are easily consumible by yeast. It’s also what they add to chocolate-covered cherry cordials to make them all gooey on the inside. (The invertase breaks down the sucrose-saturated fondant into fructose and glucose.)

Bonus question: If I had 3 quarts of 26 Brix juice and added 1 quart of cherry juice to bring it to 23 Brix, what was the Brix of the cherry juice?

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One Response to Strawblog: Day 2: Chaptalization, or the boring math part.

  1. Vinogirl on 14 July 2009 at 11:31 pm

    What a great experiment!

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