The iodine test is not really a very good way to determine if your mash is done, or "done enough" for you. It is relatively straight forward to calculate the maximum weight of extract (sugar plus proteins, and other dissolved matter) that can be created in your mash. It's also not difficult to determine the weight of extract actually created in your mash. At 100% conversion, these two values are equal. In other cases, the percent conversion completed (conversion efficiency) is determined by this formula:
Percent conversion = 100% * Weight of extract created in mash / Max potential weight of extract
or, Fraction converted = Extractactual / Extractmax
To calculate the potential weight of extract, start with the ppg of the grain bill (the weighted average of the ppg's of each of the grain bill components), and convert that to percent of the grain that can be converted to extract. Since potentials are given based on dry grain weight, and most grain contains about 4% moisture, we have throw in a multiplier of 0.96 to correct for the fact that we are weighing moisture containing grain. If 100% of the grain could be converted then it would have a potential of 46.173 ppg (this is the ppg of sucrose which is 100% sugar.) The formula for potential extract weight is:
Extractmax = Grain weight * 0.96 * Grain bill ppg / 46.173
This can be converted to the maximum possible wort SG in the mash, first in °Plato, and then SG. °Plato is just the weight percent of extract in the wort, which is equal to the weight of extract / (weight of extract + weight of strike water). Water has a density of 8.33 lb/gal at 68°F (20°C). The formula for SG in °P becomes:
°Pmax = 100° * Max potential extract [lb] / (Max potential extract [lb] + Strike volume [gal] * 8.33 lb/gal)
The formula to convert °P
max into SG
max is:
SGmax = 1 + °Pmax / (258.6 - (°Pmax / 258.2) * 227.1)
We can now get an approximate % conversion using the following formula:
% Conversion = 100% * (SGactual - 1) / (SGmax - 1)
The above formulas can all be put into a simple spreadsheet, so you don't have to do any calculations by hand.
Prior to taking the SG measurement of the wort in the mash, you must make sure the wort is homogenized throughout the entire volume of the mash vessel. For simple BIAB, stirring the mash aggressively prior to sampling is adequate. With a false bottom, continuous recirculation, or vorlaufing (2 - 3X the volume under the FB) followed by a stir, will give you sufficient homogenization prior to sampling. Systems with a malt pipe have special challenges when it comes to homogenizing the wort, but we won't take that up here.
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For the more pedantic geeks among us, the more rigorous calculations for % conversion follow.
Measure the SG
actual of the wort in the mash (after homogenizing.) Then convert SG
actual to °P
actual using the following formula:
°Pactual = (1111.14 * SGactual) - (630.272 * SGactual^2) + (135.9975 * SGactual^3) - 616.868
Now calculate Extract
actual from °P
actual as follows:
°Pactual / 100° = Extractactual / (Extractactual + Strike Water Weight)
°Pactual / 100° * (Extractactual + Strike Water Weight) = Extractactual
Extractactual = (°Pactual / 100°) * Extractactual + (°Pactual / 100°) * Strike Water Weight
Extractactual - Extractactual * °Pactual / 100° = (°Pactual / 100°) * Strike Water Weight
Extractactual * (1 - °Pactual / 100°) = (°Pactual / 100°) * Strike Water Weight
Extractactual = (°Pactual / 100°) * Strike Water Weight / (1 - °Pactual / 100°)
Since we calculated Extractmax above, we can now calculate the percent conversion as:
% Conversion = 100% * Extractactual / Extractmax
We can manipulate the equations a bit more and come up with another formula for % conversion that depends only on Plato measurements. This formula is convenient if you normally measure SG in Plato.
% Conversion = 100% * (°Pactual / 100°) * Strike Water Weight * (1 - °Pmax / 100) / ((°Pmax / 100) * Strike Water Weight * (1 - °Pactual / 100°))
% Conversion = 100% * °Pactual * (100 - °Pmax) / (°Pmax * (100 - °Pactual ))
Brew on