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Alcohol Determination for Fortification -
Introduction to Fortification
Timing of fortification
- When producing sweet fortified wines the fermentation has to be arrested at a specific time by the addition of alcoholic spirit to prevent the yeast fermenting the wine to dryness.
- The timing and volume of the alcoholic spirit is critical to achieve the final desired alcoholic strength and sugar concentration.
Volume of alcoholic spirit required spirit
- To calculate the timing of the fortification the formula below can be used.
X = 0.33S + 1.262W - 0.262G
X = is the sugar level or oBé of the fermenting must when fortification is required.
S = the final desired alcoholic strength (%v/v) of the wine after fortification.
W = the final desired sugar level or oBé of the fortified wine.
G = the initial sugar level or oBé of the grapes used.
To understand this formula lets run through the following example.
Timing of fortification - Example
- The volume of spirit required can be calculated by applying the calculations below based on the Pearson's-square.
X = V(C - A) / B - C
X = the volume of fortifying spirit required in liters (? L)
V = the volume of must or wine to Bé fortified litres (678 L)
C = the final alcohol content aimed for (18.0%v/v)
A = the required alcohol content of the fermenting wine at fortification (4.32%v/v - calculated above)
B = the alcoholic strength of the fortifying spirit (95%v/v)
X = 678 L (18.0 - 4.32) / 95.0 - 18 = 120 L
Therefore 120L of 95%v/v alcoholic spirit has to be added.
Final volume of wine is 678 L+ 120.5 L = 798 L
- Assume -
- we start with grapes with a sugar concentration of 13.0
- and want to finish with a wine with -
- a final alcoholic strength of 18%v/v (S)
- and a sweetness level of 4.0 oBé as measured by hydrometry (W)
- First we need to realise that the final 4.0 o Bé required, does not reflect the true final o Bé or sugar concentration as would be determined for a sugar/water solution, due to the presence of alcohol.
This is because alcohol has a lower density than water and will obscure the 'true' oBé by 0.262 oBe/1 %v/v of alcohol.
i.e. 18%v/v * 0.262oBé / 1%v/v = 4.68 oBe (obscuration).
or obscuration = S * 0.262
therefore the final true Baumé or sugar concentration of the fortified wine will be 4.0 + 4.68 = 8.68 oBe.
or final true oBé = 0.262S + W
The initial 13.0oBé minus final 8.68oBé is 4.32oBé (13.0 - 8.68 = 4.32)
i.e. G - (0.262S + W)
Therefore the wine should be arrested by fortification after 4.32 oBé has been fermented to alcohol to leave 8.68 oBé of sugar.
The formation of 4.32 %v/v alcohol, (@ 1%v/v alcohol/1 oBé), will obscure the true Bé of 8.68 oBé by -
4.32%v/v * 0.262oBé / 1%v/v = 1.12 oBé obscuration
i.e. (G - 0.262S + W) * 0.262
Therefore the Be to fortify at is the true Bé - the obscured Bé
i.e. 8.68oBé - 1.12oBé = 7.56oBé
i.e. (0.262S + W) - 0.262(G - 0.262S + W)
i.e. 0.262S + W - 0.262G - 0.068S + 0.262W
i.e. 0.33S + 1.262W - 0.262G
i.e 0.33 * 18 + 1.262 * 4.0 - 0.262 * 13
= 7.56 oBé
The Pearson's Square can be use to calculate the volume of alcohol that needs to added to a solution (must/wine) to achieve the desired final alcoholic strength.
X = is the final desired alcohol concentration (18.0%v/v)
A = alcohol concentration (%v/v) of the must/wine to be fortified (4.32 %v/v)
C = the alcoholic strength/concentration of the fortifying spirit (95.0%v/v)
D = the difference between X and A (18.0 - 4.32 = 13.68)
B = the difference between C and X (95.0 - 18.0 = 77.0)
|4.32|| || || ||77.0|
| ||\|| ||/|| |
| || ||18.0|| || |
| ||/|| ||\|| |
|95.0|| || || ||13.68|
Then the volume ratio of the fortifying spirit to must/wine = B/D = 77.0/13.68 = 5.63 = 1/5.63
i.e. for 678 L of must/wine add 678L * 1/5.63 = 120L
i.e. 120L of 96%v/v fortifying spirit will need to be added to 678L of must/wine, (@ 4.32%v/v alc.), to finish with a fortified wine of 18%v/v alcohol.
Note: the final volume will not be exact due to shrinkage of volume caused by changes in alcoholic concentrations, however, these can be ignored.