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Alcohol Distillation -
TheoryBatch distillation and fractional distillation
Batch Distillation (Brandy Spirit Production)
Fractional Distillation (SVR Production)
Quality Assesment - Acetaldehyde
- Alcoholic distillation is basically the process of separation the more volatile component, alcohol, from the less volatile component, water, from a water/alcohol solution, by heating the solution and condensing and collecting the alcohol rich vapours released, as a high alcohol strength liquid (spirit).
- The starting material for most distillations, such as wine for brandy and beer for whisky, are usually weak alcoholic solutions (5%-7% v/v ethanol)
- This seems counter intuitive, as it would seem that if the desired product is alcohol, the starting solution should have a high alcohol concentration.
- The reason for starting with a relatively low alcohol starting material is that alcohol molecules (A) and water molecules (W) interact with themselves (A-A, W-W, the reasons why they form liquids) and between each other (A-W, the reason they form solutions).
The concentration of the species A-A, W-W and A-W depends on the alcohol (A) concentration.
- Like most substances, like attracts like, in the same way that alcohol molecules will have a stronger affinity for other alcohol molecules (A-A) and water having a strong affinity for itself (W-W).
- In a weak alcoholic solution, fewer molecules of alcohol will have an opportunity to interact with each other, being separated by many water molecules.
This results in fewer, stronger A-A formations, and more of the weaker A-W formations, making it easier for the more volatile alcohol molecules (A) to be liberated from the solution on heating.
- On the other hand the collected distillate, from a batch/pot distillation, will be high in alcohol, and form many A-As, making it much harder to concentrate the alcohol even further through a second distillation.
In fact, a water/alcohol solution with a alcohol concentration of 95.6% alcohol sees the number of A-A molecules becoming so high that the alcohol becomes as reluctant as water molecules to vaporize of, in fact they will have the same volatility and no more separation or concentration of the alcohol becomes possible by standard distillation.
Solutions of this nature are called azeotropic solutions.
- The phenomenon, where volatilities of substances deviate from that predicted by Raoult's law, is refered to as non-ideal behaviour, or deviation from ideality, which happens to be a positive deviation with regards to a water/alcohol solution.
- So far we have assumed that the only alcohol present in wine is ethanol, however there many other important, if minor quantities of other alcohols present. (see other alcohol)
- In a batch/pot distillation, the alcoholic vapours coming of the bulk of the liquid are condensed above the liquid on the still outlet, on the way to the condenser.
Equilibriums with complex and interesting volatility deviations from the ideal are established there between the many alcohols and other components involved, all involving the phenomenon described above.
Pot or batch distillationSpirit Quality Assesment
- A pot still consists of a pot and an outlet to a condenser. The outlet itself also serves as a condenser and to some extent as a fractionating column (see fractional distillation).
- Batch distillation as the name suggest starts with a single batch of low alcoholic starting material (5-7%v/v alc.), that is heated in a pot still and a low wine distillate of between 26-50% v/v alcohol is collected.
This may then be further concentrated by a second distillation to a higher strength brandy of up to 80%v/v alcohol.
(Brandy is derived from the dutch word brandewijn, meaning burned wine).
- The pre condensation of minor but important flavour components along the pot still's outlet, is a place where complex equilibriums are established.
These equilibriums involve radical deviations from the ideal, predicted behaviour of the volatile compounds' volatilities (see distillation theory).
The result is that the product fraction make up as they are collected from the still is unpredictable.
This is why pot still shape and batch distillation produces a unique product and requires an experienced operator to determine the points of fraction separation.
- Brandy spirit is most suitable in a fortified wine style where the complexity of the brandy flavours is desired as an integral part of the style.
- Fractional distillation relies on a fractionating column to separate out components with different volatilities found in the starting material.
- This is achieved through the column's internal architecture in providing many areas where the rising vapour can intimately interact with the condensed liquid along the length of the column.
- The support for the condensate (condensed liquid) inside the column can be simply achieved by filling the column with a high surface to volume packing material.
More complicated arrangements exist; such as a series of trays and bubble caps, where the vapour is forced to bubble through the condensate collected on the trays at various sites along the length of the column.
- The separation along the height of the column can be viewed as a series of batch distillations, concentrating the more volatile component at the top and leaving behind the less volatile component at the bottom.
- The efficiency of the column to separate out the volatile compounds is dependant on design elements, including column length.
The columns efficiency is stated as the number of theoretical plates.
- Fractional distillation can be set up as a continuous process by removing the separated components from the different positions along the column (alcohol at the top) and the removed components replaced with more of the starting material.
The starting material is added at the middle of the column in the case of wine.
- Further specialized distillations can remove components with similar volatilities such as methanol from an alcohol solution.
- Fractional distillation can cheaply produce a neutral high strength spirit of between 94-96% v/v alcohol, called SVR (Spiritus Vinum Rectificatum).
SVR is the most suitable spirit for fortification purposes where minimal dilution or interference of the wine/grape flavours are intended.
- Aldehydes, mainly acetaldehyde and some other compounds, such as keytones, are produced during fermentation.
- These compounds can be distilled over and concentrated, mainly in the more volatile, first fraction (heads) of the distillate.
- The concentration level of these compounds in the distillate can be significant if the still has not been operated properly or their concentration were already high in the starting material.
- At high alcoholic strengths of the fortifying spirit, especially SVR, these compounds exist as acetals, which have lower flavour thresholds than the original compounds.
- However, when the fortifying spirit's alcoholic strength is diluted on fortification, and finds itself in the more acidic environment of wine, the acetals revert back to the original compounds with higher flavour thresholds.
Of particular concern here are the aldehydes, other then acetaldehyde, that are quite noxious even at their low concentration levels.
- For the above reasons the quality of the fortifying spirit should be assessed by diluting it with a 1% acidified (tartaric or citric acid) water solution to the final strength of the fortified wine anticipated.
This solution should be left for 48 hours before assessment, to allow the acetals to be converted back to their original forms, through hydrolysis.
- Azeotrope or azeotropic solutions is a word derived from the Greek - 'a' + 'zein' = to boil, 'tropos' = turning.
meaning "to boil unchanged" or the solution has turned into a "constant boiling mixture".
- When the volatilities of two components in a two part solution become the same no more separation can occur and the boiling point remains static. Ethanol and water form a constant boiling mixture at 95.6%v/v EtOH with a boiling of 78.2 oC, a temperature near the boiling point of pure ethanol (78.5oC) and far away from that of water (100oC) at atmospheric pressure (101.325 kPa).