It appears they use the last retort (thumper to us) to load up with low-volatile acids to produce the esters. This means the acids do not come across to the distillate. Traditional Rum stills will have 1-3 thumpers.To solve this problem, the brilliant Jamaican chemist HH Cousins developed a process to boost the ester count in rums in a more economical way.
The ‘lees’ in the retort at the end of distillation retains a considerable amount of the acids from the fermentation. Volatile enough to make it to (and concentrate in) the retort, not volatile enough to make it to the rum. The acids are recovered by adding lime (calcium oxide) to the lees to produce the calcium salts of the acids. This concentrated acid mixture after precipitation of calcium sulphate (by adding sulphuric acid) is added to high strength rum (i.e. lots of alcohol) and placed in the high wines retort where the esterification process (alcohol + acid) takes place. The resulting distillate is now supercharged with esters – up to 7,000 – and this distillate is used to ‘top up’ the rums produced in the normal way to reach the levels such as DOK at 1,600.
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