Rum primarily originated in the Caribbean, although the first references to liqueurs obtained from sugar cane are from India. Sugar cane was introduced to the Caribbean by Christopher Columbus in 1493.
The chief producing countries are Barbados and Santo Domingo. Nowadays the coastal planes of Guyana (Demerara) are rich in estates producing sugar cane (Saccharum officinarum).
At harvest time the fields of sugar cane are set alight in order to sanitise the soil, the stems are scorched in this process and the canes subsequently wither and are harvested by machete, a strategy thought to yield a superior product when compared with rum made from cane harvested by machine.
The canes are topped to remove the leafy parts and the cane then ferried to mills. There is considerable contamination with Leuconostoc mesenteroides, which produces a gum that causes problems during extraction. It is important to avoid delays between cutting and milling, and the maximum time elapse should be less than 24 h.
During processing, the canes are cut and crushed and the juice limed, clarified and evaporated. Various fractions are generated, but the key product for rum is molasses. Four to five tons of molasses are typically obtained per 100 tons cane.
The nature of the molasses depends on cane variety, soil type, climate, cultivation and harvesting conditions. They are delivered hot to the distillery either by pipe or by tanker and are stored at 45°C. The molasses are pumped at 85-88°Brix and are mixed with water in line. Lighter flavour rums may incorporate cane juice (12-16% w/v sucrose).
Formerly adventitious yeasts were used to effect fermentation, but nowadays pure cultures of S. cerevisiae, S. bayanus and Schizosaccharomyces pombe are used. They are propagated from slopes by successively scaled up incubations using sucrose as the carbon source.
Prior to fermentation, the molasses are diluted to 45°Brix and their temperature elevated to 70°C in order to destroy contaminating organisms. The pH is lowered by the addition of sulphuric acid and the whole clarified by putting into a conical-bottomed settling tank, from which the sludge can be decanted from the cone. Ammonium sulphate is added as a source of nitrogen.
Fermentation is conducted at 30-33°C in cylindroconical vessels that may be closed or open. The final sugar content will be 16-20° Brix and this is reached in 24 h with an alcohol yield of 5-7% ABV. Some high-gravity fermentations nowadays furnish 10-13% ABV.
Distillation is conducted in pot stills that were traditionally of copper or wood but now more likely to be fabricated from stainless steel. As for whisky, there are also column stills of stainless steel or copper (Coffey stills).
Pot stills afford heavier rums that need prolonged maturation, whereas the column stills are employed for lighter rums, or to generate the neutral spirits that can be used for the production of gin and vodka. Distillates are collected at 80-94% ABV for rums and >96% for neutral spirits.
Pot distillation of rum is exactly analogous to the techniques used in the production of whisky. The pot is charged with wash at approximately 5.5% ABV and the retort charged with low wines at 51-52% ABV from the previous distillation. The fractions obtained are heads, spirits, and feints. The heads are rich in esters and are collected for the initial 5 min in the low wines receiver. The ensuing spirits are collected for 1.5-2 h at 85% ABV. When the emerging strength drops to 43% ABV, the flow is again diverted to the low wines receiver in order to collect the feints. Distillation is completed when the distillate approaches some 1% ABV.
Column distillation allows ten times more output than does pot distillation and is performed exactly analogously to the whisky process.
Rum is aged in Bourbon oak casks. It is racked at 83-85% ABV. As the main production locale is tropical, ageing is quite rapid and may be complete within 6 months. There may first have been a blending of light rums produced in column stills with heavier rums out of pots. Furthermore, there may be transfers between casks for successive maturation periods. Finally rum is chilled to -10oC and filtered to remove fatty acid esters prior to dilution to final strength and packaging.
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