The distillery is designed to produce 60,000 litres of a 96% w/w alcohol product from the anaerobic fermentation of the sugar in molasses by yeast. The following is a simple process flow diagram that gives an overview of the operations at the distillery.
To make rum one has to start with sugar from sugarcane. Molasses is a by-product from the sugarcane industry. It is the slurry that remains after most of the recoverable sugars have been extracted from the crushed cane. Generally, it is composed of 70-80% w/w solids and 20-30% w/w water. Of particular interest to the distillery is its fermentable sugar content which typically amounts to 35-55% w/w (sucrose, glucose, fructose); this is what the yeast “feed” on during fermentation.
This is a uni-cellular organism of the Saccharomyces Cerevisae family. Aerobic conditions promote propagation and growth of the yeast, while anaerobic conditions result in alcohol fermentation – the basis of the rum industry. Yeast, being living organisms, requires a controlled environment with the right amounts of vitamins, minerals and nitrogen. All of these are present in sufficient quantities in the molasses, except for the nitrogen, which must be supplied as Ammonium Sulphate (NH4)2SO4.
The molasses is dissolved in water in the ratio of approximately 1:3. We add a small amount sulphuric acid to reduce the pH to control bacterial infections. It also has the effect of assisting by reducing the amount of non-sugar dissolved solids that can be harmful to both fermentation and distillation. The resulting mixture called ‘mash’, is about 12 – 13% sugar.
4. YEAST GERMINATION, PROPAGATION & GROWTH
4.3 Germination (Growth of new yeast cells):
Clarified mash of specific gravity 1.050 – 1.060 is sent to a germinator, along ammonium sulphate. This mixture is sterilised using low-pressure steam before the yeast culture is added. This culture is either obtained as 2-6L lab grown cultures or retained contents of the propagator. Germination proceeds for approximately 12 hours before the contents of the vessel are transferred to the propagator. During this time filtered air is injected into the vessel to maintain circulation and aerated conditions.
4.3 Propagation (Increase in the number of yeast cells):
Contents of the germinator are added to the sterilised contents of the propagator (Yeast feed, mash and ammonium sulphate). Propagation continues for 18 hours, before being transferred to the growing tanks. Aerobic conditions are maintained in the vessel by the addition of filtered air to the propagator. This, as well as the water circulation through the external water jacket, maintains the temperature at 30-32ºC.
4.3 Yeast Growth (Increase in the size of the yeast cells):
Mash + (NH4)2SO4 + Propagated culture → Growing Tanks
The culture is allowed to grow for 24 hours. Injected air and circulation through the wash cooler maintain the growing temperature at 30 + 2°C. Antifoam (food grade) is also added during the growing process, before the contents are transferred to the designated fermenters.
5. YEAST FERMENTATION
Propagated Yeast Culture + Mash (s.g. ~1.096) + Nutrients → ‘Wash’ (8-10% alcohol)
Alcohol is formed according to the following equation:
C6H12O6 + Yeast → 2 C2H5OH + Heat
100 lb → 51.11 lb + 48.89 lb + 17,000 BTU
Fermentation is completed within 36-48 hours and the temperature is maintained by recirculation through a cooler. A pH of 4.2 – 4.5 and a temperature of 32-35°C are considered to be optimum for alcohol production. The above equation also shows that carbon dioxide is produced.
The purpose of distillation is to obtain the alcohol from the fermented wash (8-10% w/w alcohol composition) and ultimately refine it to produce the spirits that will be used to make the rum. The fermented wash contains not only alcohol but also many by-products that as a group are called congeners. These congeners are vital to the taste and aroma of rum.
The first column is the Wash Stripper or Beer Column; it removes water and residual solids from the ‘wash’ stream. The product from this column is heavy rum steam (80-85% ethanol by vol). This is our first product. It contains all the congeners from the fermentation. It is very flavourful and aromatic and it is inevitably aged. To make light, the heavy rum is then sent to the Purifier (Hydroselector) Column. Here the water added changes the vapour/liquid equilibrium so that the light components separate easily from the alcohol. The head goes to the alcohol recovery column, while the bottoms feeds the Rectifier Column; this stream is typically 12% alcohol. The rectifier concentrates the alcohol to be separated; a stream close to the top of the column is sent for final rectification, the bottoms is recycled to the purifier and the other cuts are sent to the Alcohol Recovery Column. This Recovery Column recovers the alcohol from all the by-product streams from the other columns.
The Final Column produces a bottoms product of 96.6% alcohol (light rum) that may be used to make rum.
The light and heavy rums are aged in oak barrels for periods of not less than two years and up to fifteen for the heavy rums. The spirits are eventually blended and sometimes colouring is added. For white rums, the residual colour from the barrel is actually removed.
The difference between rum and whisky manufacture begins with the starting point. Whisky starts with barley that must be malted before it is converted from a starch to a sugar that can be fermented. After fermentation, it is also distilled usually using a pot still or single column.
The sugar that is fermented to make cognac, come from grapes. As is the French style, only grapes grown in a particular area in France can be used. First the grapes are fermented into wine and this is distilled in special stills to yield cognac.
Brandy is made the same way but because the grapes aren’t grown in the Cognac region, it can not be called cognac. All spirits are aged in a special barrel prior to blending and bottling.