From cane juice to alcohol : Fermentation


A vital energy

Living cell turn organic compounds such as sugar into energy through oxidation. This process is called cell respiration. Most of the times, it consumes oxygen and releases carbon dioxide and water.
Under certain circumstances, the cell creates its energy without a supply of oxygen. Some of those processes are :

  • Alcoholic fermentation: the one used in the rhum making process;
  • Lactic fermentation: the process is involved in muscles when oxygen supply tends to be insufficient or in the yoghurt making process.
  • Butyric fermentation.

 

Alcoholic fermentation

Alcoholic fermentation results from bacteria or yeast activity. It lies at the roots of alcohol found in rhum agricole.
Mankind has been using yeasts from the dawn of humanity to turn sugar into alcohol (beer, wine, ...) or to make bread rise.
Yeasts used in fermentation may be :

  • Spontaneously fermention: vesou (fresh sugarcane juice) is sowed by wild yeasts occurring in the local environment. Rhums produced this way have usually richer aromas, due to the yeast diversity. However, it's a rather risky process since yeasts could be overrun by other yeasts or bacterium, generating different kinds of fermentation (butyric, lactic, ...), voiding the full batch. This process hence requires higher production control to avoid contamination;
  • Pitching: in order to be sure to select the right fermentation kind (alcoholic rather than lactic, ...) as well as a good yield, the yeast is usually pitched into vesou (fresh sugarcane juice). Yeast strains used in this process are bred ones and therefore guarantees results reproducibility;
  • Master batch: a sample from vesou is pitched and heated to foster fermentation. When the fermentation is well established, the tank content is sent to pitch the other vesou tanks. Yeast strains are also industrially bred as in the straight pitching technique.

Fermentation is an exothermic (heat generating) one. Since most yeasts cease activity or eventually die above 35°C (95°F) and rhum being produced in tropical climate, temperature control is hence vital in order to avoid reaching excessive temperatures. Limiting the temperature also avoid contamination from other yeast or bacteria strains.
To further limit the temperature, batches are more watered in the south of Martinique as temperature are usually higher and water supplies too scarce to allow tank dousing as in the north part of the island.
The fermented liquid, locally known as vin is hence lightly concentrated in alcohol (3% to 6% abv - 6 to 12 US proof), a value well below maximum concentrations usually attained by yeasts (usually under 20% abv / 40 US proof).
This very low alcohol concentration affects the way distillation is conducted.

 

Raw materials

Traditional rum and rhum agricole are both obtained through the fermentation of a solution of cane sugar. The main difference between them is in the way the sugared solution is made:

  • Traditional rhum:  The solution is made of a by-product of the sugar production, molasses. Molasses is a thick syrup which is watered down before being pitched. Molasses are frequently imported form elsewhere, Brazil being the main producer of molasses. Quality and specificity of a traditional rhum hence rather reflects the skills of the distiller than the cane growing specificities;
  • Agricole rhum: The solution is pure fresh cane juice, also called vesou. Fresh sugarcane and its juice have to be processed quickly as wild strains of bacteria or yeast found on the cane peel are prone to develop and later the quality of the product. Rhum agricole has hence to be made close to the fields where the cane grew. Rhum agricole is therefore a terroir product.

 

A.O.C. specifications

Saccharomyces cervisae
Saccharomyces cerevisiae.
WikiCommons image

According to A.O.C. specifications, fermentation must be discontinuous and conducted in open tanks whose capacity does not exceed 500 hectolitres (13'208 US gallons). In a continuous fermentation process, fermentation starts at pitching and ends when the sugar supply has been exhausted or the ethanol has reached toxic levels for the yeasts (usually under 20% abv). As for rhum the limit is the sugar supply exhaustion as the vin has an typical concentration below 6% abv.

Pitching is limited to only one yeast type: Saccharomyces and has to be conducted according to one of the following techniques:

Fermentation (from pitching to distillation) has to be made under 72 hours and temperature has to remain below 38,5 °C (101°F). Fermented solution minimal concentration is 3,5% abv (7 US proof).
A special committee appointed by the AOC regulating body also write specifications on certain fermentation characteristics such as:

  • Nitrogen and phosphate additions during fermentation;
  • acidification (pH lowering) which ensures yeast protection against bacterial contamination.