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Let’s talk about the fermentation process

Gregoire Erchoff

In my previous post I talked about sugar – where it come from etc. I think the fermentation process is really key to understanding the character and specificity of rum as it relates to the organoleptic nature of the drink. According to most master blenders and rum distillers, the majority of flavour and aroma is born during the fermentation process. They will then be separated, isolated or discarded during distillation.

What is the raw material that is to be fermented? Sugar of course, but in what form?

Juice /syrup/molasses/HTM… and what do these contain?


(1) High test Molasses


Molasses is a viscous by-product of the refining of sugarcane into sugar. The word comes from the Portuguese melaço, ultimately derived from mel, the Latin word for “honey”.[1] Molasses varies by amount of sugar and method of extraction, and age of plant.

Sirup is the result of a first boiling. It is called syrup first, and has the highest sugar content. Syrup is also referred to as “cane syrup”.

Sugarcane juice is the juice extracted from pressed sugarcane. The juice is obtained by crushing peeled sugar cane in a mill. It can be a hand cranked machine, or powered.

High test molasses, which is also called cane high test molasses, invert molasses and cane invert syrup, is manufactured by evaporation of the partly inverted cane juice. Strictly speaking, high test molasses is not a molasses but an inverted syrup.

If we are talking about Authentic Caribbean Rum, most of them are made from Molasses and only some from sugarcane juice or syrup

Preparing the wash:

This operation has a two-fold objective. First, to yield a good quantity of alcohol. Second, to bring out all of the secondary aromatic flavours from the alcoholic fermentation.

Composition of the wash:

The composition is the process of blending the sugar material (molasses or juice or concentrate) with water or sometimes vinasse (left over from the first distillation with no alcohol content). To get a concentration of sugar around 100 g/l. the temperature must not exceed 37degrees to ensure a good fermentation.

According to the tropical temperature of the wash at the end of the fermentation process, it can be between 26 to 30degrees.

Weak density could result in the unwanted growth of bad bacteria, which could disturb the fermentation process.


Acidification of the wash is necessary to avoid the development of many bacteria. However, an over acidification of the wash could stop the yeast from working.

In the old days the acidification of the wash was obtained by introducing vinasse instead of water into the wash.

Molasses.jpgPreparing the wash :

The molasses is diluted with water or sometimes vinasse for the Grand Arome type of rums like Jamaican rums. Water heavy in minerals should be avoided. Liquids with bacterial flora should be discarded as well, as they could start a secondary fermentation with harmful components.

Where necessary, distillers can adjust the PH level of the wash with a mineral acid. (which totally disappears in the distillation process).

The yeast :

To start the fermentation process yeast is required. There are different types or strains of yeast – cultivated by the distillery, dried selected yeast, manufactured, and natural yeast.

Saccharomyces Cerevisae is the most common strain, used since 1945.

Fermentation is the result of the transformation of the sugar into alcohol.

The sugar from the cane is mostly sucrose. During the first step of the fermentation the sucrose is transformed into glucose and fructose. Then these elements enter into the process of glycolysis, which allows the cells to transform sugar into pyruvic acid, after which goes through decarboxylation to become ethanol. One molecule of glucose gives two molecules of ethanol.

Secondary fermentation:

During the fermentation process, and because of the different elements that are in the cane juice or molasses and water, the bacteria is subject to producing different acids (propionic, lactic, acetic, etc.)

Such acids can give unwanted flavours or special organoleptic character to the Rum.

Control of temperature of the wash:

The control of the temperature of the Wash is important for different reasons:

Over 35°c the activity of the yeast becomes slow.

Loss of alcohol y evaporation.

Secondary fermentation contents like acrolein.

At low temperature the yeast produces more ester.

The time for fermentation is around 12h for light Rum and from 24h to 36h for others.

It can take to 10 days or more for the Grand Arome type of Rum like that of Jamaica.


‘The views expressed above are the opinions of the writer and do not necessarily represent the views of WIRSPA Inc’.
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