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The Fermentation Of Molasses

July 2, 2021
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The most important step when creating rum is fermentation, which is why we spent over 18 months researching and experimenting to achieve a bold and flavoursome product. In this blog I am going to give you an insight into the fermentation process and the variables which influence our rum’s fantastic flavour.

Before we dive into the key components let’s start by looking at how alcohol and flavours are created. Fermentation is the chemical reaction of water, yeast, and sugar; but depending on how this is treated depends on how much alcohol is created and what flavours are produced. When the mix is heated up it allows the yeast to convert the sugars into alcohol. Whilst this is bubbling away a process called esterification occurs. Don’t switch off! Sounds boring I know but it’s quite simple; esterification is the formation of flavour molecules also known as esters.

The flavours that are produced are dependent on the following variables:

  • Sugar source
  • pH
  • Temperature
  • Duration of fermentation

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Sugar Source

Yeast requires a source of sugar in order for it to consume and subsequently turn into alcohol. For whisky the sugar source is typically from grains such as malted barley, for rum you need to use a product that derives from sugar cane. There are several options for rum producers, either sugar, sugar cane juice or molasses. Many larger commercial distilleries will opt to use pure sugar as their base. We experimented with this and what it does is produce a lot of alcohol, as the yeast finds it very easy to convert into alcohol, so great you might think? however very little flavour is created, not so good! As craft distillery we can never compete on scale, we therefore need to compete on flavour, making stunning products that are different to mass produced spirits. Sugar cane juice is generally only used in countries with their own supply of sugar cane as it needs to be fresh when used, hard to achieve if you live in a cold climate far away from a source of sugar cane. The other option which many craft distilleries opt to use is molasses, this is the leftover part of sugar cane which is not so easy to convert into sugar crystals, it is discarded by sugar refineries. We use molasses as our base as it contains a greater depth of flavour, it is harder for the yeast to convert into alcohol, therefore your yield is lower but you end up with a superior tasting product.


When we started our fermentation journey, we found that the pH level has a huge effect on the overall style of the finished product, which led us to play around with high and low pH fermentations. Generally speaking we found that the lower we went the sourer the flavour profile, whereas a higher pH creates a sweeter spirit. So pH is pretty easy to understand but not so easy to control.

We wanted to bring some of the sour notes into our rum, why you might ask as rum is meant to be sweet right? Well actually we feel the best rums are those that are more 3 dimensional, you want to taste the sweetness from the molasses but you want those rum flavours to linger in your mouth. Lowering the pH allows us to bring a deeper lingering flavour after the initial sweetness has hit your tastebuds.

There are several things you can do to influence the pH level. We found it’s quite common that calcium hydroxide or calcium carbonate are used to increase the pH resulting in a sweeter style of rum.  Whereas you can use citric acid to reduce the pH, which stresses the yeast further to create those slightly sourer flavours. There are other acids used within the industry to reduce pH however citric is the safest and doesn’t have a negative effect on the flavours produced whereas other acids have been known to create stale flavours or a vomit-like aroma! Despite the demand for weird and wonderful flavoured spirits hopefully no one would go this far.

Fermentation of liquid molasses in school experience


Dead simple to get this; just think back to high school chemistry lessons. Do you remember how as temperature increases, the rate of the chemical reaction speeds up? This is the basic principle that we must bear in mind when fermenting. To give you an idea, if we increase temperature by 10 degrees we will see the chemical reaction double in speed. This is great for rum production because with the chemical reaction increasing so does the number of esters produced. When you try our Jester Rum this is one of the first things you will notice, the front of mouth citrus followed by rich fruity notes as a result of all those esters.

Duration of Fermentation

Tick Tock, how long should you leave your fermentation to ferment for? What we’ve learnt is that the longer you leave it the more flavour molecules are created, which is great when we are creating a bold and flavourful rum. However, if you wanted to create a vodka then you don’t want lots of flavour, but you do want lots of alcohol. During the first 3 days of a fermentation almost all the alcohol is made; after the 3rd day you will make maybe 0.5 – 1.5% if you’re lucky. Which is great, a nice quick fermentation with loads of alcohol – fantastic! But it won’t be until you distil the wash that you will realise that there is very little flavour, in fact 60% of flavour molecules are produced in the 4th 5th and 6th days. Unfortunately, many commercial rum distilleries also have short fermentations, because they want maximum yield.

When a fermentation is left for a long time you will get a fantastic fruity middle mouth feel, this is a great but rare characteristic to find in a spirit. So guess how long we leave our fermentation for? 7 days, reducing what we can produce but making a product we are so proud of.

To sum all of this up, when fermenting our Jester Rum we opted for a flavour rich sugar source, opted for higher temperatures to generate more flavour molecules, opted for a long fermentation to allow the flavours to develop and controlled the pH to allow these flavours to sing and linger on the palette. The difference with Jester Rum then? You guessed it Flavour! That’s what craft is about.

Cheers, Sam