Making Rum from Molasses (Journey Into Rum Part 1)

One of my main distilling tasks at the Shakespeare Distillery was to help develop a rum and a spiced rum recipe. Here’s our journey making rum from molasses.

Molasses Delivery

The start of the Shakespeare rum project began in earnest one fine morning when a lorry driver delivered 5 tonnes of molasses to the distillery. The molasses in the tanker was 25-30°C when it entered the lorry tanker. At that temperature the consistency of the molasses was like water, so it was easy to pour from container to container.

Our molasses was sourced from the Tate and Lyle sugar refinery in London. The lorry driver said he delivered this same molasses to BrewDog as well, so it seemed like it was a good base material to start with. The intermediate bulk containers (IBCs) filled very quickly with molasses. Since the molasses was warm it took less than 10 minutes to fill a 1000L IBC.

We wheeled the four IBCs full of molasses back into the distillery. Afterwards, I took a sample of molasses from an IBC to do a small-scale rum distillation with.

Molasses Wort Preparation

Before I started working at the distillery, my boss, David, had done 30 small rum experiments already. The molasses he used was from a different supplier. The quality of that molasses was very low; it had a strong bitter taste and I think there were a lot of unfermentable sugars in it. Unfermentable sugars are sugars that cannot be fermented by the yeast. In other words, the yeast is unable to convert these sugars into alcohol. The more unfermentable sugars there are in the molasses, the less alcohol will be produced. This new molasses was better tasting and sweet, so we hoped it would produce a better tasting rum as well.

Refractometer Measurements

David was using a refractometer to measure the starting sugar content of the molasses wort. A refractometer operates on the idea that light passing through a sample of wort will refract light at different angles. The degree of light refraction depends on the concentration of dissolved sugar in the wort. A refractometer is useful for doing a quick initial reading, but things get less accurate as soon as the fermentation begins. Fermentation produces alcohol, which means the sample will have both sugar and alcohol in it. The presence of alcohol will change the refractometer reading making it inaccurate.

Since David had no other measurement devices at the time, he used the refractometer to measure the liquid before the fermentation started AND after it ended. The data, which was in Brix, was used to calculate how much alcohol was produced from the fermentation. We couldn’t trust those numbers since the refractometer isn’t accurate enough on its own.

Luckily, we now had more than the refractometer to use. Our new iStill fermenter came with a tilt sensor that measures the specific gravity of a liquid.

Calculating Our Ingredients

We weighed out 1L of molasses, then proceeded to calculate how many litres of molasses would be needed to make up 4.6L of our molasses wort. Of the previous 30 small rum experiments David did, there was one in particular that we really liked during the taste test. So in the following set of experiments, we replicated that previous test batch.

After weighing the molasses, we weighed out the water needed to make 4.6 litres of molasses wort at 14°Brix. In our little plastic jugs (aka, our makeshift fermenters), we slowly added citric acid to get the pH down to 3.5 – 4.0. We needed 75% less citric acid than we did for the previous test batches using molasses from the different supplier. This was good news as it meant that we would not have to buy as much citric acid to lower the pH. The reason we were trying to lower the pH, or make the wort more acidic, was because we liked the flavours that the yeast produced during fermentation in a more acidic environment.

Afterwards, I pitched in dried baker’s yeast. “Pitching in the yeast” is just another way of saying that we added in yeast.

Fun fact: the dried baker’s yeast that you buy in the grocery store is produced on nutrient sources rich in sugar. The sugar source used is usually molasses. That is why you will see a lot of home distillers turn to baker’s yeast when they are making rum.

Molasses Wort Fermentation

David, being the resourceful man that he is, purchased aquarium heaters to maintain the fermentation at the right temperature. We set the fermentation temperature to around 35°C, then popped on the lids and air locks.

An air lock is a funny little plastic device with water inside. During fermentation, yeast will eat up the sugars in the wort, and convert the sugars into alcohol. Carbon dioxide gas will also be released during the fermentation process. Carbon dioxide will rise up into the airlock. The pressure from the carbon dioxide will build up until it pushes through the water in the airlock, causing the water in it to burp and bubble during the fermentation. The escaping CO2 will help to relieve the pressure inside.

A very active fermentation will also be very noisy since you will be able to hear carbon dioxide gas escape through the air lock. The water inside an airlock will also prevent the surrounding air, which isn’t pressurized, from getting into a fermentation vessel. In this way, carbon dioxide gas is able to leave a fermentation vessel, but air is unable to get inside a vessel.

This was the reason our fermentation vessels had air locks on them. We then sealed the tops of the fermentation vessels with a plaster gun to make sure no air could seep into the fermentation vessels. CO2 would only be able to leave through the airlock.

Molasses Wash Fermentation Experiments

For our first fermentation vessel, we prepared 4.6L of molasses wort at 14°Brix. It had a starting pH of 3.5-4, a temperature in the mid-thirties, and we pitched in dried baker’s yeast. During the course of the fermentation, we came in every day to shake the fermentation vessel for 30 seconds to make sure everything was mixed.

The second fermentation vessel was similar to the first, except we did NOT shake the fermentation vessel. The third fermentation vessel was the same as the first one, except we first rehydrated the baker’s dried yeast in some warm water for 15 minutes before pitching it into the fermentation vessel. The three molasses wort fermentations were then left to ferment out.

This was the first part of our journey making rum from molasses. Read about our molasses fermentation in the next post.

Watch it on YouTube Below

Related Content

• Journey Into Rum Part 2