What is molasses and what does it do in rum?

Molasses is what remains at the end of sugar extraction, when no more crystal sugar can be obtained from the thickened juice of sugar cane or sugar beet. The cane is pressed, the juice boiled down, and the sugar crystallises out in steps. What is left in the centrifuge after the last crystallisation is molasses.

The Food and Agriculture Organization of the United Nations puts the yield at about 3.0 percent per tonne of cane. That yield fluctuates with the season, the variety and the mill settings: the FAO cites a range of 2.2 to 3.7 percent. From a thousand tonnes of cane, roughly thirty tonnes of molasses are therefore released.

Besides sugar there is all sorts in it: on average 20 percent water, 12 percent ash, 4.5 percent nitrogenous compounds and 5 percent non-nitrogenous acids. It is precisely this residual fraction that gives a molasses rum its own character.

Molasses contains on average 35 percent sucrose, supplemented with 9 percent fructose, 7 percent glucose and 3 percent other reducing sugars. Together that amounts to more than half of the weight.

For a distiller, that is the attractive side of the by-product. The sugar is already concentrated, so the molasses only has to be diluted and fermented. With fresh cane juice, you have to get rid of water instead.

• Sucrose: 35 percent
• Fructose: 9 percent
• Glucose: 7 percent
• Other reducing sugars: 3 percent
• Other carbohydrates: 4 percent

The remainder consists of water, ash, acids and nitrogen-containing compounds. Those substances are not fermented into alcohol, but they feed the yeast and yield aromatic by-products during distillation.

Rum makers use molasses because the sugar in it is already concentrated, it keeps all year round and it becomes available as a by-product of the sugar mill. Fresh cane juice can do none of the three.

European law leaves little room to adjust rum afterwards. Annex I, category 1 of Regulation (EU) 2019/787 prescribes that rum is distilled to less than 96 percent, so that the recognisable characteristics of rum are preserved. Adding alcohol, diluted or undiluted, is prohibited. Flavouring is not allowed. For colour, only caramel is permitted.

Anyone who wants to use the designation traditionnel or tradicional must go further: distil below 90 percent and reach a volatile substances content of at least 225 grams per hectolitre of 100 percent alcohol. Such a producer may not sweeten the product either. Those 225 grams are exactly where the flavour compounds from the molasses are found.

Rhum agricole does not come from molasses but from freshly pressed sugarcane juice. The French specification for the Rhum de la Martinique designation of origin expressly prohibits enriching that juice with by-products of sugar production, syrup and molasses included.

The rules are tight. Fermentation may last at most 120 hours, counted from filling the vat until distillation. Distillation takes place on a continuous column with at least 15 plates in the exhausting zone and 5 to 9 fully copper plates in the concentration zone. The daily distillate must come out between 65 and 75 percent, and for sale to the consumer 40 percent is the minimum.

The designation has existed since 1996. A rhum vieux spends at least three years on oak wood and is bottled on Martinique.

Regulation (EU) 2019/787 allows only three raw materials for rum: molasses or syrup from cane sugar production, or the sugar cane juice itself. Sugar beet molasses falls outside this and therefore does not yield rum, however similar the product may look.

Annex I of that regulation lists 44 categories of spirit drinks. Rum is number 1. Besides the ban on added alcohol and flavouring, there is a limit on sweetening: the end product may contain at most 20 grams of sweetening products per litre, expressed as invert sugar. The minimum alcoholic strength is 37.5 percent.

The term agricultural may not be used freely. It is reserved for rum distilled exclusively from fermented sugar cane juice, and only in combination with a geographical indication of a French overseas department or the autonomous region of Madeira.