rumMix yourself a Rum and Coke cocktail using a dash of dark rum and a classic cola then watch a harmless frothy scum form around the added ice cubes. Then mix another Rum and Coke but using pale rum (Barcardi is suitable but a supermarket own-label will be just as good at two-thirds the price). No dirty looking froth forms this time, but why is that?

Distilling molasses, the gooey burnt sugar syrup left over when sugar cane is turned into sugar, produces a colourless and highly alcoholic spirit. Adding back a bit of the dark brown molasses turns this 'white' rum into a sweeter, darker version of the original.

The added molasses contains dissolved calcium and magnesium (the same stuff that furs up the kettle in hard water areas). Adding cola causes a chemical reaction to occur which turns these into solids. These float on the surface of the drink and form the scum which is really difficult to wash off unless you leave the glasses to soak.

White rum doesn't have the added molasses so there is nothing for the cola to react with. Hence no scum and a visually more attractive drink.

Before the ice cubes melt perhaps I could interest you in one last observation? Had you noticed how there always seems to be bubbles of air trapped in the cubes? No? Well unless you've been using the blemish-free fake plastic ice cubes seen only in billboard drinks adverts then your cubes will be imperfect. All tap water contains dissolved air and this gets trapped as the water freezes.

PG Supermarket Rum Price Comparisons

Own-label dark rums

Asda Dark Rum (1 litre) £15.90
Asda Spiced Rum (70cl ) £14
Sainsbury’s Basics Dark Rum (70cl) £10
Sainsbury’s Superior Dark Rum (1 litre) £15.50
Tesco Dark Rum (1 litre) £16.50
Tesco Everyday Value Dark Rum (70cl) £10

Own-label white rums

Asda White Rum (1 litre) £15.90
Asda Carta Blanca Superior White Rum (1.5 litres) £22.50
Sainsbury’s Basics White Rum (70cl) £10
Sainsbury’s Superior White Rum (1 litre) £15.50
Tesco White Rum (1 litre) £16.30
Tesco Everyday Value White Rum (70cl) £10

This article also appeared as Paula's Wines of the Week on MatureTimes.co.uk