Why does dark rum form a frothy scum on the top of the drink when mixed with cola, but white rum does not?
鈥� Dark rum is essentially white rum plus colourings and flavourings. Recipes vary, but the traditional flavouring of dark rum is partly molasses, which is the residue left after the juice of sugar cane has been boiled and concentrated, and after as much sugar as possible has been extracted. Molasses is a good source of several nutrients, including calcium and magnesium salts. It also contains tarry caramelised compounds and degraded organic constituents of plant cell walls such as waxes, fatty acids and polysaccharides.
Cola drinks amount to flavoured solutions of phosphoric acid, and when you mix phosphoric acid with solutions of earth-alkali ions such as calcium and magnesium, they form insoluble phosphates. In a clear water solution these would precipitate clouds of insoluble phosphates, but in low concentrations in a mix such as rum, you get tiny crystals and lots of bubbles bound into scum by materials such as those fatty acids and waxes. This mix readily congeals on the surface of the drink.
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If you leave the glass to dry, the scum might require elbow grease or a good detergent to remove it.
鈥淩um recipes vary, but the traditional flavouring of dark rum is partly molasses鈥�
Jon Richfield Somerset West, South Africa