Why do crisp ginger biscuits go soft if left exposed to the air for a couple of days when other baked products, such as cakes and bread, go hard?
Krista Nelson Rokeby, Tasmania, Australia
The difference between a cake and a biscuit is similar to the difference between bread and toast. Bread starts out soft and moist but dries out over time and becomes an unpleasant mix of soft and dry. Toast is made by drying out bread to make it crisp. As toast ages, it absorbs moisture, making it rubbery and unpleasant.
Biscuits are the toast of the cake world and were originally made by baking cakes twice to dry them out for storage. If you leave a cake and a biscuit in a standard kitchen, the moist cake will dry out and the dry biscuit will become soggy, both approaching the same state, just from different directions.
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Claire Gregson Portadown, County Armagh, UK
Biscuits are essentially dried cakes, so absorb ambient moisture. Cakes are much more moist, so evaporate water to the surrounding air. Just eat and enjoy.
David Jackson Liverpool, UK
Biscuits start out with a very low moisture content of between 1 and 3 per cent, depending on the type, whereas this is around 15 to 30 per cent for cakes. In an atmosphere of moderate humidity, water will diffuse out of a cake and into a biscuit until equilibrium is reached, not only with the entrapped air, but also the starchy, sugary matrix of the product.
In the 1990s, biscuit and cake manufacturer McVitie’s fought a UK tax claim on its Jaffa Cakes. Chocolate-coated biscuits are subject to value-added tax (VAT) in the UK, whereas chocolate-coated cakes aren’t, so a huge amount of money was at stake.
McVitie’s (my employer at the time) won the case, partly because cakes, including Jaffa cakes, become dry when they go stale, whereas biscuits go soft.
Gerald Dorey Oxford, UK
This question is an example of the profound cake-biscuit existential problem exemplified by the Jaffa Cake. This chocolate-coated confection has provoked much discussion: a BBC radio programme that discussed the subject even invoked Ludwig Wittgenstein’s ideas of the futility of “family resemblance” tests, whether a categorisation is merely a semantic reflex and whether a non-binary category might be applied.
Lawyers are more practically minded, and in 1991, a UK VAT tribunal decided that the critical factor was whether the item absorbs moisture over time and becomes softer, as with biscuits, or gives up moisture and hardens, as with cake. This confirmed that the Jaffa Cake is a cake, and is therefore liable for a lower rate of tax than it would have been if classified as a biscuit.
Clearly, the ambient humidity is critical. I would suggest that in countries with weather that is hot and humid enough to melt the chocolate on a Jaffa Cake, no cake would harden, nor would a biscuit stay hard for long. The reverse would be true in deserts. In both areas, the cake-biscuit duality disappears.
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