Science Questions

Why does a biscuit go soggy more quickly in a hot, milky drink than it does in a hot, watery drink? Nigel

Chris: This was actually the subject of a proper scientific study. It was done by Len Fisher who I think is at the University of Nottingham. “How to dunk a biscuit’ was the title of the book he wrote. The reason that it goes soft when you dunk it in anything is because biscuits are largely made of starch molecules (that’s the flour) and those are long chains of molecules. In between them are sugar crystals that stick them together along with some fat as well.

When you put the biscuit in something hot like hot tea the heat from it melts the fat. Also the water soaks into the holes between the molecules and dissolves the sugar. This makes the whole structure become very weak and the biscuit falls apart.

There’s a certain dunking time; for Rich Tea the dunking time was about four seconds. Digestives last longer, they can tolerate up to ten seconds of dunk before they start to fall apart. The only difference they found between the milky tea and non-milky tea was the flavour. They found that milky tea makes the biscuit flavour much more intense. The reason, probably, is because milk has got a lot of fat in it unless you’re drinking liquid water (the white skimmed stuff). The fat intensifies the fat-soluble flavours in the biscuit, helps them to evaporate and go up your nose. You’ll get more flavour and more retro-aroma, as it’s called, from a biscuit dunked in milky tea than one dunked in black tea. The only possible exception I can think is that perhaps the fat in the milk is helping to liquefy the fat in the biscuit a little bit faster.

Dave: In order to keep the fat suspended in milk there are lots of things called surfactants. They’re actually like washing-up liquid in that it can lift fat up off your washing-up. They might reduce surface tension in the water so it could mean that the water can get into the biscuit quicker. If it’s absorbed faster it will have more time to dissolve the sugar.

November 2007