Why does a biscuit go soggy more quickly in a hot, milky drink than it does in a hot, watery drink?
Chris - Biscuit dunking duration was actually the subject of a proper scientific study in recent years. It was done by Len Fisher who is based at the University of Bristol. He published his findings in his book "How to dunk a biscuit". The reason the nation's favourite snack goes soft when you dunk it in anything is because biscuits are largely made of starch molecules (thatís the flour), which are long chains of molecules. 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 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.
Different biscuits with different physical properties and dimensions have different "dunking times" - in other words how long it takes before biscuit-meltdown occurs. For Rich Tea the dunking time is 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 youíre 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. Youíll get more flavour and more retro-aroma, as itís 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.
Another possibility is that it could be down to the surfactants in the milk. In order to keep the fat suspended, milk contains chemicals called surfactants. These behave a bit like washing-up liquid, which lifts fat off your washing-up by making oils dissolve. A consequence of surfactant action is that reduce the surface tension in the water so that could mean that the water can get into the biscuit quicker. If itís absorbed faster it will have more time to dissolve the sugar. Therefore the dunking time for a given biscuit in a cup of tea made with full-fat milk might be lower!