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Biscuit Dunking Physics


Biscuits (or what the Americans call "cookies") have been around for about 2,300 years, but only very recently have we started applying science to biscuits . I'm not talking about how to best make biscuits, but the serious business of how to dunk biscuits. Thanks to physics, we have discovered which biscuits are best for dunking, and we've even discovered that tea and coffee are not the best liquids for dunking your biscuits in.

Biscuits started off in Rome around the 3rd Century BC. The word biscuit comes from the Latin "bis coctum" which means "twice-baked". Back then, a biscuit was a thin unleavened wafer, quite hard, and with a very low water content - hence the name "twice-baked". The advantage of the low water content was that the biscuit would have a long shelf-life, because it wouldn't get mouldy. Way back then, it was a few thousand years before tea and coffee would made it to Europe, so the ancient Romans would soften their hard biscuits by dunking them in wine.

But nowadays the preferred liquids are hot tea or coffee. But there are a lot of different factors to consider when you want to dunk a biscuit. After all, you don't want your biscuit disintegrating, leaving you with an unattractive sludge on the bottom of your hot cuppa. Some new-fangled modern biscuits have a central creamy section which is prone to melting leaving behind two tasteless wafers, while other biscuits will simply collapse into a sloppy mess. Some biscuits are too stiff and rigid to enjoy easily before dunking, but pleasantly edible after dunking. But structural integrity is only part of the story - there's taste as well. For example some biscuits are boring and tasteless before you dunk them, but delicious after you dunk them.

So in 1998, Dr. Len Fisher (working out of the University of Bristol in the UK) decided to look at the Physics of Biscuit Dunking.

Now a biscuit is basically dried-up grains of starch, which are glued together with sugar. The hot liquid will swell and soften the grains of starch - which is good. But the hot liquid will also dissolve the sugar, so that eventually the wetted biscuit loses so much structural integrity that it will collapse under its own weight.

The reason that it gets wet is because a biscuit is porous. It is riddled with interconnecting hollow channels. Once the tea or coffee gets access to these channels, capillary action sucks the liquid deeper into the channels. Len Fisher used an old equation from 1921 to predict how long it would take for the liquid to rise in your favourite biscuit.

But he did more than just scribble equations - he did experiments, involving gold, a belt-sander, a microscope, an X-ray machine, and sensitive weighing scales. He found that the best dunking time for a gingernut biscuit was 3 seconds, but 8 seconds for a digestive biscuit.

Overall, his personal recommendation was to use a wide-brimmed cup filled almost to the top, to do horizontal dunking (so that only the bottom side got wet), and then to quickly turn your dunked biscuit upside down so the stronger dry side gives structural integrity to the wet side.

But in 1999, Len Fisher decided to come back and look more deeply at flavour. He wanted to work out which liquid is the best for dunking. To his surprise he found that the best dunking liquid is not hot tea or coffee - but a milk drink.

Now there are two factors to flavour - taste (where the flavour chemicals excite the taste buds on your tongue) and smell (where the flavour chemicals excite the olfactory epithelium in your nose).

To analyse the smells, his team inserted a tube into one nostril of the eager volunteers. The different flavour chemicals from each sample of air were separated and then analysed.

And sure enough, dunking your biscuit into a milky drink gives you up to 11 times more flavour release than from eating the dry biscuit alone.

Why? Well, the answer lies in the fat in milk. Milk is basically little tiny droplets of fat which are suspended in water. These droplets of fat do two things. First, they absorb the flavour molecules really well. Second, these little fat droplets also hang around in your mouth, so that the flavour and aroma chemicals can sit on your tongue AND be released up to your nose.

But hot non-milky drinks tend to carry the flavour molecules straight down your gob and into your gut before the taste sensors on your tongue and the olfactory epithelium in your nose have had a chance to fully appreciate them.

His team also found that the worst drink to have with your biscuit is a soft drink such as lemonade. The flavour does not stay the same, it actually goes down by a factor of 10.

But these remarkable research results don't come easy - you've got to be prepared to work right through your morning tea break.

Tags: physics

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Published 03 February 2000