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The Science of Dunking

Can science do anything to bring the dedicated biscuit dunker into parity with the dunker of doughnuts? Could science, which has added that extra edge to the achievement of athlete and astronaut alike, be used to enhance ultimate biscuit dunking performance?

by Len Fisher

Doughnuts might have been designed for dunking. A doughnut, like bread, is held together by an elastic net of the protein gluten. The gluten might stretch, and eventually even break, when the doughnut is dunked in hot coffee, but it doesn’t swell or dissolve as the liquid is drawn into the network of holes and channels that the gluten supports. This means that the doughnut dunker can take his or her time, pausing only to let the excess liquid drain back into the cup before raising the doughnut to the waiting mouth. The only problem that a doughnut dunker faces is the selection of the doughnut.

Biscuit dunkers face much more of a challenge. If recent market research is to be believed, one biscuit dunk in every five ends in disaster, with the dunker fishing around in the bottom of the cup for the soggy remains. The problem for serious biscuit dunkers is that hot tea or coffee dissolves the sugar, melts the fat and swells and softens the starch grains in the biscuit. The wetted biscuit eventually collapses under its own weight.

Can science do anything to bring the dedicated biscuit dunker into parity with the dunker of doughnuts? Could science, which has added that extra edge to the achievement of athlete and astronaut alike, be used to enhance ultimate biscuit dunking performance and save that fifth, vital dunk?

These questions were put to me by an advertising company wanting to promote ‘National Biscuit-Dunking Week’. As someone who uses the science underlying commonplace objects and activities to make science more publicly accessible, I was happy to give ‘The Physics of Biscuit Dunking’ a try.

The first question that we asked was ‘What does a biscuit look like from a physicist’s point of view?’ We decided to be reductionist about biscuits, attempting to understand their response to dunking in simple physical terms and leaving the complications until later. When we examined a biscuit under a microscope, it appeared to consist of a tortuous set of interconnected holes, cavities and channels (so does a doughnut). In the case of a biscuit, the channels are there because it consists of dried-up starch granules imperfectly glued together with sugar and fat. To a scientist, the biscuit dunking problem is to work out how hot tea or coffee gets into these channels and what happens when it does.

One of the tasty heroes of our story

With this picture of dunking in mind, I sat down with some of my colleagues in the Bristol University Physics Department and proceeded to examine the question experimentally. Solemnly, we dipped our biscuits into our drinks, timing exactly how long they took to collapse. This was Baconian science, named after Sir Francis Bacon, the Elizabethan courtier who declared that science was simply a matter of collecting a sufficient number of facts to make a pattern.

Baconian science lost us a lot of biscuits, but did not provide a scientific approach to biscuit dunking. Serendipity, the art of making fortunate discoveries, came to the rescue when I decided to try holding a biscuit horizontally, with just one side in contact with the surface of the tea. I was amazed to find that this biscuit beat the previous record for longevity by almost a factor of four.

Paradigm shifts often arise from unexpected observations, but these observations need to be verified. The more unexpected the observation, the harsher the testing. In the words of Carl Sagan: ‘Extraordinary claims require extraordinary evidence’. No one is going to discard the whole of modern physics just because someone has claimed that ‘Yogic Flying’ is possible, or because a magician has bent spoons on television. If levitation did prove to be a fact, though, or spoons could really be bent without a force being applied, then physics would have to take it on the chin and reconsider.

One long-lived horizontal biscuit dunk was hardly likely to require a paradigm shift for its explanation. For that rare event to happen, the new observation must be inexplicable by currently known rules. Even more importantly, the effect observed has to be a real one, and not the result of some one-off circumstance.

The other hero of our tale (the biscuit - not the baby)

One thing that convinces scientists that an effect is real is reproducibility – finding the same result when a test is repeated. The long lived biscuit could have been exceptional because it had been harder baked than others we had tried, or for any number of reasons other than the method of dunking. We repeated the experiments with other biscuits and other biscuit types. The result was always the same – biscuits that were dunked by the ‘horizontal’ technique lasted much longer than those that were dunked conventionally. It seemed that the method really was the key.

What was the explanation? One possibility was diffusion, a process whereby each individual molecule in the penetrating liquid meanders from place to place in a random fashion, exploring the channels and cavities in the biscuit with no apparent method or pattern to its wanderings. The movement is similar to that of a drunken man walking home from the pub, not knowing in which direction home lies. Each step is a haphazard lurch, which could be forwards, backwards, or sideways. The complicated statistics of such movement (called a stochastic process) has been worked out by mathematicians. It shows that his probable distance from the pub depends on the square root of the time. Put simply, if he takes an hour to get a mile away from the pub, it is likely to take him four hours to get two miles away. If the same mathematics applied to the flow of liquid in the random channels of porous materials such as biscuits, then it would take four times as long for a biscuit dunked by our fortuitous method to get fully wet as it would for a biscuit dunked ‘normally’. The reason for this is that in a normal dunk the liquid only has to get as far as the mid-plane of the biscuit to be fully wetted, since the liquid is coming from both sides. If the biscuit is laid flat at the top of the cup, the liquid has to travel twice as far (i.e. from one side of the biscuit to the other) before the biscuit is fully wetted, which would take four times as long according to the mathematics of diffusion.

The American scientist E.W. Washburn found a similar factor of four when he studied the dunking of blotting paper – a mat of cellulose fibres that is also full of random channels. Washburn’s experiments, performed some eighty years ago, were simplicity itself. He marked off a piece of blotting paper with lines at equal intervals, then dipped it vertically into ink (easier to see than water) with the lines above and parallel to the liquid surface, and with one line exactly at the surface. He then timed how long it took the ink to reach successive lines. He found that it took four times as long to reach the second line as it did to reach the first, and nine times as long to

reach the third line. I attempted to repeat Washburn’s experiment and the biscuits turned out to be very similar to blotting paper when it came to taking up liquid. The Washburn equation not only explains why biscuits dunked by the ‘flat-on’ scientific method can be dunked for four times as long as with the conventional method - it can also be used to predict how long a biscuit may safely be dunked by those who prefer a more conventional approach. Only one assumption is needed – that the biscuit will not fall apart so long as a thin layer remains dry and sufficiently strong to support the weight of the wet bit. But how thin can this layer be? There was only one way to find out, and that was by measuring the breaking strength of dry biscuits that had been thinned down. I consequently ground down a range of biscuits on the physics department’s belt sander, a process that covered me with biscuit dust and which caused much amusement among workshop staff.

Whole dry biscuits, I found, could support up to two kilogram’s of weight when clamped horizontally at one end with the weight placed on the other end. The thinned-down dry biscuits were strong in proportion to their weight, and could be reduced to two percent of their original thickness and still be strong enough to support the weight of an otherwise saturated biscuit (between ten and twenty grams, depending on the biscuit type).

All that was needed now was to calculate how long the biscuits could be dunked while still leaving a thin layer, either in the mid-plane of the biscuit for a conventional dunk or on the upper surface of the biscuit for a ‘scientific’ dunk. The calculation was easily done using the Washburn Equation plus the values of the effective channel radius for different biscuits. For most biscuits, the answer comes out at between 3.5 seconds and 5 seconds for a conventional dunk, and between 14 and 20 seconds for a ‘scientific’ dunk.

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First Science 2014