[(11 2 Let’s get physical
Catherine Fellows talks to Dr Peter Barham about the science ofcake making.
If you have driven past lidinburgh‘s‘ (‘ameron 'l'oll shopping centre recently. you may haye noticed an inordinately large piece ofcake. It is a billboard ad for this year‘s Edinburgh International Science Festival. where Dr Peter Barham. Lecturer in Polymer Physics at Bristol University. is delivering a lecture on the Black Forest (iateau.
‘I happen to be a person who likes cooking.‘ is his perfectly reasonable justification. ‘We scientists are eager to communicate with the public. We want to share our enthusiasm. A lot ofthe work that I do is very esoteric. but a great deal of what I know about polymers is directly applicable to cooking. ('ooking is something people relate to -- the kitchen is a scientific laborator y in which everyone has conducted experiments. The preparation of a Black Forest (iateau involves all kinds ofscientific processes and the finished result should be spectacular.‘
Barham's talk will begin with a briefexplanation ofterms to overcome the language barrier between people and science. ‘lnsteadoftalking about eggs and flour. we talk about proteins and starches.’ he says. going on to describe what happens when you make a cake. ‘Basieally. all the action is in the eggs. 'l'hey are composed of large protein molecules in solution. When you beat them up. you change the state of the molecules. Imagine that they are like balls of wool; w hen you beat. you unravel them into sepai ate strands. forming one large tangle. At the same time you are incorporating lots of air. In culinary terms this is a mousse. we call it a foamed gel.
‘You add sugar to the eggs belot c beating because the more v iscous the liquid. the easier it is to ‘stretch'. and the quicker the molecules will ‘denature‘. People vs ho say yeur tneringues won't work if you has e even a trace of yolk are talking absolute nonsense; you will iust have to beat much harder. Not only does the yolk make the solution more runny. but the cholesterol in it suppresses the denaturing process. One w ay of speeding tip the process. and the way they used before the daysol electric yy hisks. is to heat the eggs by doing your beatingoy er a
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bum mime. When the molecules get hot they vibrate so much that they shake theittselvcs apart .’
As he is talking. l)r Barham will demonstrate his theses by making a cake on the spot. ‘lt cart be a bit tricky. what with the electric whisk drowning me out. but 1 usually manage to get it into the oven. ’l‘hen I use the Blue Peter trick and produce "one I prepared earlier" on
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which to illustrate the decoration stage.’
Whipping cream is a whole different ball of wool from eggs. ‘('ream is a suspension of fat droplets in water: when you beat it you reverse the structure. The fat droplets become longer and thinner until eventually they become a network. the continuous phase. with the discontinuous water droplets
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suspended in it. For this to work you need at least 30 per cent fat; over 50 per cent and you have butter which is always in the ‘water-in-fat' stage. Manufacturers ofwhipped cream often substitute some ofthe fat with seaweed derivatives which perform the same function.‘
As he smothers the cake in cream and cherries. Dr Barham will reveal that the physicist is a culinary