Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 16/10/2007 13:10:24
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Despite being a former Physics teacher I got stuck on a problem. Why is it that chocolate chips in cookies melt if you touch them…but they survive the baking process?
Asked by Ian Guest, Sheffield
Read this Question from our Podcast (http://www.thenakedscientists.com/HTML/content/questions/question/1756/)
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We also received a related question:
"Hello, this is Simon Griffiths from Brisbane, Australia. I have an interesting question for the Naked Scientists. Why don’t Cadbury’s Flakes melt? I’ve tried microwaving and double-boiling with no luck. And I though it would be a tasty experiment for the Naked Scientists."
Diana: So we have several issues here - one of them is that cooking chocolate has different melting properties to ordinary chocolate:
Stephen: Hi my names Stephen Euston. I’m a Food Scientist in the School of Food Science at the Herriot Watt University. I’ve been asked to talk about the reasons why chocolate melts at different temperatures. When you make chocolate you roast cocoa beans to extract something called chocolate liquor. To make eating chocolate from that you normally add extra fat, cocoa butter, and that reduces the melting point of chocolate so that it becomes closer to body temperature. So it melts in your mouth when you eat it. With baking chocolate you don’t add extra fat and that gives it its higher melting point.
Diana: Then there’s the special case of the Flake…
Paul: Hello, my name’s Paul Bidder, I’m an Independent Food Technologist and I’ve spent some time working in the confectionary industry. I think the explanation for this is that the distribution of fat within the Flake type of chocolate is different from normal chocolate. Basically, chocolate is a mixture of very tiny ground particles, sugar cocoa and milk solids. That’s surrounded by the cocoa butter or cocoa fat. When the cocoa butter melts – if the fat is very well distributed it allows the fat to lubricate the individual particles and so they can slide over each other quite easily and that will give you a nice runny chocolate. If, on the other hand the fat within the chocolate isn’t as well distributed the particles of chocolate won’t slide over each other and not then give you a nice runny chocolate. It just happens that with the case of Flake, the manufacturing process is slightly different and the fat is not as well distributed as it would be in the case of ordinary chocolate.
Diana: But how do chocolate chips stay chip-y in biscuits and cakes?
Paul: This is slightly different, I think from the situation of the Flake in that, when chocolate does melt it doesn’t melt in exactly the same was as an ice cube would do if you heat it up. If you melt chocolate without applying any force to it, it actually tends to retain its shape and that’s because of the structure of all the particles that are there within the matrix of the fat. The biscuit around it provides enough support for the chocolate chip to remain chip-shaped through the baking process.
Diana: And why might cooking permanently change chocolate after it has reset?
Paul: It may be that during the baking process, the fat distributes itself around within the chocolate chip a little bit more effectively and so perhaps will melt a little bit easier. A more likely explanation is that the fat within the chocolate chip becomes mixed with some of the fat from the biscuit and when you get two different types of fat mixing together the actual physical effect is that it reduces the melting point of fat mixture and that may account for why the chocolate, after it has been baked appears to melt easier than the uncooked chocolate chip.
Diana: So to sum up: don’t cook with flakes and watch out for those cocoa fat matrices, because the wrong sort can make you very sticky!
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A few possibilities (big word for guess)
The fat content of the chip somehow prevents the m3elting, or slows down the melting process. Which then helps it solidify when cooler.
The chip is added at a later stage...
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maybe the pressure exerted from the cooking cookie holds the form of the chip. Baking soda and such. Yet when its touched by someone who can't wait until they've cooled, the pressure from the touch is enough to cause the chocolate chip to be disturbed and lose its shape.
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Possibly the chips are held together by their surface tension until the cookie material has bound together in the cooking process.
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I speculated that it's to do with fat - cooking chocolate has a different composition to eating chocolate. Since biscuit and cake mixtures usually contain a lot of fat, does the fat from the baking flow ooze into the chip and thereby alter its behaviour?