Choc Chips Survive Baking
Sun, 21st Oct 2007
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from the show Particle Physics - The Secrets of the Universe
Ian Guest, Sheffield asked:
Despite being a former Physics teacher I got stuck on a problem. Why is it that chocolate chips in cookies melt if you touch themÖbut they survive the baking process?
We also received a related question:
"Hello, this is Simon Griffiths from Brisbane, Australia. I have an interesting question for the Naked Scientists. Why donít Cadburyís Flakes melt? Iíve 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. Iím a Food Scientist in the School of Food Science at the Herriot Watt University. Iíve 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 donít add extra fat and that gives it its higher melting point.
Diana: Then thereís the special case of the FlakeÖ
Paul: Hello, my nameís Paul Bidder, Iím an Independent Food Technologist and Iíve 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. Thatís surrounded by the cocoa butter or cocoa fat. When the cocoa butter melts Ė 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 isnít as well distributed the particles of chocolate wonít 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 doesnít 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 thatís 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: donít cook with flakes and watch out for those cocoa fat matrices, because the wrong sort can make you very sticky!