The Science of Chocolate (Beckett, Stephen T.)

Abstract

Royal Society of Chemistry: Cambridge, 2000. xiii + 175 pp. ISBN 0-85404-600-3. £ 18.95.Like most people, I am fond of chocolate. My favorites are the dark bittersweet chocolates, but I enjoy eating it in almost any form. However, until I read The Science of Chocolate, I didn't realize how interesting chocolate is as a material. Consider: chocolate is a solid at room temperature, but melts into a smooth viscous liquid at body temperature ("melts in your mouth, ..."). What other food has this property? I can't think of any. When you break a chocolate bar, it snaps, so there must be something crystalline in it to give that rigidity. Most of us have put some chocolate away for another day, forgotten about it, then returned months later to find the glossy brown surface marred by a white "bloom". The candy is usually still edible, though it doesn't taste quite as good. I've always wondered why this happens and precisely what the white stuff is.The Science of Chocolate is a concise, readable survey of the history, manufacture, biology, physics, and chemistry of chocolate. The author, Stephen T. Beckett, works for Nestlé and is well versed in his subject. While the book is probably best suited to those studying food science and technology, there is a lot of interesting chemistry in its pages. Chocolate is an interesting example of many of the principles that are taught somewhere in the chemistry curriculum.One of the important issues in chocolate manufacture is controlling the flow properties of liquid chocolate. Much of the taste and sensual pleasure in eating chocolate comes from its smooth flow in the mouth. Chocolate is a complex, composite material containing cocoa particles, sugar particles, and fat, mainly cocoa butter and milk fat. To get good flow properties, the cocoa particles must be ground finely--otherwise the chocolate is gritty--but also with a distribution of particle sizes. The solid particles, however, must be coated with fat to get them to flow, so there are important problems in surface chemistry to consider. While the cocoa particles are lipophilic, the sugar particles are not, so an emulsifier, usually lecithin, is needed. The mixture is a Bingham fluid requiring a nonzero shear stress to get it moving, but also shear thinning, which means that the viscosity decreases as the shear rate increases. Ketchup is another familiar example of a Bingham fluid.The familiar snap when a chocolate bar is broken occurs because the fats are partly crystalline. Since cocoa butter is a mixture of triglycerides, its phase behavior is complex. There are at least six different crystal phases: the desirable form for confections, called form V, has a melting point about 33 °C. The molten chocolate must be "tempered" to produce seed crystals so that the final product contains mainly this desirable crystalline phase rather than the lower-melting forms. But there is a more stable, and more dense, phase, form VI, which can be produced in the final product in a slow solid-solid transformation. If this occurs, some of the fat will be forced to the surface, causing the white fat bloom that makes the confection unattractive and less pleasant to eat.This is fascinating science, all contained in the humble chocolate bar. Once I started reading The Science of Chocolate, I found myself carried along by the connections to things I have learned and taught over the years. I will be able to use chocolate as an example in several courses that I regularly teach. This is a book that I will recommend to students to show them how the basic science they are learning is used to manufacture and improve one of their favorite foods. I can also imagine using this book as the basis of a non-majors course or a senior seminar; Chapter 10 contains 18 experiments for students to perform, so it could easily be a lab course as well. Of course, our library should have a copy. Best of all, it makes the eating of chocolate a richer experience.