Valentine, Chocolate and Science

taken from The Science of Chocolate
by Stephen T. Beckett
Royal Society of Chemistry: Cambridge, 2000. xiii + 175 pp.
ISBN 0-85404-600-3. reviewed by Jeffrey Kovac


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. 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.