These are experiments about the chemistry of chocolate, especially the 6 polymorphs of chocolate. It can help students understand that natural substances are a mixture of many different compounds or it can be done just for fun.

1. Chocolate
Chemistry for all the Senses
Solveig Böhme & Sara Munktell
Demonstrative Experiments 2014
1 Aim
The aim of this experiment is to illustrate how melting time and temperature influence
materials properties after heat treatment by studying the "chocolate system". How
cooling rate and the presence of seed crystals effect the recrystallization will also be
discussed.
2 Introduction
2.1 Early History
Chocolate can be traced back to the first Indian civilizations in southern Mexico and
Central America about 3500 years ago. In Indian cultures (e.g. Mayas, Incas and Aztecs)
chocolate was consumed as a drink similar to today‘s hot chocolate. It was brought to
Europe in the 16th century by the Spanish and quickly became fashionable among the
European aristocracy.
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2. Chocolate - Chemistry for all the Senses September 26, 2014
2.2 Chocolate Bars
In 1828, the Dutch chemist Conrad van Houten developed a process to press cocoa butter
(fat) from cocoa beans. This became the basis to manufacture chocolate bars which were
first produced in England as a mixture of cocoa butter, powder from non-pressed cocoa
beans and sugar. Those three are still the basic ingredients of dark chocolate.
Later, in 1876, the Swiss Henry Nestlé added milk powder to the mixture and produced
the first milk chocolate. White chocolate was not invented until 1930 when the cocoa
powder was omitted from the milk chocolate production process.
3 Theoretical Background
3.1 Cocoa Butter
Chocolate consists for the most part of cocoa butter (about 55 wt%). Fats are triesters
of glycerol with fatty acids (carboxylic acids with long aliphatic chains). Cocoa butter
fat is quite unique in nature since it almost only contains three different fatty acid
chains: oleic acid, stearic acid and palmitic acid. Thus, the structure is much more
homogeneous than other natural fats and the molecules can be stacked like chairs leading
to the crystallization of cocoa butter.
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3. Chocolate - Chemistry for all the Senses September 26, 2014
However, the aliphatic chains can be oriented at various different angles towards each
other and the glycerol backbone which enables crystalization in different structures.
Therefore, cocoa butter is able to form six distinct crystal structures (polymorphs) with
different properties, e.g. melting points. Only one form, polymorph V, melts just below
body temperature at 32-34 ◦C, gives chocolate its shiny gloss as well as the familiar
"snap" when biting or breaking it.
Polymorph Melting point Properties
I 17.3 ◦C soft, crumbly, melts easily
II 23.3 ◦C soft, crumbly, melts easily
III 25.5 ◦C firm, poor snap, melts easily
IV 27.3 ◦C firm, poor snap, melts easily
V 33.8 ◦C glossy, firm, good snap,
melts near body temperature
VI 36.3 ◦C hard, takes months to form in solid state
3.2 Chocolate Tempering
Which polymorph is formed depends mainly on the temperature at which the chocolate is
melted as well as on the cooling rate. When cooling is carried out quickly after heating
above 50 ◦C a mixture of polymorphs I-V forms which makes the chocolate soft and
matte. It also melts very easily at room temperature and the taste is affected.
Moreover, "chocolate bloom" is observable. Since some polymorphs melt below room
temperature those liquid fats will be pushed out of the solid structures and gather as
white chocolate bloom on the surface.
In 1879, the Swiss Rudolf Lindt invented an industrial process to temper chocolate in
a way that only polymorph V is obtained. Chocolate is melted only at 35 ◦C. At this
temperature not all form V crystals are melted and some seed crystals remain. When the
melt is slowly cooled to room temperature polymorph V crystallizes completely. Today
it is even common to add shredded chocolate as polymorph V seed crystals.
It is more difficult to temper milk and white chocolate since milk fat is added via
milk powder; fat molecules in different sizes and shapes are introduced and hinder the
stacking to a crystal structure.
4 Instructions
4.1 Materials
• Dark chocolate (at least 40 % cocoa)
• Another kind of chocolate to your liking (such as white or milk)
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4. Chocolate - Chemistry for all the Senses September 26, 2014
• Cooking plate, thermostat, thermometer, spoon, glasses, water bath, microscope,
alumina foil
• Liquid nitrogen
4.2 Procedure
Open your mind and experience chocolate with all your senses. Watch, touch, listen,
smell and last but not least taste!
Take a row of the dark chocolate and break it to pieces. Note how the chocolate feels
and sounds when you break it and what its texture and colour looks like. Shred the
chocolate and divide it between two beakers.
Melt one batch slowly at maximum 35 ◦C in a thermostat, measure the temperature
continuously to make sure you do not raise it above 35 ◦C. Be careful not to contaminate
the chocolate sample with water. Put a drop of melted chocolate on a microscope glass
and let it set at room temperature. Put another drop between two glasses and cool
rapidly in liquid nitrogen. Study the two samples in a microscope and observe any
eventual differences.
Spread some of the chocolate on a foil tray and cool rapidly with liquid nitrogen. Pour
out the rest of the chocolate on another tray and leave it to set in room temperature.
Compare the two samples by breaking and tasting.
Melt the other batch and heat it to 50-55 ◦C in a water bath, keep it there for at least
30 minutes. Repeat the same procedures as for the 35 ◦C batch, but save some of the
chocolate in the beaker. Let the beaker cool down to about 40 ◦C and add some more
shredded chocolate. Stir in and let it homogenise, then pour out on a tray and let it set
at room temperature. Compare with the chocolate melted at 35 ◦C and the untreated
chocolate.
Repeat the experiments with another chocolate type of your choice.
4.3 Risk assessment
Risk of stomach ache, if you eat too much chocolate. Allergies: Chocolate may contain
traces of nuts and peanuts. Risk for hot/cold burns when handling hotplates and liquid
nitrogen. LD50 for dark chocolate is about 6 kg (roughly 31x200g bars).
References
1. Beckett, S.T., The Science of Chocolate, The Royal Society of Chemistry: Cam-
bridge, U.K., 2000.
2. Tannenbaum, G., Journal of Chemical Education, 2004, 81, 1131-1135.
3. www.compoundchem.com, 2014-09-19.
4. Photo: www.telegraph.co.uk, 2014-09-19.
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5. T HE CHE MIS T RY OF CHOC OL AT E
IS CHOCOLATE AN APHRODISIAC?
Phenylethylamine occurs naturally in the brain,
and is dubbed ‘the love drug’ due to its ability to
produce feelings of well-being and contentment.
It is also present in significant concentrations
in chocolate, but since it is broken down after
ingestion, it has been ruled out as causing a
significant aphrodisiac effect.
Tryptophan is a chemical in the brain linked to
the production of serotonin, the neurotransmitter
that produces feelings of elation. It is present
in chocolate, but only in small quantities, and
again is most likely not causing any aphrodisiac
effect.
2014 COMPOUND INTEREST - WWW.COMPOUNDCHEM.COM
WHY IS CHOCOLATE TOXIC TO DOGS?
Theobromine is a mild stimulant, similar
in effect to caffeine, found in chocolate.
This compound is harmless to humans at
the levels found in chocolate - a fatal dose
would require eating tens of kilograms of
milk chocolate!
On cats & dogs, however, theobromine has
a much more potent effect; small doses can
lead to vomiting & diarrhoea, whilst as little
as 50g of dark chocolate could kill a small
dog.THEOBROMINE
N
N
N
HN
O
O
CH3
CH3
NH2
HN
O
OH
NH2
PHENYLETHYLAMINE
TRYPTOPHAN