Better Physics through Chocolate: a soft condensed matter perspective (presented at MIT Senior Week 2011)

1. Better Physics through Chocolate:
a soft condensed matter perspective
Naveen N. Sinha
PhD Candidate, Weitz Lab
Harvard School of Engineering and Applied Sciences

2. Why chocolate and physics?

3. Why chocolate and physics?
Cacao beans undergo a remarkable transformation,
which can inspire new questions in soft matter physics.

4. How does an astringent cacao
bean become flavorful cocoa?
(more chemistry than physics)

5. kallari.com

6. kallari.com

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9. kallari.com

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12. kallari.com

13. kallari.com

14. kallari.com

15. Over the centuries, people have invented new ways to
deliver the flavor and texture of chocolate.

16. For most of its history, cocoa beans were used for thick,
strongly-flavored beverages.

17. Like milk or cream, hot chocolate is an oil-in-water
emulsion, which makes it a viscous liquid.
The cocoa solids likely
help stabilize the
emulsions and prevent
separation.

18. The Mesoamerican drinks were bittersweet, flavorful,
and thicker than much of todays’ hot chocolate.
Atole: blue corn atole, unsweetened 99% chocolate,
honey, chili, salt.
Pepper allspice: unsweetened 99% chocolate, New
Mexican raw wildflower honey, hibiscus flowers, spices,
Catarine chili, and Mexican vanilla.
Rose Almond: unsweetened 99% chocolate, agave nectar,
almond, roses, Chipotle Morita chili, and Mexican vanilla.
Kakawa Chocolate House, Santa Fe, NM

19. The European beverages were sweeter and
incorporated new ingredients, including milk.
1631 Spanish: 73.5% chocolate, evaporated cane juice,
almonds, hazelnuts, roses, spices, Ceylon cinnamon, Red
Chimayo chili, and Mexican vanilla.
1680 English: milk, 61% chocolate, egg yolks, sherry,
orange blossoms, cinnamon.
1790 Jeffersonian: 73.5% chocolate, cane sugar, nutmeg,
Mexican vanilla.
Kakawa Chocolate House, Santa Fe, NM

20. By reincorporating additional cocoa butter,
chocolate can be molded into its familiar bar form.

21. In bar chocolate, the aqueous continuous phase of the
hot cocoa is replaced with solid cocoa butter.
The cocoa solids are
stabilized by a network of
cocoa butter crystals.
Taza Chocolate

22. The create this solid dispersion, cocoa beans are
ground and combined with additional cocoa butter.
Roast Winnow Grind Re-grind
develop flavor remove husks extract cocoa add sugar,
butter cocoa butter
Taza Chocolate

23. Trader Joe’s 72% Taza 70% bar
100 mm 100 mm
Callebaut 100% Cocoa butter
100 mm 100 mm
23

24. The melting transition of the cocoa butter in chocolate
is responsible for its distinctive texture.
Cocoa butter
(fats)
Cocoa solids
(protein, carbs)
Chocolate dissolved in acetone

25. I’ve used oscillating shear wave rheology to
quantitatively study changes in the texture.
Purely elastic
T = 1/w
Stress
Strain time
w
Purely viscous
Stress time
Strain

26. The rheometer can help quantify the liquid to solid
phase transition in cocoa butter.
 elastic modulus
 viscous modulus
-- axial force

27. Cocoa butter has a narrow melting range between
those of olive oil and coconut butter.
Olive oil Coconut butter

28. Like other edible fats, the fats in cocoa butter are
triglycerides made from fatty acids and glycerol.
HO
O
stearic acid
HO
OH
O OH glycerol
oleic acid OH
HO
O
palmitic acid

29. Like other edible fats, the fats in cocoa butter are
triglycerides made from fatty acids and glycerol.

30. The differences in the melting point are due to the
composition of triglycerides.
100%
Poly-unsat.
80%
Weight %
60% Mono-unsat.
40% Saturated
20%
0%
Peanut oil Cocoa Coconut Olive oil
butter butter

31. The fraction of solid cocoa butter decreases as the
temperature increases, so it quickly melts in the mouth.
J. Engmann and M. R. Mackley, “SEMI-SOLID PROCESSING OF CHOCOLATE AND COCOA BUTTER”
Trans IChemE, Part C, Food and Bioproducts Processing, 2006, 84(C2): 95–101

32. The chocolate can also be tempered, for a glossy
appearance that is resistant to bloom.
200 mm 200 mm
Tempered Not tempered

33. Tempered chocolate also breaks cleanly and with an
audible snap, from the release of elastic energy.
200 mm 200 mm
Tempered Not tempered

34. Cocoa butter, like many edible fats, occurs in several
different crystalline phases.
a b b
more stable,
higher melting point
K Sato and S Ueno, "Polymorphism in Fats and Oils", Bailey’s Industrial Oil and Fat
Products, Sixth Edition. 2005.

35. Each phase has a different melting point. The b form
has the highest melting point is most desirable.
T ( C)
36
33
b
30
27
24 b’
21
18
a
15

36. The chocolate is super-cooled, then re-heated, to
maximize the number of b crystals.
T ( C)
36
33
b
30
27
24 b’
21
18
a
15
Time

37. The goal of tempering is as many and as small crystals
as possible throughout the cocoa butter.
50 mm 50 mm
Tempered Un-tempered
Marty and Marangoni, “Effects of Cocoa Butter Origin, Tempering Procedure, and
Structure on Oil Migration Kinetics” Crystal Growth & Design, Vol. 9, No. 37
10, 2009

38. More recently, chocolatiers are experimenting with
novel ways of presenting chocolate.

39. Water and chocolate can be whipped into a
stable foam, known as chocolate chantilly.
2 parts chocolate
1 part water
1 part chocolate
1 part water

40. The chocolate dessert at Alinea features frozen
chocolate foam and square chocolate sauce drops.

41. I’ve had some success replicating this effect in lab…