chocolate physics
DESCRIPTION
Better Physics through Chocolate: a soft condensed matter perspective (presented at MIT Senior Week 2011)TRANSCRIPT
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Better Physics through Chocolate:a soft condensed matter perspective
Naveen N. SinhaPhD Candidate, Weitz LabHarvard School of Engineering and Applied Sciences
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Why chocolate and physics?
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Why chocolate and physics?
Cacao beans undergo a remarkable transformation, which can inspire new questions in soft matter physics.
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How does an astringent cacao bean become flavorful cocoa?
(more chemistry than physics)
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Over the centuries, people have invented new ways to deliver the flavor and texture of chocolate.
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For most of its history, cocoa beans were used for thick, strongly-flavored beverages.
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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.
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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
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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
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By reincorporating additional cocoa butter, chocolate can be molded into its familiar bar form.
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Taza Chocolate
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.
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Roast Winnow Grind Re-grind
extract cocoa butter
add sugar, cocoa butter
remove husksdevelop flavor
Taza Chocolate
The create this solid dispersion, cocoa beans are ground and combined with additional cocoa butter.
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23
Trader Joe’s 72% Taza 70% bar
Callebaut 100% Cocoa butter100 mm 100 mm
100 mm 100 mm
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Cocoa butter(fats)
Cocoa solids(protein, carbs)
The melting transition of the cocoa butter in chocolate is responsible for its distinctive texture.
Chocolate dissolved in acetone
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I’ve used oscillating shear wave rheology to quantitatively study changes in the texture.
StressStrain
StressStrain
w
Purely elastic
Purely viscous
time
time
T = 1/w
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The rheometer can help quantify the liquid to solid phase transition in cocoa butter.
elastic modulus
viscous modulus
-- axial force
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Olive oil Coconut butter
Cocoa butter has a narrow melting range between those of olive oil and coconut butter.
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HO
O
HO
O
HO
O glycerololeic acid
palmitic acid
stearic acid
Like other edible fats, the fats in cocoa butter are triglycerides made from fatty acids and glycerol.
OHOHOH
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Like other edible fats, the fats in cocoa butter are triglycerides made from fatty acids and glycerol.
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Wei
ght %
Peanut oil Cocoa butter
Coconut butter
Olive oil0%
20%
40%
60%
80%
100%
Poly-unsat.
Mono-unsat.
Saturated
The differences in the melting point are due to the composition of triglycerides.
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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
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Tempered Not tempered
200 mm200 mm
The chocolate can also be tempered, for a glossy appearance that is resistant to bloom.
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Tempered Not tempered
200 mm200 mm
Tempered chocolate also breaks cleanly and with an audible snap, from the release of elastic energy.
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a ’b b
Cocoa butter, like many edible fats, occurs in several different crystalline phases.
K Sato and S Ueno, "Polymorphism in Fats and Oils", Bailey’s Industrial Oil and Fat Products, Sixth Edition. 2005.
more stable, higher melting point
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36
33
30
27
24
21
18
15
T (°C)
a
b’
b
Each phase has a different melting point. The b form has the highest melting point is most desirable.
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36
33
30
27
24
21
18
15
T (°C)
Time
The chocolate is super-cooled, then re-heated, to maximize the number of b crystals.
a
b’
b
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Un-temperedTempered
Marty and Marangoni, “Effects of Cocoa Butter Origin, Tempering Procedure, and Structure on Oil Migration Kinetics” Crystal Growth & Design, Vol. 9, No. 10, 2009
The goal of tempering is as many and as small crystals as possible throughout the cocoa butter.
37
50 mm 50 mm
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More recently, chocolatiers are experimenting with novel ways of presenting chocolate.
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Water and chocolate can be whipped into a stable foam, known as chocolate chantilly.
2 parts chocolate1 part water
1 part chocolate1 part water
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The chocolate dessert at Alinea features frozen chocolate foam and square chocolate sauce drops.
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I’ve had some success replicating this effect in lab…
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