Better Physics through Chocolate:a soft condensed matter perspective

Naveen N. SinhaPhD Candidate, Weitz LabHarvard School of Engineering and Applied Sciences

Why chocolate and physics?

Why chocolate and physics?

Cacao beans undergo a remarkable transformation, which can inspire new questions in soft matter physics.

How does an astringent cacao bean become flavorful cocoa?

(more chemistry than physics)

kallari.com

kallari.com

kallari.com

kallari.com

kallari.com

kallari.com

kallari.com

kallari.com

kallari.com

kallari.com

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

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

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.

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

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

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

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.

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.

23

Trader Joe’s 72% Taza 70% bar

Callebaut 100% Cocoa butter100 mm 100 mm

100 mm 100 mm

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

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

The rheometer can help quantify the liquid to solid phase transition in cocoa butter.

elastic modulus

viscous modulus

-- axial force

Olive oil Coconut butter

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

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

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

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.

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

Tempered Not tempered

200 mm200 mm

The chocolate can also be tempered, for a glossy appearance that is resistant to bloom.

Tempered Not tempered

200 mm200 mm

Tempered chocolate also breaks cleanly and with an audible snap, from the release of elastic energy.

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

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.

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

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

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

Water and chocolate can be whipped into a stable foam, known as chocolate chantilly.

2 parts chocolate1 part water

1 part chocolate1 part water

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

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