How to Temper Chocolate like a Materials Engineer

[Music]

what do you

see speaking as a materials scientist

and engineer i see crystal grains

microstructural features attempt the

width of a human hair

but what is this this is a micrograph of

chocolate

and this is a micrograph of steel

it turns out that what makes a

chocolatier a blacksmith is what makes

you

an engineer what what does that mean

we’re going to explore how food and

cooking can be leveraged as

really effective case studies to better

understand physical phenomena

and engineering processes we’ve all been

taught

and probably have a basic understanding

of the fact that all matter is made up

of the same ingredients

atoms following the same basic rules of

physics

how these fundamental puzzle pieces fit

together and interact

determines everything and what they’re

doing on a nano and

micro scale affect macroscopic

properties that we

observe every day something as simple as

how atoms stack together

can drastically change things like

mechanical strength

conductivity melting point and a panoply

of other characteristics

but how can we internalize that

relationship across length scales

and how can we manipulate it let’s

consider

an analog a rubik’s cube contains

individual repeating blocks

or atoms in a crystal structure because

of symmetrical repetition across

all three dimensions because of this

geometry

we with relatively little applied

external force can cause

rotation in a few different planes but

off-axis

no such luck a poly crystal is made up

of a sea of crystal grains or extended

crystal structures

at different angular orientations

different dimensions different

geometries

and different repeat units as well

potentially while the building blocks

are different these polycrystalline

materials chocolate and steel

can be processed and strengthened in

very similar ways

this is purely because of the

universality of crystal structure

as a tendency in matter and the reality

that structure

plays a huge part in the properties so

how can we

control structure manipulating what the

building blocks are

only gets you so far but processing

changes the game

to normalize and stabilize we do

something called

tempering this just means heat treating

our materials to make the crystal

structure or the types of crystals

within uniform you can see the

difference for steel

and for chocolate while tempering

chocolate is the achilles heel for menu

baker

tempering steel is a typical

manufacturing process and is something

you’ll hear a lot about in something

like

knife making while the composition of

these materials stays the same during

this process

we as process engineers through

selectively heating and cooling

can facilitate crystal faces we want and

eliminate

those we don’t this is why swords can be

sharp without shattering

why airplane wings can be strong without

snapping why buildings stay upright

but also why some chocolate is shiny

with a snap

and some isn’t let’s think about what’s

in chocolate to understand what’s going

on

a bit better there are cocoa solids

fats sugars and an emulsifier

which brings everything together in an

emulsion or a mixture of things that

normally

don’t want to mix when this mixture

crystallizes uniformly

the macroscopic result is a chocolate

that solidifies quickly

has a shiny finish or a smooth

well-aligned surface

and desirable mechanical properties in

other words

it’s strong and thermally stable too

there are intermediate structures that

may be stable most of the time

but will phase segregate out under

conditions of heat or humidity

in chocolate this is called fat bloom

and is not that desirable

to avoid this we temper following this

phase diagram as we thermally process

some chocolate

we can start to see why this is by

melting completely we can ensure that

our solidification is starting from

scratch

rather than biased toward any

pre-existing nucleation sites

solidification isn’t instantaneous it

happens over time as molecules in our

melt

lose thermal energy and start to glom

onto other low energy molecules

the unstable crystal phase that results

in bloom as it decomposes

forms pretty easily alongside our good

crystals during a simple heating cycle

so this is where we have to get tricky

and one hold a temperature

too high for these unstable type 4

crystals to be maintained

but low enough for these type 5 crystals

the ones we want

to continue to grow and two induce the

phase we want

by providing a template to crystallize

around this is called seeding

mapping this whole process onto

microstructure and macro properties

we can start to see why this works

rather than following a checklist on

faith

and hoping that her chocolate temper’s

all right you may be wondering why i’ve

been talking about chocolate this whole

time

when there’s so much more important

stuff going on in the world right now

to focus on and sure there is

but we as a culture are in the midst of

a crisis of confidence

with science and the truth-seeking

process and when science feels

monolithic and far away

we see trends and dismissal of expertise

and disinterest to critically examine

reality

stem education in the us as it stands

now

is far from perfect schools do a great

job of making real science and real

engineering feel lofty and unattainable

while classroom science is dry and

mundane

now i won’t try to convince you that

particle accelerator research directly

impacts your troops to the grocery store

but we all live in the same world in a

society where problem solving nominally

benefits

everyone even if indirectly and where

reality cannot be siloed

we need more people more kinds of people

in stem

but we all benefit if the baseline

familiarity and understanding of

fundamental science

and the scientific method increases too

but how do we do that if not in school

the laboratory

in other words the kitchen

as a kid i was not in love with science

classes at school

but i would spend hours at home in the

kitchen

concocting i got to build up and flex

analytical and critical skills by

following recipes

making mistakes and improvising

what i was doing was building scientific

intuition

and an appreciation for the scientific

method even in elementary school

i was learning how process engineering

impacted properties

every time i broke a custard overworked

a cake

under baked a pie i was building

intuitive confidence

and competence that i rely on now as an

mit student

and an early career researcher in

materials physics

chocolate tempering isn’t the only

parallel between real science and the

kitchen laboratory

optical fibers are drawn just like sugar

glass or cotton candy

vulcanized rubber tires work just like

jello and the list can go on

and on the democratized science lab lets

us question

investigate and understand

cooking can feel like a trivial joy but

it connects people

and it is an opportunity to flex

decisive creative

and analytical muscles by sharing the

joys of a sourdough starter

you can spread scientific literacy and

engagement

because anyone can cook and anyone

can be an engineer now is the time to

teach

and to learn through experimentation