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