The Thermodynamics of Home-Made Ice Cream
Donald L. ~ibbon' and Keith Kennedy
Calgon Corporation, Box 1346, Pittsburgh, PA 15230
Nathan kading2
Stanford University, Palo Alto, CA
Mardsen Quieroz
Department of Mechanical Engineering, Brigham Young University, Provo, UT
Introduction: Why Ice Cream?
One of a free societys most serious problems is an ignorant
citizenry. Ignorance leads to poor decision-making
when the citizens are called on to make choices, whether
those choices are made at the ballot box or at the local
store. Intentional secrecy, the "What you don't know won't
hurt you-I know what's best for you" approach, received a
lot of publicity during the Iran-Contra affair. In matters
scientific, however, it is commonly not intentional secrecy
that is the problem. Citizens remain largely ignorant of the
technological workings that form the basis of their society
because of a widely held perception that "science is too
hard" to understand.
But much of fundamental science, well taught, is within
the grasp of most people. It can be done, as recent articles
on Jerl Walker's success in teaching physics at Cleveland
State University so eloquently testify. But teaching, to be
effective, has to be tailored to the particular group of students
being taught. If principles are taught abstractly,
most students of any age will simply not make the effort to
understand. But principles taught in a *amework of common
experience often become quite obvious. It's the context,
not the principle, that makes learning difficult.
Quite obviously, if citizens understand how things work,
what the principles are on which the devices of daily life
are based. then all sorts of benefits fall out. Unrealistic
hopes for miraculous "technological fures" fade, while innovation
becomes more likelv. Rates of obsolescence would eo
down, andequipment, both personal and public, would l&t
lower, because it would be used more effectively. Thus it is
cle&li in society's best interest to do a better jib of teaching
science to its citizens.
How best to go about implementing that fundamental
idea? One good ap~roach is to brim science closer to home,
in fact, bring it iiht into the kitchin! We propose here the
princide that eatine the results of the experiment is an
impoAant aid to derstanding the results! At least the
experiments are rarely complete failures that way.
Ice Cream as the Basis for a Chemistry Course
We suggest that making ice cream can be the basis of an
entire course in chemistry. Consider the principles on
which the process is based.
First of all, ice-cream making is fundamentally a matter
of controlling heat transfer. What's warm has to be made
cold. The mechanism that makes it all possible in a small
manually operated system is the process of depression of
freezing point in the system ice-water-salt. To begin to understand
what we just said in a fundamental way, one
must first consider all sorts of concepts:
Presented at the 1987 Annual Meeting of the American Association
for the Advancement of Science, Chicago, IL.
'Author to whom correspondence should be addressed.
'Undergraduate student.
temperature itself
the nature of heat and heat versus temperature
the nroeess of meltine
phase diagrams
the effects of dissolved solids on liquid freezing points.
Back of all these processes and definitions, the students
should be encouraged to ask, ''Why? Why? Why does this
happen?
As the ice cream "freezes", many other process are taking
place. Of course, the most obvious one is nucleation of ice
crystals in the ice cream mix. Successful ice cream making
requires maximum number and minimum size of ice crystals,
so that the ice cream is "smooth". These two criteria
require maximum rate of heat transfer. Why? What is the
role of stirring the mix? It not only raises the rate of heat
transfer. but also adds air to the mix. alterine - the entire
originalliquid-to-metal-to-liquid heat lransfer situation.
What is the final result of the orocess? The startine mix
does not become a solid.block of iee. What is it? It turns out
that the ice cream never fullv freezes: it remains about
loq liquid at its hardest. What part is liquid? What is the
difference between a iscous liquid and a did? Does that
difference matter? Why and where?
This listine is onlv a tinv fraction of the issues that can
be discussedYduring"the of making ice cream in the
classroom. And the interest certain to be aroused during
these discussions can be focused on a more formal "setting"
of those principles. Lab sessions can become the most interesting
part of the course. The end result of such a course
is students who are both motivated and knowledgeable
about the world they see on tidaily basis.
Experimental Setting: Getting There Is More Than Half
The Fun
The major experimental equipment has to be an ice
cream freezer and some method of measuring temperature.
We had access to a digital readout with inputs for
several thermocouples, but long thermometers would have
done the job. My ice cream freezer is an old hand-cranked
one. From vast experience, I know that it takes about 11
min to freeze a batch, if the ice-salt ratio is about right. We
drilled several holes in the side of the bucket and wiredlepoxied
in a thermocouple to monitor the brine temperature.
One of the most severe challenges, one not surmounted in
these experiments, is how to determine the temperature of
the mix inside the central tub while it is being stirred by
the paddles; some form of microtelemetry seems to be the
answer. However, we performed three major experiments
and talked about dozens of others.
Heat Capacity
We wanted to determine the heat capacity of the mix, to
see how much heat had to be removed from the mix to
freeze it. So we did an elementary calorimetry experiment.
We added ice to a salt-water mixture, stirred until it was
658 Journal of Chemical Education
Temp. (c;5] 10
Brine Temperature
-5
Mix Temperature
Elapsed Time (minutes)
Figure 1. Temperture equilibrium diagram for making marshmallowchocolate
chip ice cream.
quite cold (about -13 'C in one experiment), removed the
remaining ice to avoid complication of melting, measured
the temperature of a batch of ice cream mix, then put the
mix into the brine in a steel container. We wrapped the
brine bucket in towels to insulate it, and began stirring
both the brine and mix, all the while measuring the temperature
versus time in both containers. The plot of one
batch of data is shown as Figure 1. Let's look at that graph
and see what we can learn.
First, we had reached equilibrium between the two materials
in slightly over 11 min. The change in temperature
of the brine was from -10 'C to +2 'C
AtB=-10-(+2)=1 'C (1)
The change in temperature of the mix was from +20 'C to
+2 'C
AtM=ZO-2=lS .C (2)
We know the volume of the brine (VB) and the volume of
the mix (VM). We can weigh both the brine and the mix to
get their densities in gramsfliter
DB = weight of brine
v~
DM = weight of mix
v~
Heat capacity is measured in calories per degree per
gram and will he designated HB and HM. For the overall
heat transfer, we have
(Total heat last)rl = (Total heat gained,,
HBxAtBxVBxDg=HMxAtMxVMxDM (5)
He and HM are both unknown. One way to quantify the
experiment would be to run it brine-against-water, where
H,.* has the well-known value of 1 calPClg. That would
give us HB and then we wuld calculate HM . Now, knowing
HM , we can calculate how much heat has to be removed to
make ice cream.
First we'd have to make some. Determining the
ingredients' starting and final temperatures would allow
us to calculate AtM. Then, remember that question above
about the ice cream never being fully solid? Only part of it
actually freezes. We wuld make an assumption about its
actual ice content (for purposes of discussion, let's assume
70%), and assume that the heat of crystallization of that
part was approximately that of water.
The ice cream making urocess consists of molinn the mix
down to the freezing of water, then freezing whatever
part of it will actually become solid, then continuing tocool
160
PULSE
RATE
(Bmts/Smnd)
loo
80
Pulse Rate as a
Function of Time
C
024e.e.1012
Iar ndt, lll,h rn~dt,
MIXING TIME
(minutes)
Figure 2. Pulse rate of person cranking the freezer plotted against time.
the whole batch to its fmal temperature. Therefore, for 1 L
of ice cream mix, starting at +20 'C and ending at -10 'C,
we have the following:
(Total heat rern~ved)~ = HM x 20'C x 1 L x DM
To get through all of these calculations, the only thing we
would need, in addition to what we already know, is the
heat of crystallization of water.
Viscosity
We also wanted to monitor the rate of change of viscosity
during the freezing process, presuming that this was related
to the rate of nucleation and growth of ice crystals in
the mix. It was proposed that the pulse rate of the cranker
might be a direct function of viswsity, so we checked that
out. It is an experimental fad that the mix gets harder to
crank as it gets colder and harder. The cranker's pulse is a
way to calibrate or quantify that fad. The data from this
experiment are shown as Figure 2. Dozens of questions
come out of this: the nature of precision, how to fit curves
to data; what do we expect the relationship to be (straightline,
ever-increasing, etc.) and why; how to design experiments
to check on only one thing, ete.
A second way to check on the viscosity of the mix might
be with an inductive ammeter on the power cord of a motorized
freezer. These meters measure the current flow in
a single wire of a two-conductor pair by simply clamping
around the wire. As the resistance to flow (viscosity) of the
mix goes up, it takes more power to turn the motor. Power,
measured in watts, is equal to volts (a fixed value) multiplied
by amps (the current flow). As the viscosity goes up,
the power required goes up. We shouldn't do this for long
on small motors without overload protection (such as a
Waring Blendor) or we'll bum them out. However, ice
cream f