Hot Potato!
By: David Weston
Larissa Cannon
Background
• Potatoes cook by absorbing heat through
radiation and convection. The heat is then
transferred through the potato by
conduction.
• The “perfect” temperature for a baked
potato is 210 °F
• Aluminum foil has a shiny side and a dull
side. These two surfaces have different
emissivities, meaning that the potato will
absorb different amounts of radiative heat
when the different sides of the foil are
facing outward while cooking.
Hypothesis
• A potato will cook
faster if the shiny side of
the aluminum foil is
touching the potato
than if the dull side is
touching the potato.
Approach
qconv
qrad
qcond
Aluminum foil
Potato
Approach cont…
•
We estimated the potato as a cylinder and solved for the time
required to heat it to a specified “ideal” temperature of 210°F
Assumptions
• We assumed that the aluminum foil would
reach very near the temperature of the oven
almost instantaneously
• We assumed the potato to be a perfect
cylinder
• We assumed the presence of the aluminum
foil would be tantamount to an instantaneous
change in surface temperature for the
potato and used an infinite Biot number to
calculate the transient heat transfer
Experiment
• We wrapped one
potato with the shiny
side in and the other
potato with the shiny
side out.
• After allowing the oven
to heat to 350°F, each
potato was heated
separately for the
predicted time
required to reach
210°F.
Experiment cont…
• Once the potato
reached the predicted
time, we measured its
center temperature
using a thermocouple.
• Our prediction? The
potato with the shiny
foil touching it will finish
cooking in less time!
Results
Side
Touching
Potato
Predicted
time
Required
to reach
210°F
Predicted
temp
Actual
temp
Shiny
24.2 min.
210 °F
122.6 °F
Dull
33.2 min.
210 °F
143.0 °F
NOT 210 °F
like we
predicted
Results cont...
• Since the measured
temperatures did not
match our predicted
values, we measured
the temp of each
potato after the SAME
amount of time
Shiny side
wins!
Improvements
• Core a perfect cylinder out of the
potato so that the transient heat
transfer equations are more applicable
• Use a thermocouple that can withstand
the heat of the oven so that we can
leave it in the potato and get the
temperature profile for the entire
heating process
Conclusion
• Yes, if the shiny side is touching the
potato, it will cook faster, so wrap it
right!
• The shiny side cooks faster because it
absorbs more radiation from the oven
and reflects more radiation back into
the potato due to the difference in
emissivity between the shiny and dull
sides of the aluminum foil.