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Coffee Thermodynamics

June 17, 2011

When I was a graduate student, my student friends and I debated a seemingly simple problem. If you want your coffee to reach a drinkable temperature faster, do you add milk immediately, or do you wait a particular interval, t. Yes, it's just the type of thing scientists discuss while their neighbors are discussing sports scores. One of the professors heard the conversation, and he added another variable to the mix. From what height is the milk poured?

There's quite a bit of thermodynamics involved here. The idea that black bodies radiate faster would seem to indicate that keeping the coffee black longer is the best strategy; but is milk also white in the infrared, where it really counts? At such low temperatures (yes, 100°C is "low"), radiation is not that important. Conduction dominates, so liquids with a higher differential temperature with respect to ambient will cool faster. This also indicates that keeping the coffee black for a longer time is better.

The fat content in milk will likely reduce convection in the hot liquid, keeping it hot for a longer time. This also indicates that keeping the coffee black for a longer time results in a cooler final liquid. But what about cooling caused by evaporation? At this point, I would vote for adding the milk later, but this is definitely a case where one good experiment is better than a lot of theory. This problem has been researched by many, and it does appear that keeping the coffee black longer does lead to a cooler final liquid.[1-3]

My graduate school musings were in the early 1970s, but my present research shows that this problem originated in 1958 at Cornell University. Two students, Darwin Novak and Robert Seidel, did this study as a senior project. They found that for the particular initial states of their coffee and milk, waiting 310 seconds to add the milk brought the liquid to a safe drinking temperature 85 seconds faster. They presented their paper, "The Mechanisms of Cooling Hot Quiescent Liquids," at a meeting of the American Institute of Chemical Engineers in Philadelphia, Pennsylvania.[4]

A cup of black coffee

A cup of black coffee. Wait 310 seconds before adding milk.

Photograph by Julius Schorzman, via Wikimedia Commons)

I've mentioned latent heat of fusion, also called the enthalpy of fusion, in the context of solar energy storage in a previous article (Solar Salt, July 30, 2010). You need to apply quite a bit of heat to many materials to melt them, and they give up this heat when they solidify. This latent heat of fusion principle is apparently at the root of a new product to enhance the coffee drinking experience.

The products is called Coffee JouliesTM, with notable homage to James Prescott Joule. Coffee Joulies are stainless steel nuggets that are dropped into coffee to cause initial cooling and a final even drinking temperature. The heat capacity of the stainless steel accounts for some of the initial cooling, but it's latent heat of fusion that does the real work. One nugget is required for every five ounces of coffee.[5-9]

These nuggets are filled with what the manufacturer reveals to be a phase change material with a melting point of 140°F (60°C). Because of its application, this material is food-grade and completely edible. Thus, something like m-Phenylenediamine (C6H8N2), which has a melting point of 60.85 °C (141.53 °F) and a nicely large heat of fusion of 173.844 kJ/kg, is not going to make the cut.

The Coffee Joulies cool your coffee to a drinkable 140°F three times faster than normal.[5] The phase change material will then maintain this 140°F for quite a while. The Coffee Joulies add an extra fifteen percent to the liquid volume.[5]

An interesting thing about this product, aside from its application of chemical thermodynamics to the coffee problem, is its funding. The company was launched through Kickstarter funding. Kickstarter is a "crowd funding" site in which individuals can pledge money in support of a creative project. If a project exceeds its funding goal, the pledges are redeemed. This funding model circumvents the problems that small projects and creative projects have in securing investment.

Dave Petrillo and David Jackson, inventors of Coffee Joulies, needed to raise $10,000 to go from prototype to product. In the end, they raised more than $300,000. After all, everyone can relate to coffee. They're manufacturing their product in a silverware factory near my home town in upstate New York. Jackson, in an interview on NPR, said,
"If we weren't on Kickstarter, we'd have to basically build some ourselves, sell them to raise some more money, build some more, and it would have taken at least a year of doing that to get anywhere close to where we are now... And we just completely leapfrogged that within a month."[7]

Success does not come randomly. "The Daves," as they are called, lived near each other as they were growing up in Pennington, N.J., where they would make various mechanical gadgets such as kite buggies and robots.[10] They both have mechanical engineering degrees, and Jackson earned a Master's degree at Stanford. Their most important business decision was locating Sherrill Manufacturing, a hollow-handle flatware manufacturer that was established in an abandoned manufacturing plant of Oneida Limited.[8]

Oneida Limited had employed 2,000 people in Sherrill in its heyday, but stopped operations there in 2004.[8] Two former employees restarted the plant in 2005 with about a hundred employees, which dwindled to just fifteen.[6] The similarity of manufacture of the hollow handles of the flatware and the Coffee Joulies will likely make this an easy production. The production target is about ten thousand Coffee Joulies per week.[8] Sherrill Manufacturing is supporting manufacture by underwriting half the cost of tooling in hope that it will get repaid when the company takes off.[6]

You can pre-order Coffee Joulies at their web site, www.joulies.com. I haven't tested them, myself, since they are not yet available. They may be a little pricey, especially since a scientist-sized coffee would need at least three of them. However, a purchase of Coffee Joulies is your affirmation of an important principle of thermodynamics.

I have one more story about the intersection of coffee and science. My friends at Bell Labs built a unique coffee station in the mid-1970s. When coffee sits on a hot plate for an extended period, it starts to taste bitter. The apparent reason is oxidation of the compounds that comprise coffee. The Bell Labs people applied a flow of nitrogen gas over the coffee pot to exclude oxygen and keep it fresh.


  1. Mary Ellen Verona, "The Coffee Cooling Problem," Maryland Virtual High School of Science and Mathematics Web Site.
  2. S. M. Blinder, "The Coffee Cooling Problem," Wolfram Demonstrations.
  3. W. G. Rees and C. Viney, "On Cooling Tea and Coffee," American Journal of Physics, vol. 56, no. 5 (1988), pp. 434-437.
  4. Steve Smith, "Brew-haha? Coffee Cools More Quickly If You Wait to Add the Cream," National Engineers Week Foundation Press Release (No Date).
  5. Hot stuff: The metal coffee beans that keep your drink warm for hours, Daily Mail Reporter (UK), May 2, 2011.
  6. Jenna Wortham, "A Web Edge for Makers of Real Stuff," The New York Times, April 20, 2011.
  7. Tamara Keith, "Arts-Funding Site Gets 'Kickstart' From Business Bids," NPR Morning Edition, May 31, 2011.
  8. Cassaundra Baber, "Coffee Joulies' young inventors bring new life to Sherrill plant," Utica Observer-Dispatch, May 30, 2011.
  9. Anton Olsen, "Coffee Joulies – Your Coffee, Just Right," Wired, April 4, 2011.
  10. Coffee Joulies FAQ Page.                                   

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