The Physics of Coffee & Cream

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Every Seattleite has been in this situation: On a cold, rainy December morning, you get your coffee to go from Vivace, Stumptown, or Starbucks, and then watch out the window for your bus. The bus, you know, might be a minute or two late, and you’ll have to wait a few minutes. You want to keep your coffee as hot as possible during your wait so that it’s still piping hot when you step out the door. You grab a lid for your cup, pausing at the cream. Should you add the cream to your coffee now, or will that only cool your drink faster? Maybe you should add your cream at the last minute, before you dash out the door.

The basic physics of heat provides the answer: you should go ahead and add the cream to your coffee now. Coffee with cream cools about 20% slower than black coffee, for three reasons:

  1. Black coffee is darker. Dark colors absorb heat faster than light colors (just think about wearing a black T-shirt versus a white T-shirt on a hot, sunny day). But dark colors also emit heat faster than light colors—absorption and emission are essentially two sides of the same coin. So by lightening the color of your coffee, you slow its rate of heat loss slightly.
  2. Stefan-Boltzmann says so. The Stefan-Boltzmann law says that hotter surfaces radiate heat faster—specifically, the power of emission is proportional to the temperature (in kelvin) raised to the fourth power. So let’s say you have two cups of coffee that start at the same temperature. You pour cream in cup #1 and the coffee drops in temperature immediately. But the rate at which it loses heat also drops. Meanwhile, the hotter black coffee in cup #2 cools so rapidly that within five minutes the two coffees are at about the same temperature. But you still haven’t added the cream to coffee #2! When you do, it cools even more; cup #1 is now the hotter of the two.
  3. Viscosity versus evaporation. This is the clincher. Adding cream thickens the coffee (adds viscosity), so it evaporates slower. You’d be surprised just how much heat evaporation carries away. Slow the rate of evaporation and you avoid a lot of that heat loss. (This is also one big reason that coffee stays warm longer with a lid on the cup.)

So, next time you’re caught in the rain, put the cream in your coffee right away. Your fingers will thank you.

Watch our high-speed video above of cream being poured into coffee at 2,000 frames per second.

Coffee Without Compromises

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Copyright Scott Rao. Used with permission.Few cookbooks devote a single page to instructions on how to make great coffee. That’s a shame, for two reasons. First, a special meal, hours or days in the making, shouldn’t end with a mediocre cup of joe. Yet even today, in the era of ubiquitous Starbucks, the coffee served at some of the best restaurants in the world wouldn’t pass muster with the average street vendor in Seattle or in Portland, or just about anywhere else where people value their coffee.

Another reason it is painful to see fine coffee making neglected at restaurants is that major advances in technology, technique, and understanding have actually transformed the art in recent decades, in much the same way that Modernist ideas, techniques, and ingredients have revolutionized food. While Ferran Adrià, Heston Blumenthal, Wylie Dufresne, and other Modernist chefs were pushing through the limits of cooking, pioneering baristas like James Hoffmann, Scott Rao, Andy Schecter, Tim Wendleboe, and others were working out the art and science of espresso and brewed coffee.

So when we were planning Modernist Cuisine, we decided early on that it should include a whole chapter on coffee. We think it’s crucial that all chefs understand the basics of what matters most in coffee making, from the roasting of beans all the way through the artful presentation of steamed milk. Preparing excellent coffee takes skill, and doing it consistently requires dedication and practice, but also a solid understanding of the details.

The coffee chapter in Modernist Cuisine, which runs to nearly 50 pages, is a good starting point. But for even more insight and instruction on the subject, you could check out the work of some of these masters in the field who are as committed to making uncompromising coffee as we are to food. The best books I’ve found on this topic are The Professional Barista’s Handbook and Everything But Espresso, both by Scott Rao.

Copyright Scott Rao. Used with permission.Rao’s explanations of the how’s and why’s of coffee are as important for baristas and coffee enthusiasts as Harold McGee’s work has been for chefs and serious cooks. Through a balance of clear technical explanations and practical tips, they open your eyes to subtleties and phenomena you didn’t even know existed and show you how to control them where you can. For people who want to improve the quality and consistency of the coffee they make, these two books are the ones to buy.

Several online forums and blogs also provide great sources of information. The forums at CoffeeGeek.com and Portafilter.net, for example, are overflowing with great ideas and research (as well as some not-so-great opinions). Andy Schecter, a frequent contributor to CoffeeGeek.com, was the first to publish espresso brewing ratios, for example, which have been highly influential in the barista community.

I also get a lot out of James Hoffmann’s blog at jimseven.com. James and I met back in 2005, when I was the head development chef at The Fat Duck, and James was preparing for the World Barista Championship (which he won in 2007). If you’re in London, make sure to check out his coffee-roasting company, Square Mile Coffee Roasters.

Tim Wendelboe, who along with James and Chuck Lambert helped us in writing our coffee chapter, also has a book out now.

Whether you’re a professional barista or only make coffee for yourself at home, you’ll probably be surprised by how much more pleasure you can derive from this simple beverage with a relatively small investment of time and money.

Chris Young is a coauthor of Modernist Cuisine: The Art and Science of Cooking

Coffee: Because, Damn It, We’re from Seattle

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Nathan Myhrvold on coffee at Seattle Town Hall from Modernist Cuisine on Vimeo.

When Nathan Myhrvold recently spoke at Seattle’s Town Hall, he made sure to dedicate at least a few minutes of his speech to the topic of coffee. This was for the same reason there’s a whole chapter of MC dedicated to the subject: “Because, damn it, we’re from Seattle!” he exclaimed. Watch the video, and you might learn a thing or two…even if you’re a Seattleite, born and raised.

Coffee and Cream

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To keep your coffee hot for as long as possible, should you add cream right away or wait until just before you drink it? Will the addition of cream make coffee cool faster or slower?

The Modernist Cuisine team sought to answer these important questions. It turns out that coffee with cream added actually cools about 20% more slowly than plain black coffee, so it is best to add the cream immediately. But why is this?

A sidebar in Modernist Cuisine reveals the three major principles of physics that determine why coffee with cream cools more slowly. Check out the high-speed videos below and see if you can guess these three factors!