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Coffee Acoustics and Espresso Foam

October 31, 2016

While taste and smell are the primary senses used for quality control in the kitchen, the other senses are used as well. The sense of touch is used when you depress the center of a cake to test for doneness, and sight to determine when you've entered the Maillard reaction stage. And then there's hearing.

Polish angel wings (chruściki)My favorite example of the Maillard reaction.

These are Polish chruściki, usually made for Christmas.

(Wikimedia Commons photo by Blazej Pieczynski.)

If you listen beyond all the machine noises, there are some useful kitchen acoustics. Hearing is used in the kitchen when checking the progress of baked bread, where a nice "thump" when tapping with a spoon indicates doneness. The temperature of fry oil is often tested by the sound made when a droplet of batter is placed in the pan. One beloved kitchen sound is the pop of popcorn, with the initial pop and last pop signalling critical stages in the process.

A 2014 study, and recent follow-up research, by mechanical engineers at the University of Texas at Austin looked at an acoustic method of quality control in coffee roasting.[1-3] Just as popcorn pops, coffee beans will crack while being roasted, and experienced coffee roasters listen for these sounds to monitor the roasting process. As we all know, and as I've discussed in a recent article (The Future of Work, March 3, 2016), most jobs will be automated by mid-century, and this research points to a way to either lighten a coffee roaster's workload, or put him out of work, depending on management whim.

Bagged coffee beansAs I've found, the key to good coffee is grinding your own beans, and storing bags of coffee beans in the freezer once they've been opened.

You don't need expensive beans - Supermarket beans make wonderful coffee. I mix these beans for my personal blend.

(Photo by author.)

At the end of the coffee roasting process, two distinctive sounds are emitted; namely, a low frequency, high amplitude first crack, followed later by a higher frequency, lower amplitude second crack. Also, the popping rate of the second crack is higher.[1-2] The second crack signals the breakdown of cellulose in the coffee beans, the release of oils, and a resultant darker roast.[3] If heating is stopped soon after the first crack, the result is a highly acid light roast. Somewhat longer heating after the first crack allows sugars to caramelize.[3]

In the UTA experiments, sounds were recorded from 62 first crack events and 241 second crack events while roasting espresso blend beans in a home roaster.[3] It was found that the first crack is 15% louder than the second, it occurs at about five octaves lower frequency, with a popping rate just a fifth of the second. Some audio analysis software could easily differentiate between the two.[2]

Roasted coffee bean crack rateRoasted coffee bean crack rate.

The second crack is easily distinguished.

(Graphed from data in fig. 3 of ref. 1 using Gnumeric.)[1]

More than a billion servings of coffee are brewed each day worldwide, with the United States responsible for about 400 million cups. It's estimated that about a quarter of coffee consumed is espresso. One reason for this is that it's the base coffee for such popular drinks as caffè latte, cappuccino, caffè macchiato, and caffè mocha. Espresso is typically made from dark roast coffee having a considerable concentration of coffee oils.

Espresso is brewed by forcing high pressure hot water through finely ground and compacted (tamped) coffee. This process results in a thick beverage of dissolved and solid materials, with the oils forming a colloid. This colloid is evident by a surface foam called the crema, and experienced baristas note the appearance of the crema for quality control of the espresso-making process.

Espresso cremaCrema on a cup of espresso coffee.

Baristas often stir an espresso to give a pleasing appearance, as shown.

(Modified Wikimedia Commons image by HSwaff.)

In 2011, Researchers from Illycaffè S.p.A, Trieste, Italy, published a review article on espresso coffee foam.[4] As they stated in their article, "Only recently, some aspects of the Physics and Chemistry behind the espresso coffee foam have attracted the attention of scientists."[4] As they found, carbon dioxide generated by roasting is an important factor, and the quality of the crema depends on the quantity of carbon dioxide generated and the influence of coffee compounds.[4]

The importance of carbon dioxide is such that "espresso brewing can be described as "a quick way to transfer carbon dioxide from roasted and ground coffee to a small cup by means of hot water under pressure."[4] The important factors are therefore,[4]
• The carbon dioxide generated by roasting.
• Maintaining the carbon dioxide in the bean by proper
  packaging.
• Maintaining the carbon dioxide in the ground coffee.
• Solubilizing the carbon dioxide in water.
• Releasing the carbon dioxide into the beverage.

As the review authors conclude, the foam volume and persistence (see figure) are the consequences of the carbon dioxide present in the coffee, with roasting and preparation aiding its release into the beverage. The carbon dioxide content and lipid content appear to be more relevant than coffee species for foam creation and foam stability.[4]

Espresso foam persistenceEspresso foam persistence as a function of roasting degree.

Triangles are pure Arabica, and squares are pure Robusta.

(Graphed from data in fig. 7 of ref. 4 using Gnumeric.)[4]

References:

  1. Preston S. Wilson, "Coffee roasting acoustics," J. Acoust. Soc. Am., vol. 135, no. 6 (June, 2014), article no. EL265, DOI: 10.1121/1.4874355. This is an open access publication with a PDF file available at the same link.
  2. Jay R. Johnson, Preston S Wilson, "Additional studies of the acoustics of coffee roasting," J. Acoust. Soc. Am., vol. 139, no. 4 (April, 2016), pp. 2165-2165, DOI: 10.1121/1.4950420.
  3. How to Roast Coffee by Ear, APS News, vol. 25, no. 8 (August/September, 2016). A PDF file is available here.
  4. Ernesto Illy and Luciano Navarini, "Neglected Food Bubbles: The Espresso Coffee Foam," Food Biophysics, vol. 6, no. 3 (September, 2011), pp. 335-348, DOI: 10.1007/s11483-011-9220-5. This is an open access publication with a PDF file available at the same link.

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