Coffee Roasting Acoustics

Pres Pr esto ton n S. Wi Wils lson on:: JA JAS SA Ex Expr pres ess s Le Lett tter ers s

[ http://dx.do http://dx.doi.org/10.112 i.org/10.1121/1.4874355 1/1.4874355]]

Published Online 13 May 2014

Coffee roasting acoustics Preston S. Wilson Department of Mechanical Engineering, The University of Texas at Austin, 1 University Station C2200, Austin, Texas 78712 [email protected] [email protected] texas.edu

emitted by coffee beans during the roasting Abstract: Cracking sounds emitted process were recorded and analyzed to investigate the potential of using the sounds as the basis for an automated roast monitoring technique. Three parameters were found that could be exploited. Near the end of the roasting process, sounds known as “first crack” exhibit a higher acoustic amplitude than sounds emitted later, known as “second crack.” Firs Fi rstt cr crac ack k em emit itss mo more re lo low w fr freq eque uenc ncy y en ener ergy gy th than an se seco cond nd cr crac ack. k. Finally, the rate of cracks appearing in the second crack chorus is higher than the rate in the first crack chorus. C 2014 Acoustical Society of America V

PACS numbers: 43.20.Px, 43.60.Bf [JM] Date Received: February 25, 2014 Date Accepted: April 21, 21, 2014 2014

1. Introduction

Coffee is the world’s most widely traded tropical agricultural commodity, according to the International Coffee Organization, a global intergovernmental trade group. In the 2011/12 2011/ 12 seaso season, n, 134.4  106 bag bagss of coffee coffee (60 kg each) each) were exported by countries that produc pro ducee cof coffee fee bea beans, ns, wor worth th an est estima imated ted $30 $30.1 .1  109.1 By one es estim timate ate thi thiss yie yields lds 1.5  109 servings of coffee consumed every day, worldwide. 2 Green coffee beans must be roasted before they are used in all forms of the coffee beverage. Roasting the beans is accomplished using a variety of heating methods and at a variety of scales, ranging from fro m mas mass-m s-mark arket et ind indust ustria riall roa roaste sters rs run runnin ning g con contin tinuou uously sly (pr (proce ocessi ssing ng as muc much h as 5000 500 0 kg/ kg/h) h) to the single single bat batch ch hom homee roa roaste sterr pro proce cessi ssing ng a cou couple ple of bat batche chess a wee week k (<1 kg/h) kg/h).. To place the econo economic mic impact of global coffee coffee roasting in persp perspecti ective, ve, the cost of energy required to roast the world’s yearly supply is about $1  109 (calculated using the average 2011 consumer cost of electricity in the US, $0.1/kW-h), hence both economic and quality optimization is of interest. Controlling the roast time and temperature profile results in a range of roast levels from light to dark, greatly affecting the style, flavor, and aroma of the resulting coffee cof fee bev bever erage age.. Ter Termin minati ating ng the roa roasti sting ng pro proce cess ss at jus justt the rig right ht tim timee all allows ows the roaster (most often, a human operator) to achieve the desired darkness of the roast and its accompanying flavor profile, and hence is one of the key roast parameters. Several metrics can be monitored (time, color, aroma, bean volume, bean temperature), using process measurement instrumentation or by the person conducting the roast, to t o indicate the degree of roasting and ultimately to determine when to terminate the roast. 3 The roasting roasting pro proce cess ss can also be mon monitor itored ed by ear ear,, by lis listen tening ing for eve events nts known kno wn col collec lectiv tively ely as “fir “first st cra crack” ck” and “se “secon cond d cra crack” ck” (de (descr scribe ibed d mor moree com comple pletel tely y below), which also signify the progression of the roast. Depending on the type of roasting machine, these sounds can usually be heard by the unaided ear and are perhaps the most important way to monitor the roast for small-batch and artisanal roasting, often combined with the visual cue of bean color and the olfactory cue of aroma. For automated roast monitoring, a num number of techniques have been stud stu died, such as monitoring tor ing bea bean n vol volume ume and por porosi osity, ty,4 con conten tentt of the exh exhaus austt gas gases, es,5 an and d co colo lorr of th thee 6 beans, among others. Despite the widespread practice of monitoring the roasting process audibly, in the home, comm commerci ercial al artis artisanal, anal, and mass mass-marke -markett indus industrial trial roasting venues ven ues,, the aut author hor has fou found nd no pre previo vious us qua quanti ntitat tative ive des descri cripti ption on of the sou sounds nds

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produced during coffee roasting, and no discussion of an automated acoustic monitoring technique. Briefly, the sounds known as first crack and second crack are the result of pyrolytic processes that occur within the bean during roasting. First crack occurs at an internal bean temperature of approximately 200  C and coincides with the release of steam and gases. Each bean experiences a first crack, and hence as a group of beans is roasting, first one or two cracks are heard and then more and more occur forming a chorus of first cracks. After about two minutes, first crack ceases and there is an acoustically inactive time. Second crack occurs when the temperature reaches about 230  C and additional gas is emitted along with increased fracturing of bean material. Again a few beans begin second crack. Over about the next two minutes, a chorus builds, peaks, and declines and then second crack stops. The beans can ignite and burn beyond this phase and hence roasting rarely continues beyond the end of second crack. 7,8 The sounds sounds of firs firstt cra crack ck are qualitat qualitative ively ly sim simila ilarr to the sound of pop popcor corn n V popping poppin g while second crack sounds more like the brea breakfast kfast cereal Rice Krispies Krispies in milk. Additional qualitative audible differences between first and second crack are: first crack is louder, first crack is lower in frequency, and individual second cracks occur more frequently within the chorus than first cracks. The purpose of the present work is to quantify these effects as a preliminary step toward the development of an automated acoustical roast monitoring technique. R

2. Description of the measurements

A consumer-grade, 0.45-kg-capacity, drum-based coffee roaster with an electrical heating element element (1.6 kW) was used to roast a 0.23 kg batch of green coffee beans beans through the end of second crack. The beans were a typical blend of Arabica and Robusta beans marketed as an espresso blend. A Roland R-26 portable digital audio recorder was used to record the sounds emitted during the roasting process, specifically the first and second crack choruses previously described, including the sounds of the roasting machine itself. The roaster roaster was ope operat rated ed out outdoo doors rs on a flat con concre crete te sur surfac facee to eli elimin minate ate pot potent ential ial reverberation encountered in an enclosed space. The recorder was mounted on a short tripod tri pod (15 cm height) height) that was placed placed on the concret concretee sur surfac face, e, at a dis distan tance ce of 0.3 0.35 5m from the roaster. The recorder’s automatic gain function was disabled and one of the R26’s built-in microphones was used. A free-field calibration of the recording system was performed using a substitution method in the fully anechoic chamber at The University of Texas at Austin, so that absolute acoustic pressure could be determined from the data. The frequency response of the recording system was found to be flat within 62 dB from 20 Hz to 40 kHz. After After recording, recording, the data was transferred transferred to a lapto laptop p computer for analysis. Minute-long segments of the recordings are presented in Mm. 1 and 1 and Mm. 2, 2, for first and second crack, respectively. respectively. Note that both files were amplified by 18.2 dB for optimum playback but relative gain was preserved. Mm.1. The sound of the first crack chorus. This is a file of type “wav” (5 Mb). Mm.2. The sound of the second crack chorus. This is a file of type “wav” (5 Mb).

3. Results

Initial analy Initial analysis sis indic indicated ated that crac cracking king events coul could d be dete detected cted automatically automatically using thresholding above þ23 mPa and below 23 mPa. The analysis presented in this section tio n was con conduc ducted ted on all the cra cracks cks found abo above ve thi thiss amp amplit litude ude thr thresh eshold old,, whi which ch resulted in detection of 62 cracking events in the first crack chorus and 241 events in the second crack chorus. To qua quanti ntify fy the the qualitative assessm assessment ent tha thatt firs firstt cra crack ck is lou louder der tha than n sec second ond crack, crac k, the peak acoustic acoustic pressure (scaled (scaled to 1 m by sphe spherical rical spreading) spreading) of indiv individual idual cracking events was analyzed and histograms were formed for the events of first and second crack. These crack amplitude distributions are shown in Fig. 1, where it can be

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Preston S. Wilson: Coffee roasting acoustics




Fig. 1. Distribution of the peak acoustic pressure amplitudes recorded during first and second crack choruses. The numbers of occurrences were normalized by the maximum value for each case (20 and 80 for first crack and second crack, respectively). The total number of cracks n cracks n is listed in the legend. The acoustic amplitudes were measured at 0.35m range and scaled to 1 m assuming spherical spreading. spreading.

seen that the maximum amplitudes of first crack and second crack are 63 mPa and 55 mPa,, res mPa respec pectiv tively ely,, and tha thatt the there re are a lar larger ger number number of hig higher her amp amplit litude ude eve events nts in first crack. Hence, a simple peak finding process could be used in an acoustic process contro con troll sys system tem to aut automa omatic ticall ally y dif differ ferent entiat iatee bet betwee ween n firs firstt cr crack ack and sec second ond cra crack ck using peak acoustic pressure. To quantify the second qualitative assessment, that first crack events are lower in frequency than second crack events, averaged acoustic pressure spectra were calculated. Ten individual cracks were taken at random from within both first and second crack choruses. These events included both the cracking sounds and the noise due to the roaster. The pressure signatures were detrended, then 512-point, Hann windowed fast Fourier transforms transforms (FFT (FFTs) s) were applied and avera averaged. ged. These aver averaged aged acoustic pressure spectra are shown in Fig. 2. First crack contains more low frequency energy, with a spectral peak at about 800 Hz. Second Second crack exhibits exhibits a flatte flatterr spectrum that is lower in amplitude amplitude than first crack up through about 10 kHz, and has a spec spectral tral peak at about 15 kHz. The background background noise emitted emitted by the roaster, roaster, including including the sounds of the beans beans rot rotati ating ng in the drum, drum, and the sou sound nd of a fan circulat circulating ing heated heated air air,, but absent of any cracking events was analyzed in the same way, except using 4096-point FFTs, FF Ts, to bet better ter res resolve olve the ton tonal al com compon ponent entss of the roa roaste sterr noi noise. se. Te Ten n seg segmen ments ts of crackcra ck-fre freee nois noisee wer weree use used d to det determ ermine ine the ave averag ragee noi noise se spe spectr ctra a due to all oth other er aspects of the roasting process. Figure 2 shows broadband noise due to beans rotating in the drum and the broadband component of fan noise, which can be seen along with spectral lines due to the rotating machinery. To further emphasize the mean characteristics of the broadband noise, a smoothed spectrum is also shown in the thick black curve. In all cases, the noise level is sufficiently below the level of the cracking events.

Fig. 2. Averaged acoustic pressure spectra are shown for 10 individual crack events within the first and second crack choruses. Roaster noise spectra (total and broadband only) are also shown. All spectra were normalized by the maximum value of the first crack spectrum.



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Fig. 3. The rate of individual cracking events is shown for both first crack and second crack as a function of time during the roast. Cracks Cracks were tallied within within time intervals of 15 s and 5 s for first and second crack choruses, respectively.

These res These result ultss ind indica icate te tha thatt the mea mean n spe spect ctral ral con conte tent nt of jus justt ten ind indivi ividua duall cr crack acks, s, including inclu ding roast roasting ing proce process ss noise noise,, can be used to autom automatic atically ally diffe different rentiate iate between first crack and second crack acoustically, using a relatively low-resolution FFT, even when no special care is taken to reduce or exclude the roasting process noise. Finally, the rate of emission of individual cracks was analyzed for both first and second crack choruses as a function of roast time and is shown in Fig. 3 Fig. 3.. First crack progresse gre ssess fro from m jus justt bef before ore 400 s wit within hin the roa roast st and ends at abo about ut 600 s. Second Second cra crack ck begins begi ns at about 620 s and ends at about 730 s. The qualitative qualitative assessment, assessment, that second second crack events occur more frequently than first crack events is verified. Figure 3 shows that first crack has a peak rate of about 100 cracks per minute, while second crack has a peak rate of over 500 cracks per minute. These results also indicate that an automated technique could be used to differentiate between first crack and second crack by rate, providing a third metric for the automatic acoustic monitoring of the roast. 4. Conclusions

Sounds emitted during the coffee roasting process were measured and analyzed, including the sounds of first crack and second crack, and the background noise produced by the rotating drum and by the circulating circulating fan. Thre Threee acous acoustica ticall chara character cteristic isticss of the process were found that could be used to form an automated acoustical roast monitoring technique: first crack is louder than second crack (by 15% in peak acoustic pressure), first crack is significantly lower in frequency than second crack (by a factor of nearly 19), and second crack events proceed at a higher rate (by a factor of about 5) than first crack events. Other roasting noise does not impact the use of these signals. Thes Th esee fa fact ctor orss we were re qu quan anti tifie fied d fo forr an es espr pres esso so bl blen end d an and d on onee pa part rtic icul ular ar ro roas asti ting ng machine. Future work should include analysis of all the various types of green coffee beans of interest and different roasting processes and machines. Acknowledgments

The author acknowledges support of The University of Texas at Austin Department of Mechanical Engineering and the Fluor Centennial Teaching Teaching Fellowship #2. References and links 1

N. Luttinger and G. Dicum, The Dicum, The Coffee Book: Anatomy of an Industry from Crop to the Last Drop (The New Press, New York, 2006). 2 International Coffee Organization, I.C.O. Organization, I.C.O. Annual Review 2011/12 (International 2011/12 (International Coffee Organization, London, 2013). 3 J. A. Hernndez, B. Heyd, and G. Trystram, “Prediction of brightness and surface area kinetics during coffee roasting,” J. Food Eng. 89, 156–163 (2008). 4 P. Frisullo, J. Laverse, M. Barnab, L. Navarini, and M. A. Del Nobile, “Coffee beans microstructural changes induced by cultivation processing: An x-ray microtomographic investigation,” J. Food Eng. 109 , 175–181 (2012).

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E. R. Dutra, L. S. Oliveira, A. S. Franca, V. P. Ferraz, and R. J. C. F. Afonso, “A preliminary study on the feasibility of using the composition of coffee roasting exhaust gas for the determination of the degree of roast,” J. Food Eng. 47, 241–246 (2001). 6 J. A. Hernndez, B. Heyd, and G. Trystram, “On-line assessment of brightness and surface kinetics during coffee roasting,” J. Food Eng. 87, 314–322 (2008). 7 K. Sinnott, The Sinnott, The Art and Craft of Coffee: An Enthusiast’s Guide to Selecting, Roasting, and Brewing Exquisite Coffee (Quarry Coffee (Quarry Books, Beverly, MA, 2011). 8 J. Freeman, C. Freeman, and T. Duggan, The Blue Bottle Craft of Coffee: Growing, Roasting, Drinking, with Recipes (Ten Recipes (Ten Speed Press, Berkeley, CA, 2012).



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Coffee Roasting Acoustics

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