International Cooperation in a High Tech Domain.
Would anyone want to do high tech research on coffee, a plant that has been cultivated and processed for centuries? It may surprise many to learn that the answer is a resounding ''yes''. It should be less surprising that this beverage, with such a rich international and intercultural history has provoked motivation for collaboration between researchers in pluridisciplinary domains in Europe and the Americas. This collaboration has been begun in response to concerns both of production and processing to increase quality, as well as to address questions of safety and environmental impact of roasting. The techniques used revolve around the measurement of gases produced as a way to investigate complex chemical reactions in the heart of the beans or cherries themselves, reactions impossible to study by more conventional techniques.
This research project began when Prof. da Silva in Brazil contacted us to see if we could evaluate the quality of ground coffee by studying the aromatic (odor carrying) compounds in coffee. Brazilian coffee producers are concerned about rivals introducing ground corn and other cheap agricultural products into ground coffee in order to cut costs. Although the result of this dishonest practice may be discerned in the final product by the connoisseur, the marketed product can not easily be systematically controlled. In order to perform these measurements, Prof. da Silva modified a system for ultrasensitive measurements in solid samples. Once these measurements were completed, he considered using a system similar to one developed in The Netherlands to measure ammonia in agricultural settings. He contacted us to see if we would evaluate the concept once he learned that we were working on a similar and more flexible version of the system. The original research goals of our group in Paris have involved the conception of a compact trace gas sensing system for ''on site'' pollution measurements. This challenge of working with coffee beans under diverse operating conditions gave us the chance to demonstrate just how flexible the system could be.
These first measurements demonstrated the utility of the technique by proving that the gas released by corn or other additives could be distinguished from that released from coffee. This study has been brought to term, and further quality concerns that Prof. da Silva wished to study included measurements of gas produced by coffee cherries when they are dried and shelled. This information can be used to evaluate not only drying procedures, but also how the shelled cherries can better be stored in order to prolong freshness and reduce spoilage. The next step once we had proven that our system was precise enough was instigated by Rob Urgert, at WAU, also in The Netherlands. WAU supports research into every facet of agricultural production and food processing, handling and storage. Rob's doctoral research concerns the medical implications of over-roasted coffee, including chemicals called Methyl Esters and Perenobenzynes which get transformed at high temperatures and may or may not be carcinogenic (see notes 1,2). Rob approached Prof. Bicanic at WAU, one of our long time collaborators, who suggested that we could modify our gas sensor by combining it with a small roaster to measure gases produced by the beans.
Although the effects of these chemicals are not yet known, we saw the opportunity to show that our system could identify the bean roasting characteristics based on the gases alone. Some work has been done in gas-liquid phase with a different technique, but this didn't allow « on line » measurements (see note 3). We wanted to control temperature, and be able to stop or increase the process to achieve different roasts (say a French, or a darker Brazilian) in a repeatable fashion. We could further use this combination roaster-gas sensor to measure the composition of the gases produced, to estimate the environmental impact associated with different roasting procedures. Tom A. Griesemer, founder of Stauf's, a gourmet roasting shop and coffee house has mentioned that there is also an environmental impact associated with roasting. An example is the increasing number of small batch roasting coffee houses in the San Francisco bay area. These shops are less strongly regulated than their large counterparts because they individually contribute very little to the atmospheric conditions, but might start causing problems with their combined exhaust. While Tom has been looking at afterburners to reduce the quantity of harmful or greenhouse gasses in the roasting exhaust, we feel that it might also be a good idea to learn more about what part of the roasting process produces them. These various applications all share the same basic technology of gas sensor, which relies on what we call Photothermal Spectroscopy. In order to explain why the technology is so versatile and sensitive, we will need to briefly look at how it works, before discussing the results. Optical and Photo-thermal techniques for gas measurements.
Many different methods to detect and analyze gases based on optical techniques are currently used for environmental or safety control. In general, light must be absorbed by the gas to be studied (typically in the air). This absorption is just like the heating inside a car on a sunny day. With the windows up, the air in the car heats up but cannot escape. The trick of our technique is to allow us to make measurements in open air, important to study pollution in the atmosphere. This means that the measurements must be quick enough to measure changes while air is moving, in order to carefully track the sources of pollution gases. These techniques are called ''Photothermal'' because they rely on the conversion of light to heat (from photo=light and therm=heat in Greek), and were first demonstrated by Alexander Graham Bell over 100 years ago (see note 4). In fact, he predicted that a communication device called the ''photophone'' would be more important than the telephone. These techniques have many advantages as we can reach detection limits on the order of a few parts in a trillion (or 10-12).
We can think of this as trying to find a large house by looking from space at the entire United States. We can achieve such sensitivity since the signal recorded is directly related to the energy actually absorbed by the sample, and so we can make measurements even when there is a very small amount of pollution in the air. A diagram of the our sensor setup (see notes 5,6) is shown in Figure 1 . We see that the light which heats the air is provided by one (CO2) laser, called a ''pump laser''. We have two different ways to actually measure the amount of light absorbed. This depends either on the change of optical properties, like light shimmering above a hot stretch of road (which is actually a mirage). This is measured with a second laser which is not absorbed, called a ''probe laser''. We have improved capabilities of the system by using a pump laser whose wavelength (or color) of light may be adjusted. Changing this wavelength permits us to distinguish between different pollution substances in the air; if we think of the car heated by sunlight, we realize that cars with a light interior heat up less than those with a dark interior. This is because different materials absorb light differently. They heat up more or less depending on the amount of light absorbed. In this way, by measuring the heat produced in our air sample by several different wavelengths, we can determine what materials are present. In air, we can distinguish the different pollution gases from one another. For instance we can determine the quantity of nitrous oxides (NOx), greenhouse gasses (COV), ozone (O3), at the same time, even when the humidity and dust are present in high quantities. This sensitive has also been used for testing of cracks in aircraft equipment and characterization of diamond films and integrated circuit (IC) devices.
To study coffee roasting, we built up a small oven capable of temperature controlled heating of 1 to about 20 beans at a time. This oven is integrated to our detection cell by a Pyrex tube, and we pump gently to assure that the gases will actually find their way in. This oven, built up with the aid of Laurent Cohen-Scali, one of our students, is shown diagramaticaly in Figure 2. Laurent performed some of the first measurements, and was replaced at the end of his internship by Yann Fusero. The beans we used were donated by Dave Swanson, and we conducted studies with samples of Kenya AA and Ethiopian Sudimo Horse. Measurements of cherry drying were conducted with non cultured samples of Diveria and Canifora cherries from Martinique, and are being continued with cultivar (Icatu) cherries donated by Dr. Filho, at IAC, Brazil.
A typical measurement of gas released during roasting is shown in Figure 3. As previously explained, we vary the pump laser wavelength to distinguish between different gases. At the point marked H2O on the Figure, we see humidity on the inside of the Pyrex tube. This observation is confirms our measurement and the analysis made by spectral analysis. We then increase the temperature to go to a different phase of roasting. Had we held the temperature at below 225°C, the beans remain brown, but do not roast properly. When we go above this threshold temperature, we see a large signal increase and hear the characteristic « popping » sound associated with proper roasting. This signal corresponds to production of a gas called ethylene (C2H4), which is known to act as a signal (see notes 7-9) in many plant species*, and may be an indicator of rapid heating and expansion. We can use this increase in signal to identify the point at which the roasting should be stopped, or to study different roasting ''profiles'' to chose the optimal combination of temperature and time for a given time/temperature equation (see note 10). Coffee roasters may use this information to better process their beans, in order to lower costs, improve process efficiency, and reduce (presumed) health risks associated with over-roasting.
Figure 4 shows an example of the cherry drying measurements. Although this figure shows the quantity of gas released, what is more striking is that such a measurement may be performed with a single cherry. In other words, this technique may be used to track cherry drying on an extremely sensitive scale. With differing philosophies regarding how cherries should be prepared for roasting, we hope that this will offer promise to improve the processing of coffee, once again to reduce costs and provide a better product.
 K. Bonaa, E. Arnesen, and D. S. Thelle, British Med. J. 297 (6656) 1989
 R. A. Woutersen, A. van Garderen-Hoetmer, and J. Bax, Carcinogenesis, 10 (2) 1989
 E. B. Rathbone, G. D. Patel, R. W. Butters, J. Agricult. Food Chem. 37 (1) 1989
 A. M. Bell, Am. J. Sci. 20 (305) 1880
 B. Zimering and A. C. Boccara, Rev. Sci. Instrum. 67 (5) 1996.
 B. Zimering and A. C. Boccara, Appl. Opt. 36 (15) 1997
 F. B. Abeles, P. W. Morgan, M. E. Saltveit, Ethylene in Plant Biology, Academic < Press, San Diego, CA, ISBN 0-12-041451-1.
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 E. J. Woltering, D. Somhorst, and P. v. d. Veer, Plant Physiol 109 1995
 B. Zimering, L. Cohen-Scali, Y. Fusero, and A. C. Boccara, Instrument Science and Technology (submitted for special issue on photothermal measurement techniques for environmental and agricultural applications)
Information on the chemistry involved in coffee roasting and harvesting is available in books such as Olman's Encyclopedia of Industrial Chemistry.
We wish to acknowledge both scientific aid and the donation of samples (beans and/or cherries) from those who have helped us, and without whom this research would not even have been begun. Their respective contributions are described in the text, and they can be contacted at the addresses below. Financial support for this work has been provided by ADEME, an adjoint agency of the (French) Minister of the Environment.
Laurent Cohen-Scali and Yann Fusero, ESPCI, 10, rue Vauquelin, 75005 Paris, France.
Professor Edson Correa da Silva, San Paolo University, San Paolo, Brazil (firstname.lastname@example.org).
M. Rob Urgert and Professor Dane Bicanic, at Wageningen Agricultural University (WAU), Wageningen, The Netherlands
(Rob.Urgert@ET3.VOED.WAU.NL and Dane.Bicanic@USER.AENF.WAU.NL). web page http://www.aenf.wau.nl/nat/research/pas.html
Tom A. Griesemer, Stauf's Coffee Roasters, 1277 Grandview Ave. Columbus, OH 43212 (email@example.com). web page http://www.staufs.com/
D. Swanson, Coffee Bean Corral, PO Box 148, Mayer, AZ 86333 (firstname.lastname@example.org). web page http://www.bslnet.com/accounts/dcoffee/www/
Dr. Oliveiro Guerreiro Filho of Instituto Agronomico de Campinas (IAC), Campinas, Brazil.
Cantinol Mauricia of Martinique Island, French West Indies.
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