Chapter 37
Capillary Electrophoresis of Roasted Coffees 1
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Jennifer M. Ames , Louise Royle , and Allan G. W. Bradbury
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1
Department of Food Science and Technology, The University of Reading, Whiteknights, Reading RG6 6AP, United Kingdom Kraft Jacobs Suchard, R&D Inc., Unterbiberger Strasse 15, D-81737 Munich, Germany 2
Arabica coffee beans were roasted either at 240-290°C for 3 min or for 2-5 min at 265°C. Aqueous extracts prepared from them, and from unroasted beans, were separated into high molecular weight ( H M W ) and low molecular weight ( L M W ) fractions by ultrafiltration, using a membrane with a nominal molecular weight cut-off of 5 000 daltons. Absorption at 460 nm for the aqueous extracts increased with degree of roast. Extracts and fractions were analyzed by capillary zone electrophoresis (CZE) using different buffers. Beans roasted to similar colors gave very similar C Z E data, regardless of roasting conditions, the electrophoretograms (e-grams) being characteristic of the degree of processing. A broad peak was obtained for the H M W fraction which increased with degree of roast and which was indicative of a complex mixture of compounds. It contained the majority of the colored material. The data suggest that the aqueous extracts of the roasted coffees are qualitatively similar, regardless of the roasting regime.
Capillary electrophoresis (CE) is a family of powerful separation techniques which has several advantages over alternative better established methods. These advantages include a very high separation efficiency, typically 4 000 000 theoretical plates per meter, compared to around 100 000 plates per meter for an H P L C column (/), injection of nanolitres of sample, separation in aqueous buffer and short run times. The absence of organic solvent means that detection at 200 nm is possible with no loss of sensitivity, permitting the detection of analytes with weak chromophores. The most frequently used mode of C E is capillary zone electrophoresis (CZE), in which positively and negatively charged analytes are separated in buffer according to their charge to mass ratios. 364
© 2000 American Chemical Society
Parliment et al.; Caffeinated Beverages ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
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365 The application of C E to the analysis of food components has been reviewed (24). The determination of caffeine (5) and phenolics in coffee (6) has been reported. Roasting is an essential processing step for the conversion of green coffee beans to a desirable beverage. The purpose of roasting is to develop the characteristic flavor and color of roast coffee. Additional consequences of roasting include loss of dry matter (typically 5-8%) (7), evolution of carbon dioxide, loss of water, reduction in levels or loss of components (including sugars, amino acids, chlorogenic acids and trigonelline), and formation of melanoidins (which give roast coffee its characteristic color) (8). A l l melanoidins are notoriously difficult to characterize (9-10). Their structures are unknown, but recent work by Hofmann (//) suggests that small colored molecules may play a crucial role in their formation. Matrix assisted laser desorption mass spectrometry ( M A L D I - T O F - M S ) has been successfully used to elucidate the structures of polymers prepared from pyrrole monomers (12), while solid state nuclear magnetic resonance spectroscopy ( N M R ) has been used to characterize some functional groups of melanoidins (13). Although melanoidins constitute at least 20% of the solids of a roast coffee brew, relatively little is known about their structure and properties. In a series of detailed studies, Maier (14) used resin and gel permeation chromatography to separate coffee melanoidins into fractions and a wide range of functional groups was identified. Amino acids and monosaccharides were released from the melanoidins on acid hydrolysis. Steinhart and Packert (15) have used gel permeation chromatography and thin layer chromatography to separate coffee melanoidins into distinct fractions. C Z E has the potential to separate coffee melanoidins according to their charge:mass ratios and to monitor changes in charge:mass ratio with degree of roasting. C Z E with p H 9.3 borate buffer has already been used to profile total Maillard reaction products from model systems. Components of lower molecular weight migrated as sharp peaks while the higher molecular weight material migrated as a single broad band (16). The aim of this study was to apply C E to the analysis of two series of coffees to ascertain i f the technique could distinguish any differences that might exist between fractions from beans subjected to different regimes but to the same color. The first series was roasted for different times at the same temperature, and the second was roasted for the same time at different temperatures.
Experimental Sample preparation Colombian Arabica coffee beans were roasted using the conditions given in Table I. The colors of the beans were measured using a Color-Tester LK100 unit (Dr Bruno Lange GmbH, Berlin) and are shown in Table I.
Parliment et al.; Caffeinated Beverages ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
366 Table L Roasting regimes and Lange units for the coffee samples Sample Code
Temperature (*C)
Time (min)
Lange Units
240 260 280 290 265 265 265 265
3 3 3 3 2 3 4 5
43 24.5 13.3 7.1 4.5 25.8 11.8 6.3 3.9
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CO/TO
C2 C3 C4 C5 T2 T3 T4 T5
Beans (10 g) were ground i n ' a coffee grinder for 1 min. Aqueous extracts were prepared by adding boiling water (50 mL) to grounds (2.5 g), stirring for 10 s and standing for 10 min. Extracts were filtered through paper and then a 0. 2 μιη P V D A filter. Aqueous extracts (200 \\L) were separated into high molecular weight (HMW) and low molecular weight ( L M W ) fractions using Microcon ultrafiltration units with low binding regenerated cellulose membranes (nominal molecular weight cut-off of 5 000 daltons) (Millipore, Watford, U K ) at 5 000g for 50 min. The retained sample (
