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Variation in caffeine concentration in single coffee beans Glen p. Fox, Alex Wu, Yiran Liang, and Lesleigh Force J. Agric. Food Chem., Just Accepted Manuscript • Publication Date (Web): 27 Sep 2013 Downloaded from http://pubs.acs.org on September 29, 2013
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Journal of Agricultural and Food Chemistry
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Variation in caffeine concentration in single coffee beans
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Glen P Foxa*, Alex Wub, Liang Yiranb and Lesleigh Forceb
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a
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Innovation, Toowoomba, Qld 4350 Australia.
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b
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Qld 4072, Australia.
The University of Queensland, Queensland Alliance for Agricultural & Food
School of Agriculture and Food Sciences, The University of Queensland, St Lucia
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*Corresponding Author: Dr Glen Fox
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Tel: +61 746398830
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Fax: +61 746398800
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Email
[email protected] 15 16
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ABSTRACT
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Twenty eight coffee samples from around the world were tested for caffeine levels to
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develop near infrared reflectance spectroscopy (NIRS) calibrations for whole and
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ground coffee. Twenty-five individual beans from five of those coffees were used to
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develop a NIRS calibration for caffeine concentration in single beans. We used an
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international standard high performance liquid chromatography method to analyse for
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caffeine content. Coffee is a legal stimulant and possesses a number of heath
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properties. However, there is variation in the level of caffeine in brewed coffee and
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other caffeinated beverages. Being able to sort beans based on caffeine concentration
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will improve quality control in the level of caffeine in those beverages. There was a
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range in caffeine concentration was from 0.01 mg/g (decaffeinated coffee) up to 19.9
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mg/g (Italian coffee). The majority of coffees were around 10.0 mg/g to 12.0 mg/g.
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The NIRS results showed r2 values for bulk unground and ground coffees were greater
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than 0.90 with standard errors less than 2 mg/g. For the single bean calibration the r2
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were between 0.85 and 0.93 with standard errors of cross validation of 0.8 to 1.6 mg/g
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depending upon calibration. The results showed it was possible to develop NIRS
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calibrations to estimate the caffeine concentration of individual coffee beans. One
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application of this calibration could be sorting beans on caffeine concentration to
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provide greater quality control for high-end markets. Further, bean sorting may open
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new markets for novel coffee products.
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Journal of Agricultural and Food Chemistry
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INTRODUCTION
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Coffee is one of the most-consumed beverages in the world with aroma and taste
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being the main sensory characteristics. The dominant commercial species of coffee
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are Arabica (Coffea arabica L.) and Robusta (C. canephora Pierre) and most
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commercial coffee beverages are produced from these roasted beans or blends of
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these two. However, there are differences in quality and sensory profiles between
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these species, and suitable analytical methods and trained taste panels are required to
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quantify these differences.
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In regards to quantifying chemical components, such as lipids, protein or
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caffeine concentration, expensive and time-consuming wet chemistry methods are
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required. However, near infrared reflectance spectroscopy (NIRS) has been used as a
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high through-put and cheap surrogate for quantification of quality (including caffeine
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concentration) 1-7 and sensory parameters 8. The differences detected in the chemical
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composition within and between Arabica and Robusta varieties can be useful for
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quality classification purposes and NIRS has been used successfully in this context 1.
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In addition, NIRS has also been used for quantification of green bean quality 9, 10 and
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to understand genetic, environmental and processing effects on quality 11.
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In most biological studies, a bulk sample is tested to obtain a representative
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mean value, although there is little information on the variation within the sample and
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the impact of the variation on the result. Several studies in cereal crops investigated
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the evaluation of single grains to ascertain the variation within a sample for a range of
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attributes using NIRS 12-24. To date, there has been no report of assessment of single
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coffee beans by NIRS. However, single coffee beans have been tested for specific
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purposes including optimising a wet chemistry method for caffeine concentration 25
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and identifying breeding lines for reducing caffeine concentration 2, 26.
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The aim of our study was to develop NIRS calibrations for caffeine
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concentration in bulk unground and ground coffees as well as single coffee beans
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using a high performance liquid chromatography (HPLC) method as the reference
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method to quantify caffeine levels. The NIRS calibration in bulk unground and
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ground coffees would provide a rapid, predictive method of caffeine concentration. A
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NIRS calibration on single beans would provide the opportunity to understand the
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potential variability in coffee samples, e.g. blends, or sort beans based on caffeine
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concentration to further quantify quality and potentially develop new products (e.g.
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naturally low caffeine coffee).
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MATERIALS AND METHODS
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Twenty-eight roasted Arabica and blended whole bean coffees, purchased from retail
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outlets, from a number of countries were used. Most of the coffees were Arabica type
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as per the information on the packet. Unfortunately, some of packets lacked specific
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or complete information on the origin or blend of the coffee. Table 1 shows the types
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and origins of coffees.
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Ground coffee
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A 20 g sub-sample from each coffee was ground in a standard electric coffee grinder
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for NIR scanning and HPLC analysis. The grinder was cleaned with a fine brush
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between coffees.
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FT-NIR
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Three forms of coffees (bulk unground, ground and single beans) were scanned in a
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Fourier Transform (FT) NIR multipurpose analyzer instrument (Bruker, Germany)
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from 4000 to 12000 cm at 8 cm resolution. For bulk unground and ground coffees,
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the 97 mm cup on the rotating sphere was used, where 64 scans of the rotating cup
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were averaged into a single spectrum. Coffees were scanned in duplicate.
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Five coffees were selected for single bean analysis (coffees numbered 5, 7, 11, 15,
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and 24 in Table 1). Four samples were blends which would provide a broad range
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within each coffee. A single Arabica was also included. Twenty-five individual beans
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were randomly selected from each of these five coffees and scanned in a 19 mm glass
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vial using the microsphere attachment. A chrome insert was placed on top of each
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bean to prevent light escaping, as per Bruker recommendations. The wavenumber
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range and resolution were the same as for the bulk unground and ground coffees.
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After scanning, each bean was sealed in a plastic bag and stored in the freezer prior to
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grinding for caffeine analysis.
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Opus (V 7.0) was used for calibration development with all pre-treatments selected in
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the Optimise process to find optimal calibration math treatment and factors. Savistky-
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Golay, with 1st and 2nd derviatives were used with 4, 4 gap and smoothing. Individual
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spectra files were exported in data point table format (Opus software), loaded into
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Excel, then imported into Unscrambler (V 9.8, [Camo, Sweden]) for calibration
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development. The calibrations developed from these two multivariate software
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programs were compared using the Fearn 27 calculation which tests for significant
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differences (P
