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Continuous analysis of roasted coffee beans allows roasteries to improve their roasting settings, leading to higher energy efficiency and more consistent final products. Not only is flavor affected by the roasting degree, but caffeine content can also change. Conventional analytical methods such as HPLC (high-performance liquid chromatography) for caffeine concentration determination require technical skills to operate, chemical reagents, and take from several minutes to hours to obtain the results.

In contrast, near-infrared spectroscopy (NIRS) is a fast and chemical-free alternative for caffeine, water activity, and moisture analysis in roasted coffee beans and ground coffee. The NIRS solution is easy to use and does not require any sample preparation. These analyzers can be operated nearby the roaster or in a quality control lab.

Up to 35 roasted ground coffee bean samples were analyzed on a Metrohm DS2500 Solid Analyzer with the DS2500 Holder and NIRS mini sample cups (Figure 1). Samples were positioned into the NIRS mini sample cups for the analysis in diffuse reflection mode. Data acquisition and prediction model development were performed with the software package Vision Air Complete (Table 1).

Reference values for caffeine, water activity, and moisture were obtained with the respective primary methods. Caffeine analysis followed the ISO 20481 guideline and was conducted with an ion chromatograph (IC), water activity determination followed the ISO 18787 norm, and moisture determination was performed according to DIN 10772-1.

Table 1. Hardware and software equipment overview.

Equipment Article number
DS2500 Solid Analyzer 2.922.0010
DS2500 Holder 6.7430.040
NIRS mini sample cups 6.7402.030
Vision Air 2.0 Complete 6.6072.208
Figure 1. A Metrohm DS2500 Solid Analyzer with ground coffee beans held in a NIRS mini sample cup.

The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for the different reference parameters. Correlation diagrams which display the relation between the Vis-NIR prediction and the reference values are shown in Figures 3–5 together with the respective figures of merit (FOM).

Figure 2. Selection of Vis-NIR spectra of roasted and ground coffee bean samples. Data was obtained with a DS2500 Solid Analyzer. A spectra offset was applied for visualization purposes.
Figure 3. Correlation diagram and the respective FOMs for the prediction of caffeine in roasted ground coffee samples using a DS2500 Solid Analyzer. The lab values were determined using ion chromatography according to the guidelines in ISO 20481.
Figure 4. Correlation diagram and the respective FOMs for the prediction of water activity in roasted ground coffee samples using a DS2500 Solid Analyzer. The lab values were determined according to the guidelines in ISO 18787.
Figure 5. Correlation diagram and the respective FOMs for the prediction of moisture in roasted ground coffee samples using a DS2500 Solid Analyzer. The lab values were determined according to the guidelines in DIN 10772-1.

This Application Note shows the feasibility of near-infrared spectroscopy for the analysis of several quality parameters in roasted ground coffee. One NIRS analyzer can determine the caffeine concentration (0.1–2.5 wt%) in addition to water activity and moisture content in a single measurement. Not only are results delivered in less than a minute, but no chemical reagents are required for the analysis. The time savings by using NIRS compared to the traditional analytical methods (Table 2) is immense.

Table 2. Time to result comparison for different methods used to analyze coffee.

Parameter Method Time to result
Caffeine IC System (ISO 20481) 120 min (sample preparation and measurement)
Water activity Water Activity System (ISO 18787) 15–30 min
Moisture Oven – Loss on drying (DIN 10772-1) 13 hours (sample preparation and measurement)
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Metrohm Suisse SA

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4800 Zofingen

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Internal references: AW NIR CH-0069-042023; AW NIR CH-0070-042023