Looking into individual coffee beans during the roasting process: direct micro-probe sampling on-line photo-ionisation mass spectrometric analysis of coffee roasting gases

Abstract

A micro-probe (μ-probe) gas sampling device for on-line analysis of gases evolving in confined, small objects by single-photon ionisation time-of-flight mass spectrometry (SPI-TOFMS) was developed. The technique is applied for the first time in a feasibility study to record the formation of volatile and flavour compounds during the roasting process within (inside) or in the direct vicinity (outside) of individual coffee beans. A real-time on-line analysis of evolving volatile and semi-volatile organic compounds (VOC and SVOC) as they are formed under the mild pyrolytic conditions of the roasting process was performed. The soft-ionisation mass spectra depict a molecular ion signature, which is well corresponding with the existing knowledge of coffee roasting and evolving compounds. Additionally, thereby it is possible to discriminate between Coffea arabica (Arabica) and Coffea canephora (Robusta). The recognized differences in the roasting gas profiles reflect the differences in the precursor composition of the coffee cultivars very well. Furthermore, a well-known set of marker compounds for Arabica and Robusta, namely the lipids kahweol and cafestol (detected in their dehydrated form at m/z 296 and m/z 298, respectively) were observed. If the variation in time of different compounds is observed, distinctly different evolution behaviours were detected. Here, phenol (m/z 94) and caffeine (m/z 194) are exemplary chosen, whereas phenol shows very sharp emission peaks, caffeine do not have this highly transient behaviour. Finally, the changes of the chemical signature as a function of the roasting time, the influence of sampling position (inside, outside) and cultivar (Arabica, Robusta) is investigated by multivariate statistics (PCA). In summary, this pilot study demonstrates the high potential of the measurement technique to enhance the fundamental knowledge of the formation processes of volatile and semi-volatile flavour compounds inside the individual coffee bean.

Bioactive Micronutrients in Coffee: Recent Analytical Approaches for Characterization and Quantification

“…Without the need for sample preparation or chromatographic separation, the molecular ion (M + H) + resulting from the method of DART-MS was used for the identification of caffeine in commercial instant coffee samples [149]. Caffeine has a mass to charge ratio ( m / z ) of 194, and its characterization alongside kahweol ( m / z , 296) and cafestol ( m / z , 298), two marker compounds for C. arabica and C. canephora , was achieved in the method of single photon ionization coupled with TOFMS (SPI-TOFMS) developed by Hertz-Schünemann et al [150] for the recognition of roasting gas profiles of coffee.…”

On‐line process monitoring of coffee roasting by resonant laser ionisation time‐of‐flight mass spectrometry: bridging the gap from industrial batch roasting to flavour formation inside an individual coffee bean

“…This probably reflects the fact that the process of drilling the sampling channel as well as the position the channel needs to be further examined and standardised. As there are different tissues and morphological structures in the coffee beans, a systematic investigation of the influence of the sampling point and channel depth needs to be performed in future studies. However, in general, the profiles are dominated by the masses m/z 150 and m/z 194, which can be assigned to be predominately 4‐vinylguaiacol ( m/z 150) and caffeine.…”

“…In particular, the REMPI mass spectra with logarithmic intensity axis (Figs C and F) show these differences very clearly. Although many molecular ion masses, which appear below m/z 200 in photo ionisation mass spectrometry (REMPI and SPI), can be identified/assigned, very little is known about the compounds that lead to ion signals in REMPI‐TOFMS at masses above m/z 200. In Table , the REMPI active compounds, which were previously assigned, are listed together with the observed concentration range from literature data.…”

“…organic acids, carbohydrates, amino acids and lipids). These substances are the precursor material for the generation of coffee roasting aroma, which are decomposed during the pyrolysis process . After roasting, the cells are more or less empty …”

“…Based on a previous study using SPI‐MS, a so‐called μ‐probe sampling device was used for sampling gases (1) from the inside of a coffee bean and (2) from just outside the surface (in the close vicinity of a coffee bean's surface). The sampling technology was initially developed for the analysis of combustion gases within the rod of a cigarette .…”