No abstract

A real-time automated process control tool for coffee roasting is presented to consistently and accurately achieve a targeted roast degree. It is based on the online monitoring of volatile organic compounds (VOC) in the off-gas of a drum roaster by proton transfer reaction timeof-flight mass spectrometry at a high time (1 Hz) and mass resolution (5,500 m/Δm at full width at half-maximum) and high sensitivity (better than parts per billion by volume). Forty-two roasting experiments were performed with the drum roaster being operated either on a low, medium or high hot-air inlet temperature (= energy input) and the coffee (Arabica from Antigua, Guatemala) being roasted to low, medium or dark roast degrees. A principal component analysis (PCA) discriminated, for each one of the three hotair inlet temperatures, the roast degree with a resolution of better than ±1 Colorette. The 3D space of the three first principal components was defined based on 23 mass spectral profiles of VOCs and their roast degree at the end point of roasting. This provided a very detailed picture of the evolution of the roasting process and allowed establishment of a predictive model that projects the onlinemonitored VOC profile of the roaster off-gas in real time onto the PCA space defined by the calibration process and, ultimately, to control the coffee roasting process so as to achieve a target roast degree and a consistent roasting.

a b s t r a c tLiquid-air partition coefficients (Henry's law constants, HLCs) of eight flavour compounds (volatile organic compounds, VOCs) were determined in water, over a temperature range of 4 • C to 85 • C. The HLCs were derived by using nitrogen to strip a dilute solution of a VOC and then determining the decrease in concentration of the VOC in real-time in the stripped gas using proton-transfer-reaction time-of-flight mass-spectrometry (PTR-ToF-MS). This approach provided HLCs of improved accuracy (small 95% standard deviation) over a large temperature range, especially for low volatility VOCs (HLC > 2 mol/(m 3 Pa)). The outstanding features of this approach are: (i) it is applicable for VOCs over a large range of volatility; (ii) it can be used over a wide temperature range (4 • C to 85 • C); (iii) it is automated (high-throughput); (iv) it does not require calibration or knowledge of the initial concentration of the analyte; and (v) the experimental temperature can be controlled very precisely ( T better than ±0.1 • C). The eight flavour compounds analysed in water were: (E)-␤-damascenone, 2,3-butanedione, 2-ethyl-3,5-dimethylpyrazine, 2-methylfuran, 3-methylbutanal, acetaldehyde, ethyl-3-methyl butanoate and guaiacol. Based on the measured HLCs at five fixed temperatures (4 • C, 25 • C, 45 • C, 65 • C and 85 • C), accurate non-linear analytical expressions for the temperature dependence of HLCs were derived, which were then used to calculate thermodynamic constants.

No abstract