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3 for green beans to 0.7 kg/m3 for dark roasted coffee, while the porosity increases to about 0.5 for dark roast. Coffee is one of the world’s favorite beverages and among the most important food commodities traded internationally. While multiple reasons may explain its unique success story, the scent of coffee is without doubt one of its most attractive attributes. During roasting, a coffee bean undergoes major changes in its chemical composition, and gases are formed through Maillard and caramelization reactions and pyrolytic decomposition. The volume of the beans increases up to 80% and around 20% of dry matter is lost for dark roasted coffee. (1) Concurrently, the average density decreases from about 1.3 kg/mfor green beans to 0.7 kg/mfor dark roasted coffee, while the porosity increases to about 0.5 for dark roast. (2) At the end of the roasting process, part of the gases remains trapped inside the pores of the coffee beans and are gradually released during storage or more abruptly while grinding or extracting. Indeed, the ability to retain gases formed during roasting represents one of the most remarkable properties of coffee beans. (2)

2 ), 7.3% carbon monoxide (CO), and 5.3% nitrogen (N 2 ), the rest (less than 1%) being volatile organic compounds (VOCs). While the gas composition may depend on roast profile, degree and other parameters, no such detailed information is currently available. Since CO 2 is by far the most abundant component of the released gas, for the sake of simplicity, the total release of gas from coffee is approximated hereinafter by the release of CO 2 . The composition of gas released from whole beans during grinding has been reported by Clarke and McRae (3) as 87% carbon dioxide (CO), 7.3% carbon monoxide (CO), and 5.3% nitrogen (N), the rest (less than 1%) being volatile organic compounds (VOCs). While the gas composition may depend on roast profile, degree and other parameters, no such detailed information is currently available. Since COis by far the most abundant component of the released gas, for the sake of simplicity, the total release of gas from coffee is approximated hereinafter by the release of CO

While CO 2 is one of the smallest molecules in coffee, it is possibly also greatly underestimated for the multiple roles it plays in many properties and characteristics of coffee, from roasting all the way to the cup:

(i) 1–2% of the weight of freshly roasted coffee. (2,4,5)

(ii) The release of gases creates a protective atmosphere in packaging. A proper understanding of the amount of released gases is important for packaging and preserving the quality of coffee during storage.

2 retained inside the porous structure of the coffee is an indicator of its freshness. (iii) The amount of COretained inside the porous structure of the coffee is an indicator of its freshness. (6,7)

2 is directly released into the gas phase and can form what is called the “crema”. (iv) During extraction, part of the COis directly released into the gas phase and can form what is called the “crema”. (8,9)

(v) CO 2 can be formed during coffee extraction when coffee acids are neutralized by the hydrogen carbonate in the water.

These examples illustrate the many roles of gases (mainly CO 2 ) formed during roasting and released all the way to the cup. Considering the importance of entrapped gases and its release, it appears highly warranted to develop a precise, quantitative insight into the amount and temporal release of gases from roasted coffee. We have developed and applied a novel methodology for measuring the total amounts of gases released from a coffee sample. In particular, the rate of release from roasted whole beans and ground coffee, and applied it to a range of differently roasted Arabica and Robusta coffees. The resulting data were modeled using the Weibull distribution.

In previous works, the total amount of gas or CO 2 and the degassing rates from roasted whole beans and ground coffee have been measured using three different analytical approaches:

(i) The pressure increase was measured in a closed container during storage of roasted beans (10−12) and related to the amount of released gases.

2 in roasted coffee to be determined. 2 . While it might overestimate the amount of CO 2 that can potentially be released from beans, it might give an appropriate measurement for the CO 2 that can be liberated during water extraction. (ii) Quantitative extraction and subsequent trapping on a column allowed the residual COin roasted coffee to be determined. (5,13,14) This method aims at measuring the total amount of CO. While it might overestimate the amount of COthat can potentially be released from beans, it might give an appropriate measurement for the COthat can be liberated during water extraction.

2 from roasted whole beans and roasted and ground coffee was measured via monitoring the increasing CO 2 concentration in a closed container with a large headspace volume, using IR absorption spectroscopy. (iii) The release of COfrom roasted whole beans and roasted and ground coffee was measured via monitoring the increasing COconcentration in a closed container with a large headspace volume, using IR absorption spectroscopy. (5)

2 after roasting reported by Wang et al. 2 released during storage but also differed in the amounts of CO 2 formed during roasting. They concluded that due to the longer roasting time, a slow roast at lower roasting temperature led to more CO 2 released during the actual roasting cycle than a higher temperature fast roast (both to the same roast degree). The cumulative amount of CO 2 released (during roasting and during storage) was speculated to be similar for both roasting speeds. It has also been reported that the total amount of trapped gases in roasted coffee beans is dependent only on roasting degree, not speed, when measuring residual CO 2 content. Various amounts of gas released and degassing rates of whole roasted beans have been reported in the literature, (i) from as low as 0.7–4.3 mg/g (time not reported) by Shimoni et al., (ii) to 9–10 mg/g (in 60 days) by Geiger et al., (iii) 6.5–14 mg/g (in 30 days) using the IR method and up to 16 mg/g total trapped (residual) COafter roasting reported by Wang et al. (5) Degassing time of up to 75 days was examined by Baggenstoss et al. (10) The differences in the reported values are likely due to variations in the roasting profiles, methods and degassing times used by the various authors, as well as the coffee processing type, variety, and origin. In addition, it was found that the cooling mode after roasting (air cooling vs water spraying/quenching) has an impact on the degassing of whole coffee beans. (10) Geiger et al. reported that coffee beans roasted to the same gravimetric roasting mass loss, at two different roasting speeds, differed in the amount of COreleased during storage but also differed in the amounts of COformed during roasting. They concluded that due to the longer roasting time, a slow roast at lower roasting temperature led to more COreleased during the actual roasting cycle than a higher temperature fast roast (both to the same roast degree). The cumulative amount of COreleased (during roasting and during storage) was speculated to be similar for both roasting speeds. It has also been reported that the total amount of trapped gases in roasted coffee beans is dependent only on roasting degree, not speed, when measuring residual COcontent. (5,13) However, the degassing rates after roasting are highly dependent on both roasting speed and roasting degree. (5)

2 lost within the first few minutes after grinding have been reported to be as high as 40–50% 2 content, losses during grinding were 26–30%, 33–38%, and 45–59% for coarse, medium, and fine grind sizes. 2 , based on measured degassing release rates by IR spectroscopy, 2 from the roast and ground coffee is not entirely explained but is likely a combination of different processes. 2 content and the porosity of the bean, maximum internal pressures of 4.4 atm, 2 dissolved in coffee oils are likely the dominant processes. 2 sorption, and the coffee oil, despite its high CO 2 solubility, likely plays only a smaller role in sorption of CO 2 . After grinding, the rate of degassing is greatly increased. In fact, the amounts of COlost within the first few minutes after grinding have been reported to be as high as 40–50% (15) and 59–73%, (13) relative to the total amount of entrapped gases. Wang et al. reported that by measuring residual COcontent, losses during grinding were 26–30%, 33–38%, and 45–59% for coarse, medium, and fine grind sizes. (5) The amounts of lost CO, based on measured degassing release rates by IR spectroscopy, (5) show losses of 0–14%, 0–28%, and 40–65% for coarse, medium, and fine grind sizes (estimate from the degassing profiles). The amounts and duration of degassing from roast and ground coffee have been reported as 4–8.6% (in 30 h) by Anderson et al., 4–12% (in 200 h) by Wang et al., (5) and a degassing time of 75 h by Baggenstoss et al. (10) The exact mechanism of the release of COfrom the roast and ground coffee is not entirely explained but is likely a combination of different processes. (13,14) On the basis of the total COcontent and the porosity of the bean, maximum internal pressures of 4.4 atm, (14) 8 bar, (3) and 25 atm (7) in the coffee beans have been reported. After grinding, the degassing is caused by hydrodynamic pressure driven viscous flow in combination with molecular and Knudsen diffusion. According to Anderson et al., (14) the dominant mechanisms for gas diffusion depends on the internal pressure, with a change of the (dominant) mechanism reported at around 60–120 min of degassing time. After the first initial rapid degassing, the process slows down and effective diffusivities in the later stages indicate that surface diffusion and diffusion of COdissolved in coffee oils are likely the dominant processes. (13,14) The water content in the roasted coffee is too low to account for a bulk of the COsorption, and the coffee oil, despite its high COsolubility, likely plays only a smaller role in sorption of CO (13) The process of degassing has been analytically described by diffusion in a sphere, (13,14,16) and empirical models, such as the Weibull function, have been used to model the observed kinetics. (5,17)

The degassing results reported in the literature are roughly in agreement but are spread over a large range and often differ in values and rates, probably due to different methods and different roasting/degassing conditions being used. Considering the importance of degassing and CO 2 content in packaging, grinding, extraction, and for the quality in the cup, there is a strong need for an improved and robust analytical approach to obtain accurate and precise data for a range of roasting conditions, for Robusta and Arabica coffee, from whole and ground coffee.