Kinetics of cold brew coffee extraction

Four coffee samples were used in this study. See Table 1 for grind size distribution and roasting temperature characteristics for each of the samples.

Table 1 Summary of grind size distribution by percent mass (100.0 g of grinds used in each analysis) and roasting temperature, as reported by the coffee vendor. Full size table

The grain size distributions of the four samples show that the “medium” grind coffees had wider particle distributions, both containing about 5% of particles, by mass, that are larger than 3350 µm. The “coarse” grind coffees showed no 3350 µm portion, and have a narrower particle distribution with more than 70% of particles, by mass, being retained on the 841 µm sieve. Coffee beans are naturally porous. The pore space within each grain of coffee is considered the intragranular pores. The space between grains of coffee is referred to as the intergranular pores. Earlier studies found that particle distribution was vitally important to coffee extraction, affecting both the diffusion of compounds through intragranular pores within grinds, as well as the fluid flow between the grinds (through the intergranular pore network)14,44,45. The importance of both intragranular and intergranular pore space will be discussed further with respect to diffusion limiting processes of compound extraction kinetics.

3-CGA

The compound, 3-CGA is freely soluble in water at room temperature46. Initial 3-CGA concentration increased rapidly over the first 180 minutes and slowed until reaching equilibrium at approximately 400 minutes for all coffee roasts and grinds (see Fig. 1). Moroney et al.45 attributed the initial fast extraction of soluble coffee compounds to the extraction of compounds from the surface and near-surface volume of the solid coffee grind matrix. The slower, longer time-scale extraction of additional CGA concentration, post 180 minutes, is likely due to the mass transfer of the compound through intra-grain pores into intergrain pores, and ultimately into the bulk liquid phase. The data collected in this current work follows the Spiro and Selwood36 model well and suggests that cold brew processes of 3-CGA extraction are governed by first-order kinetics. A sample kinetic plot of ln (C ∞ /[C ∞ − C]) for 3-CGA versus time is shown in Fig. 2 where C ∞ is the equilibrium concentration of 3-CGA and C is the concentration of 3-CGA at time t. Several sources7,8,9,10 providing brewing instructions for cold brew coffee recommend prolonged brewing times upwards of 12 to 24 hours. Any water/grind interaction longer than 400 minutes (6.7 hours) did not result in additional significant extraction of 3-CGA. The mean concentrations of 3-CGA at 400 and 1400 minutes were within one standard deviation of each other (see Table 1). These data suggest 3-CGA concentrations are influenced by roasting temperature, but not grind size. Blumberg et al.11 determined that increased roasting temperatures resulted in degradation of chlorogenic acid precursors and lower extractable total chlorogenic acid concentrations. The same study also observed that chlorogenic acids extracted quickly from coffee grinds, while 4-vinylcatechol oligomers showed strong retention to the coffee grinds11. The longer steeping times associated with cold brew coffee may result in increased extraction of these catechol oligomers, which are characterized by harsh bitter-tasting properties. Over-brewing cold brew coffee may result in unpalatable extracts due to these and other relatively slow-extracting compounds.

Figure 1 Concentration of 3-CGA (top) and caffeine (bottom) over time for (■) medium roast - medium grind; (●) medium roast - coarse grind; (◆) dark roast – medium grind, (▲) dark roast - coarse grind. The vertical line at 400 minutes represents the establishment of steady-state concentration for both 3-CGA and caffeine extractions. Full size image

Figure 2 First-order plot for the extraction of 3-CGA from medium grind - medium roast coffee particles at 23.5 °C. R2 = 0.983. Full size image

Caffeine

Caffeine, unlike 3-CGA, has a limited solubility at room temperature of 16 mg/mL46. However, all four coffees analyzed showed comparable extraction kinetics to those of 3-CGA. In all coffees sampled, fast initial extraction was seen over the first 180 minutes, with a slower rate of extraction after 180 minutes. Similar to 3-CGA, caffeine also reaches nearly steady-state concentrations after 400 minutes (see Fig. 1). As with 3-CGA, all samples followed a first-order kinetic model. Spiro and Selwood36 offered a thorough mathematical model for the kinetics of caffeine extraction at room temperature, and found that the diffusion of caffeine through the intragranular pore space to be the rate limiting step in the extraction process. This analysis concluded that extraction times greater than 400 minutes do little to increase the caffeine concentration of the resulting coffee. Moreover, caffeine concentrations do not demonstrate the same sensitivity to roasting temperatures as 3-CGA, and all coffee roasts and grinds were found to have comparable caffeine concentrations at equilibrium, with the exception of the dark roast - coarse grind coffee. The relatively high standard deviations are suspected to be caused by the heterogeneous grind size distributions from the commercially sourced beans. As the packaging was handled, settling of finer particles may have caused inter-sample variability.

pH

Work by Andueza et al.47 and Gloess et al.48 both report there is no correlation between pH and perceived acidity in the flavor of coffees. However, commercial coffee vendors continue to relate acidity to coffee taste when marketing coffee to consumers. The pH of coffee studied in this work ranged from 5.40 to 5.63. Moon et al.15 observed a correlation between total CGA concentrations and pH of coffee extracts. However, data collected in this work did not provide enough evidence to support the claim by Moon et al.15 that coffee samples containing high concentration of 3-CGA tend to have high acidity or low pH.

Comparison of hot brew and cold brew coffee

There is a common marketing message that cold brew coffee is fundamentally different than hot brew coffee. This may be attributed to acidity and/or caffeine concentration49,50. This work compared the same water-to-coffee ratio using cold brew and hot brew extraction processes to identify any differences between the two methods with respect to 3-CGA and caffeine concentrations. In the coffee extraction process, Moroney et al.35 described two different extraction mechanisms that function on different timescales. The fast extraction from the surface and near-surface matrix happens much more rapidly than the diffusion of compounds through the intragranular pore network to the grain surface. Because the time periods for hot brew and cold brew are drastically different, 6 minutes vs. 1440 minutes respectively, the intragranular diffusion may limit the extractable concentration of soluble coffee compounds in the hot brew, as compared to the cold brew.

3-CGA

In Fig. 3, the cold brew extraction of caffeine and 3-CGA are shown for each of the four coffee samples, with the hot brew concentrations indicated by horizontal lines. Table 2 shows the equilibrium concentrations of 3-CGA for the hot and cold brew coffees along with the pH. In both hot and cold brew extractions, all samples show comparable average 3-CGA concentrations and pH, regardless of water temperature. The CGA molecule is not seen to be limited by the intragranular pore diffusion processes, as observed with caffeine extraction. CGA is freely soluble in water, and this facilitates its extraction at both low and high temperatures. While grain size did not impact the magnitude of 3-CGA concentrations, roasting temperature of the beans did show a noticeable effect in both cold and hot experiments. In both hot and cold brew extractions, CGA was found in higher concentrations in medium roasts than in darks roasts, supporting the work of Trugo and Macrae37 that suggests that higher roasting temperatures decomposes CGA and results in lower extraction concentrations.

Figure 3 Caffeine (▲) and 3-CGA (●) concentration as a function of time for each of the four coffee samples. Horizontal lines represent each coffee type’s hot water concentration for caffeine and 3-CGA. Error bars represent the range for each measurement. Full size image

Table 2 Concentration of 3-CGA and caffeine and pH of each cold brew coffee sample after 400 minutes and 1440 minutes of brewing time (Mean ± 95% Confidence Interval, n = 6). Full size table

Caffeine

Coarse grain samples, both medium and dark roast, showed a considerable deviation in caffeine concentrations between hot and cold brew extractions (Table 3). In both samples, the cold brew coffee was found to have the higher concentration of caffeine. Medium grain samples also showed higher concentrations of caffeine in cold brew extraction, however, the difference was not statistically significant. This result suggests that the medium grind blends, in both hot and cold extraction, experienced nearly complete extraction during their respective steeping times. The hot brew extraction saturated the intra- and intergranular pores and facilitated fast diffusion (6-minute steeping times) of caffeine across the solid matrix to generate a bulk liquid phase with nearly the same concentration of caffeine as the cold brew coffee generated in 400 minutes. Coarse grain samples, with their higher relative proportion of particles in the 841 µm range, did not reach similar steady-state concentrations in both hot and cold brews. The faster, hot water extraction was diffusion limited, and likely did not allow the full extraction of caffeine across the larger radius particles. The longer brewing times for the cold brew samples resulted in greater caffeine extraction, allowing time for completion of the rate-limiting mass transfer step in the extraction process.

Table 3 Comparison of equilibrium 3-CGA and caffeine concentrations (after 1440 min) extracted using cold and hot brew method along with pH (mean ± 95% Confidence Interval, n = 6). Full size table

Role of Grind Size and Roasting Temp in Cold Brew Coffee

Further analysis of the data indicates that the observed CGA and caffeine concentration differences between medium roast and dark roast are, in general, substantial. Both CGA and caffeine showed higher concentration in medium roast samples. Our data is in support of the works of Trugo, et al.37 and Hečimović, et al.51, both suggest that higher roasting temperatures decrease the concentration of CGA and caffeine. The only exception is the observed difference in concentration of caffeine when comparing medium roast – medium grind and dark roast – medium grind samples. Although the medium roast samples showed higher concentration of caffeine than dark roast samples, the observed difference in concentration is insignificant due to large variations in the measurements.