The Effect of Time, Roasting Temperature, and Grind Size on Caffeine and Chlorogenic Acid Concentrations in Cold Brew Coffee

Abstract

The extraction kinetics and equilibrium concentrations of caffeine and 3-chlorogenic acid (3-CGA) in cold brew coffee were investigated by brewing four coffee samples (dark roast/medium grind, dark roast/coarse grind, medium roast/medium grind, medium roast/coarse grind) using cold and hot methods. 3-CGA and caffeine were found at higher concentrations in cold brew coffee made with medium roast coffees, rather than dark roast. The grind size did not impact 3-CGA and caffeine concentrations of cold brew samples significantly, indicating that the rate determining step in extraction for these compounds did not depend on surface area. Caffeine concentrations in cold brew coarse grind samples were substantially higher than their hot brew counterparts. 3-CGA concentrations and pH were comparable between cold and hot brews. This work suggests that the difference in acidity of cold brew coffee is likely not due to 3-CGA or caffeine concentrations considering that most acids in coffee are highly soluble and extract quickly. It was determined that caffeine and 3-CGA concentrations reached equilibrium according to first order kinetics between 6 and 7 hours in all cold brew samples instead of 10 to 24 hours outlined in typical cold brew methods.

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.

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.

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.

Figure 4

The graphical image of sorting effects on pore connectivity in coffee matrix beds. Grain size distribution influences both the intragranular diffusion process, as larger grains have greater diffusion distances, and the intergranular pore network, as poorly sorted grain beds have more tortuous pore networks.