An Investigation of the Kinetics and Equilibrium Chemistry of Cold-Brew Coffee: Caffeine and Chlorogenic Acid Concentrations as a Function of Roasting Temperature and Grind Size

Abstract: Recently, both small and large commercial coffee brewers have begun offering cold-brew coffee drinks to customers with the claims that these cold-water extracts contain fewer bitter acids due to brewing conditions (Toddy website, 2016) while still retaining the flavor profile. Dunkin Donuts’ website suggests that the cold-water and long brewing times allow the coffee to reach “... its purest form.” With very little research existent on the chemistry of cold brew coffee consumers are left to the marketing strategies of Starbucks and other companies regarding the contents of cold-brew coffee. This research analyzes the caffeine and chlorogenic acid (3-CGA) content of cold-brew coffee as a function of brewing time, grind size, and roasting temperature of coffee beans sourced from the Kona region of Hawaii using high pressure liquid chromatography (HPLC). Coarse and medium grinds of both dark and medium roasts were analyzed by mixing 350mL of filtered water with 35g of coffee grinds under constant stirring at 20°C. Sampling was performed every 15 minutes for the first hour, then every 30 minutes for the next ten to twelve hours, with a final sample being drawn at 24hours. Equilibrium concentrations for both 3-CGA and caffeine were reached following 600 minutes. The caffeine concentrations ranged from 935mg/L to 1475mg/L. Variation was seen as a function of roasting temperature, and less so grind size. The 3-CGA concentrations were found to range from 345mg/L to 547mg/L. In both cases, the medium roast coarse grind coffee produced the highest concentrations of caffeine and 3-CGA while dark roast coarse grind produced the lowest concentrations of caffeine and 3-CGA. Hot brew experiments agreed well with caffeine and 3-CGA extraction concentrations in both dark roast coffees, showing very similar final concentrations. The medium roast coffees showed deviation from the hot brew coffees with respect to caffeine, indicating the need for additional experimentation to determine the role of water temperature in the availability of caffeine during extraction