UC Davis

During coffee consumption, acidity and coffee color serve as two facets of quality. The acidity within a cup of coffee must be balanced; the beverage must be “bright”, but also not too sour. The key to perfect levels of acidity is to understand not only how different acids can impact the overall sensory quality of the cup, but how the acid concentrations themselves are subject to variation over coffee types and roast levels. The first goal of this research was to elucidate the general trends of acid concentrations in coffee, across all available scientific literature, including multiple coffee types and roast levels. A meta-analysis of 121 publications yielded 7,509 distinct acid concentrations, a wealth of data to serve as a necessary reference for future research pertaining to acid concentration in coffee. Analysis of the full data set reveals several trends. First, the data indicate that four specific OAs – citric, quinic, malic, and acetic – comprise more than 88% of the total mass fraction of OAs present in roasted arabica coffee. Notably, roasted robusta is much more acidic than arabica with 2 to 5 times as much total OAs, similar amounts of citric, quinic, and malic acid, but larger amounts of formic and acetic acid. As for CGAs, in both arabica and robusta 5-CQA is the major component, and higher roast levels tended to sharply decrease the concentration of all CGAs. The total amount of CGA present was more dependent on roast level than the type of coffee (arabica vs. robusta). Overall, this meta-analysis suggests that the increases in certain OAs with roast level, especially acetic acid, might play more of a role in the sensory profile of dark roast coffees than previously suspected.In particular, the acid concentration of cold brew coffee has been an area of great interest brought about by the rise in popularity of cold-brewed coffee beverages. Major coffee retailers purport that cold brew is smoother, sweeter, and more full-bodied than hot coffee; many also believe that cold brew is less acidic and more friendly to the gastro-intestinal tract. In preparation to investigate acid concentrations, we brewed the same coffee across a wide range of temperatures and were met with a stark observation: different brew temperatures resulted in different colored coffee. Thus, the second goal of this research was to explore how roast level and brew temperature affect the color of brewed coffee through a colorimetric analysis of brews prepared across three origins, three roast levels, and three brew temperatures. We find that roast level had the strongest impact on brew color, and that brew temperature had a significant impact on color for light and medium roasts, with less impact on dark roasts. Qualitatively, the cold brewed coffees tended to be redder, while the hot brewed coffees were blacker. These results suggest that the color of coffee brews might play some role in their perceived sensory qualities and that there is an opportunity to manipulate and brand brewed coffee color through judicious choices of roast level and brewing temperature.