The Effect on Taste
Not all studies that have investigated the staling of coffee have included how these chemical reactions influence taste. However, when taste is included it is apparent how immediate staling is. Just one week after roasting, tasters in one study preferred soluble coffee that had been stored in a can with 0% oxygen over coffee stored under 2% oxygen (Harris et al. 1974). Ross and others (2006) found that sensory panelists preferred fresh coffee compared with two-week stored coffee, finding the coffee bitter, but also preferred two-week stored coffee over one-week stored coffee. A different research group, Cardelli and Labuza (2001), found that sensory testers detected a loss of quality in coffee with increases in oxygen partial pressure, water activity, and temperature, confirming that these environmental influences made their way to the cup. They deemed that oxygen had the most critical role, with a close to twenty-fold staling acceleration difference between 0% oxygen and average sea level oxygen concentrations.
Many studies on staling include a sensory evaluation of coffee aroma, as opposed to taste. Aroma testers in Steinhart and Holscher’s (1991) study noted that coffee one-week off-roast were “distinctly less odor-intensive” and showed “less aroma freshness.” The researchers determined that this was due to quickly dissipating “low boiling” components such as sulfur compounds, Strecker-aldehydes, and alpha-dicarbonyls. Sanz and others (2001) found that eight identified volatile compounds were positively correlated with the sensory rating of aroma freshness and that the greatest rate of freshness loss occurred in the first month of coffee storage (see Figure 2, below).