Modeling the effect of water activity and storage temperature on chemical stability of coffee brews

This work was addressed to study the chemical stability of coffee brew derivatives as a function of water activity (a(w)) and storage temperature. To this purpose, coffee brew was freeze-dried, equilibrated at increasing a(w) values, and stored for up to 10 months at different temperatures from -30 to 60 degrees C. The chemical stability of the samples was assessed by measuring H3O+ formation during storage. Independently of storage temperature, the rate of H3O+ formation was considerably low only when a(w) was reduced below 0.5 (94% w/w). Beyond this critical boundary, the rate increased, reaching a maximum value at ca. 0.8 a(w) (78% w/w). Further hydration up to the a(w) of the freshly prepared beverage significantly increased chemical stability. It was suggested that mechanisms other than lactones' hydrolysis, probably related to nonenzymatic browning pathways, could contribute to the observed increase in acidity during coffee staling. The temperature dependence of H3O+ formation was well-described by the Arrhenius equation in the entire a(w) range considered. However, a(w) affected the apparent activation energy and frequency factor. These effects were described by simple equations that were used to set up a modified Arrhenius equation. This model was validated by comparing experimental values, not used to generate the model, with those estimated by the model itself. The model allowed efficient prediction of the chemical stability of coffee derivatives on the basis of only the a(w) value and storage temperature.