Different analytical approaches for the study of water features in green and roasted coffee beans

摘要

Water mobility and availability in coffee play an important role in the roasting and storage process, depending on the interactions between the aqueous phase and the biopolymeric matrix. The aim of this research was to deepen the understanding of these aspects in green and roasted coffee, by means of the joint application of traditional saturated salt slurry (DES) and innovative dynamic dew-point (DDI) methods for adsorption isotherm generation. In addition, on green and roasted samples at different hydration levels, the assessment of proton transverse relaxation time by TD-NMR and dielectric properties through waveguide spectroscopy in the spectral ranges 2-3, 5-6 GHz and 17-18 GHz have been performed. Green coffee exhibited above the critical a_w a different behavior if observed by means of DES or DDI, demonstrating that its hydration is strictly time dependent because of its heterogeneous structure; this phenomenon was not detectable for roasted coffee, that showed very similar results with the two different techniques. The measurement of T_2 by using TD-NMR permitted to study the mobility of different proton pools with a high level of details, allowing the quantitative assessment of exchange/interaction phenomena occurring along matrix re-hydration and the consequent increase of water proton mobility. At low frequencies, the described dielectric technique showed high potentiality for the non-destructive estimation of water content of green and roasted coffee. At higher frequencies, the obtained signal seems to be more related to water/solid matrix interactions and water activity.