Effect of moisture and water activity on textural properties of raw and roasted coffee beans

The effect of moisture and water activity (A(w)) on the textural properties of raw and roasted (light, medium and dark) coffee beans was investigated. Water-coffee interactions were described by the sorption isotherm and the mechanical properties of the samples equilibrated in the 0-0.95 A(w) range were studied by uniaxial compression were studied. Roasted beans presented a lower strength and toughness than raw ones and, independently from roasting, resulting textural properties were highly dependent on A(w). At increasing hydration degree, an increase in strength and toughness was observed until a critical A(w) value, different for raw (0.75) and roasted (ca. 0.86) beans related to a water, was reached. This was plasticization effect. Above these critical A(w) values, a progressive softening of the bean matrix occurs. In roasted coffee beans, this plasticization determined the loss of the characteristic brittleness. The critical A(w) value, above which plasticization occurred, was determined by modeling the compressive modulus using the Fermi's distribution function.