Variations in flexural and compressive fracture behavior of a brittle cellular food (dried bread) in response to moisture sorption

Mechanical properties of dried bread (a cellular baked product) equilibrate d at different water activities (A(w)) were measured using three-point bend ing and uniaxial compression to compare their responses to moisture sorptio n. The glass-to-rubber transition was clearly demarcated in all the mechani cal property-A(w) curves, although there were differences in critical A(w) (0.32-0.56) at which the dramatic changes in mechanical properties occurred . The role of water appeared to be strictly that of a plasticizer where fle xural mechanical parameters were concerned, leading to reduced modulus and fracture stress but increased fracture strain with increasing A(w). Uniaxia l compression tests revealed moisture-induced mechanical antiplasticization effects on the material in the glassy state which resulted in maximum comp ressive fracture stress but minimum fracture strain over the A(w) range fro m 0-0.56. Compressive modulus apparently was not affected much by moisture sorption up to an A(w) of 0.43, above which it decreased sharply.