Viscoelastic Properties of Tarocco Orange Fruit

In this work, the rheometrical behaviour of Tarocco orange fruit was assessed via stress–relaxation testing under constant engineering strain (ε _z) in the range of 5–20% of the initial fruit height, each test being prolonged for 15 min and replicated 20 times. The time course of the experimental dimensionless relaxation modulus (G*) was typical of a viscoelastic solid and was reconstructed by considering two Maxwell elements in parallel to an elastic spring element. In a more restricted ε _z range (i.e. 5–10%), the dimensionless viscoelastic constants (A _i) were found to be approximately constant, the elastic component responsible for the equilibrium residual stress representing about 42% of the initial relaxation modulus. After any orange fruit had been submitted to stress–relaxation testing and decompressed for 5 min, it exhibited a practically negligible mass loss (0.04 ± 0.03%) and a permanent volume contraction rising from 1.7 ± 1.0% to 3.0 ± 0.9% as ε _z was increased from 5% to 10%. Thus, by tasting the orange fruit firmness at ε _z ≤5%, fruit response, being of the viscoelastic type, is expected to give rise to nearly irrelevant deformations.