Viscoelastic behaviors and microstructure of Sapindus mukorossi pericarp for different moisture contents

Abstract

The viscoelastic behavior of Sapindus mukorossi pericarp (the covering of the Indian soapberry fruit) was investigated. Samples were subjected to creep-recovery and oscillatory shear tests. The moisture content had a strong effect on the dynamic moduli, whereas the temperature had little effect. Both the storage and loss moduli had a weak dependency on the frequency. In general, the storage and loss moduli decreased and slowly converged with an increase in the moisture content. The storage and loss moduli decreased with an increasing temperature. The creep-recovery experiment and dynamic shear tests indicated that the pericarps with high moisture contents were more prone to permanent deformation. The dynamic shear tests further demonstrated that a low moisture content, temperature, and frequency resulted in the pericarps having a more solid behavior and undergoing elastic deformation. Images of the microstructure showed that cells in the raw pericarps were round, of similar size, and neatly arranged. In the treated pericarps, the cells gradually became smaller and finally disappeared. It was inferred that drying with hot air caused the loss of intracellular substances, which in turn strengthened the dynamic characteristics of the pericarps. The completely dried pericarps thus behaved as strong viscoelastic solid in contrast to the raw pericarps.