Strain and strain rate dependence of gellan, agar and agar-gellan gels as model systems

Gellan, agar and their combination gels (1:1, 2 g/100 g) as model systems were subjected to strains up to 0.8 while varying the crosshead speed between 0.01 and 10 mm s−1 to determine different textural characteristics (Young's modulus, rigidity constant, degree of concavity and apparent biaxial elongational viscosity, ηbe) and fracture characteristics. The compression curves usually consisted of 6 zones, and the gels were sensitive to strain rate and the extent of applied strain. Agar gel showed higher fracture force and energy compared to gellan and agar-gellan samples. However, the fracture strain for gellan was highest (44.6-70.7%) followed by agar (22.8-40.3%) and agar-gellan (24.3-37.3%) gels indicating more brittleness in agar/agar-gellan gels but toughness in gellan samples. The degree of concavity of gellan gel was less than 1 indicating strain-softening characteristics; the agar gel exhibited strain-hardening phenomenon as it was more than 1. However, marginal strain-softening behaviour was observed for gellan-agar combination gel (0.856 ⩽ n ⩽ 0.999). A power law type model linked ηbe and the biaxial extensional rate at fracture. Gellan gels are suitable to prepare chewable fabricated juicy gels, while brittle products resulted from agar/agar-gellan. It is proposed that ηbe at fracture is a good index for characterising gels in relation to product development and compression-spreading.