New studies on basil (Ocimum bacilicum L.) seed gum: Part III - Steady and dynamic shear rheology

Basil seed gum (BSG), as an anionic heteropolysaccharide, consists of two major fractions according to molecular weight (M-W); PER-BSG and SUPER-BSG which have the highest and lowest M-W. respectively. In this paper, the influence of Mw distribution on rheological properties of BSG fractions was investigated. Purified BSG produced a high viscosity solution with yield stress, which was a function of Mw. The PERBSG according to higher Mw exhibited higher viscosity and stronger shear-thinning behaviour than SUPER-BSG fraction. The thixotropy extent of BSG gel was significantly increased by the presence of both fraction interactions. The dynamic rheological data showed that all BSG fractions displayed weak gel-like behaviour and PER-BSG plays a pivotal role in the development of elastic properties of BSG as it had the lowest magnitude of tan delta (0.22) and highest storage modulus value (225.98 Pa). BSG, PER-BSG and PFBSG were deviated from Cox-Merz rule, where complex viscosity was higher than steady shear viscosity. But, SUPER-SSG followed this rule, which indicates weak ability of this fraction to form a strong network. Further purification of BSG achieved by treating the gum solution with protease was resulted in increment of storage and loss moduli. Difference in the steady and dynamic features of BSG gels produced by different Mw fractions confirmed the outstanding impact of the chain length of fractions on the degree of cross linking; high Mw BSG exhibited strong tendency to polymer entanglement formation which enhanced elastic and solid behaviour in comparison with lower Mw BSG fraction. (C) 2015 Elsevier Ltd. All rights reserved.