Comparison of some chemical and rheological properties of Iranian gum tragacanth exudate from two Astragalus species (A. floccosus and A. rahensis)

Many studies have been carried out on structural and functional properties of gums from different sources. However, there is not much information available on the properties of gum exudates from different species of Astragalus. The objective of this study was to investigate and compare the chemical composition and rheological properties of two species of Astragalus under different conditions with regard to concentration, temperature and ionic strength.

Moisture, ash and protein contents of the samples were determined according to the Iranian official standards, while determinations of minerals were done using ICP-AES. All rheological measurements were made using Physica MCR 301 rheometer (Anton Paar). In addition, the flow behavior index (n) and consistency index (m) as parameters of power law model were computed, and the temperature dependency of consistency coefficient was assessed by fitting the Arrhenius model.

The moisture and ash contents of A. floccosus were higher, and its protein content was lower, than those of A. rahensis. At all concentrations and temperatures, the results of apparent viscosity versus shear rate for each two species were found to exhibit a clearly shear thinning behavior. At all concentrations and temperatures and shear rates, A. floccosus had a higher apparent viscosity. At similar concentrations and temperatures, the magnitudes of parameters (m) and (n) for the A. floccosus was, respectively, higher and lower than that of the A. rahensis.

In different food processing operations (stabilizing of soft drinks, using fat replacers, microencapsulation, and appropriate mouthfeel), the use and application of gums is based on the exact knowledge of their functional properties. In this study, it has been shown that different species of gum tragacanth have different characteristics. Therefore, in the processing of different foods (dairy, cereal, soft drinks, sauces, etc.), appropriate species of gum tragacanth should be selected and used.