Influence of three types of Iranian gum tragacanths on rheological properties and stabilization of fat-free Doogh, an Iranian yoghurt drink

Doogh is an Iranian traditional beverage based on yoghurt. In this product, serum separation occurs during storage due to its low pH. In recent years, studies have been conducted on the stabilization of doogh using hydrocolloids. The objective of this study was to compare the effects of different species of gum tragacanth (Astragalus gossypinus, Astragalus rahensis, and Astragalus fluccosus) on the stabilization and rheological properties of this typical Iranian dairy drink during storage. Gum solutions were added to Doogh samples prepared from yoghurt made with skim milk. Phase separation was measured using graduated cylinders during storage (30 days), and particle size distribution at ambient temperature was determined by laser diffractometery. Furthermore, steady and unsteady rheological tests were carried out using a controlled shear rate rheometer (CSR) at 3°C. Finally, sensory analysis was performed with 30 panelists familiar with the product, using a 5-point hedonic scale. In the samples containing 0.3% (w/w) A. gossypinus no phase separation occurred during the 30-day period. The data on particle size distribution indicated that addition of gum tragacanth resulted in a reduction in the size of aggregated particles, specially in the sample containing A. gossypinus. The apparent viscosity in this sample was higher than in the other samples. Also, its sensitivity to frequency (range = 0.01-10 Hz) was much less, which is very important from an industrial point of view. In addition, based on the sensory evaluation data, the sample containing A. gossypinus was the most acceptable product. The higher content of uronic acid and insoluble part of A. gossypinus can be the most probable reasons for the stabilized dispersion caused by A. gossypinus as compared to that caused by the other two species at the same concentration (0.3% w/w) in doogh.