Effect of Individual and Combined Addition of Salep, Tragacantin and Guar Gums on the Stabilisation of Iranian Doogh

Annual production of Doogh (a yoghurt drink) in Iran exceeds 120000 metric tons. Iran is probably a leading producer and consumer of this type of dairy-based product in the world. Separation of Doogh into 2 phases (serum separation) during storage is a major problem which affects its market share. The present study was undertaken to investigate the stabilizing effects of salep and tragantin gums (as local gums) and guar gum (as a commercial gum), added individually or in combination, to Doogh (containing 40% yoghurt). Various gums (salep, tragacantin and guar), individually (concentrations 0.1–0.5%) and in combination (at ratios 50:50, 20:80 and 80:20) over a range of 0.1 to 0.2%, were added to samples of Doogh (composed of 40% stirred yoghurt, 59.3% tap water, 0.7% NaCl), and their physical stability (serum separation percent) were monitored over a period of 30 days at various storage temperatures (5 and 25 C). In addition, some mechanical parameters, namely homogenization (up to 200 bars) and agitation speed (9000 and 11000 rpm for up to 2 min) of the samples, as well as their biphasic separation and rheological properties were determined. Sensory evaluation was also performed using semi-trained panelists and the 9-point hedonic scale procedure. Our findings revealed that the individual gums could significantly reduce phase separation during storage (p <0.01). Gum tragacantin was the most effective, resulting in full stability of Doogh at a concentration of about 0.3%. Generally speaking, increasing the concentration of gums caused changes in the rheological behavior of the Doogh samples. Samples containing salep, tragacantin and guar gums showed a pseudo-plastic flow behavior, whereas in their absence, the Dooghs behaved as a Newtonian fluid. Mechanical operations, particularly agitation speed, led to a considerable reduction in the apparent viscosity of Doogh and, consequently, a significant increase in serum biphasic separation (p <0.01). In the organoleptic tests, the Doogh samples containing tragacantin and tragacantin-guar combinations received the highest scores. It is probable that the stabilizing effects of the gums, being non-absorbent hydrocolloids, were due to their viscosity-raising and water-holding properties. Moreover, mechanical operations (stirring and homogenization) could have disrupted the colloidal structure, leading to a decrease in viscosity and an increase in the biphasic separation.