RESUMEN
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