Efficiency and mechanism of action of hydrocolloids in preventing serum separation in Doogh

Doogh is a kind of Iranian fermented dairy-based drink produced by adding water and salt to yoghurt. The major problem of this product is serum separation during storage, because of its low pH and aggregation of caseins. In the present study, the efficiency of some hydrocolloids in preventing serum separation as well as the stabilization mechanism of gum tragacanth–a local gum–were investigated. The effects of high-methoxyl pectin, tragacanth, and locust bean gum at different concentrations, individually and in combined forms, tragacanthin, and soybean soluble polysaccharides type M, on the stability of sour Doogh were investigated. In addition, the influence of gum tragacanth and locust bean gum on the stability of prebiotic Doogh (containing inulin) was also studied. In each case the duration of the study was 30 days. In order to elucidate the stabilization mechanism of gum tragacanth, the rheological properties, zeta potential, and microstructural characteristics of the stabilized Doogh samples were measured. According to our findings, tragacanthin, tragacanth and locust bean gum could inhibit serum separation for 30 days at a concentration of 0.1, 0.2 and 0.3%, respectively, while type M soybean soluble polysaccharides caused the stabilization at a concentration of 0.6% only for 6 days. Furthermore, combinations of gum tragacanth and locust bean gum were also effective in fully stabilizing Doogh (at a concentration of 0.15% and ratios of 80:20 and 50:50). Inulin was not effective in stabilizing the sour Doogh samples. However, tragacanth and locust bean gums inhibited serum separation in prebiotic Doogh. Based on our findings, the most suitable rheological model for the control samples and the samples containing hydrocolloids were the Newtonian and Power law, respectively. Based on the results of this study, tragacanthin (the water-soluble part of gum tragacanth) seems to have a major role in stabilizing Doogh, probably by being adsorbed onto caseins (via electrostatic and steric repulsion). On the other hand, bassorin (the water-insoluble part of gum tragacanth) may help stabilization by increasing viscosity. Therefore, gum tragacanth could be strongly recommended to be used as an adsorbing hydrocolloid in sour Doogh to inhibit serum separation.