In this study, the effect of different amounts of skim milk powder (SMP) (5 to 11.5%), milk protein concentrate (MPC) (1 to 5%) and soy flour (0 to 5%) on acidity, pH, viscosity and sensory characteristics (flavor, color and overall acceptance of concentrated yogurt were investigated using mixture design. The results of acidity and pH showed that increasing the MPC and SMP decreased the acidity and increased the pH of the sample significantly, while increasing soy flour increased the acidity and decreased the pH of the samples. The results of viscosity showed that only the linear effect of SMP and the MPC on the viscosity of samples was significant and increasing of SMP and MPC increased the viscosity of samples but soy flour had non-significant effect on the viscosity of the samples. The results of sensory score also showed that with increasing SMP and MPC, flavor, color and overall acceptance of samples increased significantly, but increasing soy flour decreased the sensory score. The results of variable importance in projection in PLS regression also showed that SMP had the highest positive significance and soy flour had the highest negative significance in terms of pH, viscosity, flavor score, color score and overall acceptance.  The optimum formulation was obtained using numerical optimization algorithm in
in Design Expert software were SMP 9.8%, MPC 1.72% and soy flour 1%.

Keywords: Concentrated yogurt, Milk protein concentrate, Mixture design, Skim milk powder, Soy flour

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