Yogurt stew )Khoresh Mast( as one of the traditional and iconic dishes of Isfahan, Iran, is an artisanal combination of strained yogurt, meat, saffron, and sugar. It is distinguished by its delightful taste, vibrant color, and high nutritional value and has been nationally registered as an intangible cultural heritage. With efforts to combine science and tradition in industrial production, yogurt stew holds significant potential for commercialization. However, industrial production faces challenges such as pasteurization, grainy texture, syneresis, low viscosity, and insufficient stretchability that indicate the necessity of production optimization to enhance shelf life without compromising quality. In this study, sugar content was first selected using a linear regression based on the acidity of strained yogurt, resulting in balanced sweetness and uniform flavor. Then, thermal pasteurization was studied regarding Streptococcus thermophilus as the microbial indicator of pasteurization efficacy. A temperature of 70 °C for 40 minutes was determined to minimize microbial load (99.999%) and thermal damage and extend the product's shelf life to two months. To improve rheological properties, a synergistic combination of guar gum and salep (3:1) was selected for its significant effect on increasing apparent viscosity. These hydrocolloids also enhanced stretchability and mechanical resilience, as confirmed by the Texture Profile Analysis test. Effect of these hydrocolloids was statistically significant (p < 0.05) compared to other hydrocolloids, and their optimal usage level was set at 0.2%. Considering commercial considerations, the total solid content of strained yogurt was reduced to 20%. Response surface methodology using Box-Behnken design was employed to optimize the formulation. The effects of independent variables, including fat percentage, yogurt acidity, and the ratio of guar gum and salep, on stretchability, graininess, apparent viscosity, and syneresis were analyzed. Regression models were developed to predict responses, revealing significant effect of factors (p < 0.05). Increased acidity and reduced salep ratio enhanced stretchability, while raising acidity decreased graininess. In contrast, increasing fat content reduced syneresis, while decreasing acidity and raising fat content amplified apparent viscosity. Optimal conditions were determined using regression models and desirability functions (acidity of 125 °D, fat percentage of 2.93%, and salep ratio of 0.25). These conditions led to development of a product with a desirability score of 0.94, characterized by minimizing of graininess and syneresis, maximizing of stretchability and viscosity with desirable sensory attributes. The optimized model was validated, and the measured responses fell within the predicted range. Model accuracy was eva‎luated using the normalized mean squares error (NMSE), with the highest NMSE of 1.2 for the viscosity model, indicating acceptable overall accuracy. Optimization results, compared to pasteurized control samples, showed approximately 30% reduction in total mesophilic aerobic count, 40% improvement in stretchability, 87% reduction in graininess, 97% reduction in syneresis, 100% increase in shelf life, and 115% increase in apparent viscosity of Yogurt stew. By optimizing the production process, a product with superior quality and longer durability was produced, which has a high potential for commercialization on an industrial scale. Keywords: Guar gum; Optimization; Pasteurization; Response surface methodology; Salep; Yogurt stew (Khoresh Mast).

ميلاد فتحي , صبيحه سليمانيان زاد

هاجر شكرچي زاده , احمد ميره اي