Kinetics of Physicochemical Properties of Industrial Kashk Produced by Hot Air, solar, Oven, and Freeze Dryers

Background and objectives: Kashk is a milk product that has been commonly prepared in most parts of the Middle East and especially in various parts of Iran since ancient times. Kashk is usually prepared in two forms: liquid or paste and dry. The basis of dry Kashk is milk, which is obtained by boiling, concentrating and drying yogurt, buttermilk and fermented buttermilk. The drying method has a great impact on the sensory and appearance as well as microbial properties of the product. For this purpose, Kashk was studied using different methods of oven, hot air, freezing and solar drying and its physicochemical, sensory and kinetic drying properties.
Materials and Methods: In this study, after producing Kashk from pasteurized milk, four methods were used to dry Kashk: solar drying, hot air tunnel, oven, and the finally freeze-drying method. In order to investigate the effect of drying method on physicochemical properties of Kashk, factors such as solubility, acidity, moisture, fat, color and drying kinetics were investigated. To investigate the drying kinetics, moisture fraction (MR) was selected as a dimensionless factor for process analysis and appropriate fit was selected using empirical equations (Table 2-2) and MATLAB software. The experiments were performed in 3 replicates. Statistical analysis was performed using SAS 9.1 software and LSD test at a significance level of 5%.
Results: During drying, acidity increased compared to the original sample, and the highest acidity with a value of 0.199 g/g of dry matter was related to the solar method and the lowest with a value of 0.123 g/g of dry matter was related to the freezing method. Also, in the oven and hot air methods, the type of method and temperature did not have a significant effect on the increase in acidity. The results show that the type of drying method is effective on the solubility of the whey, and among the different drying methods, the freezing method had the highest solubility with a value of 13.33. On the other hand, in the oven and hot-air drying methods, solubility also increases with increasing temperature. The L* index of the freeze-dried Kashk samples increased compared to the original sample, and in the oven and hot air methods, the L* index increased slightly with increasing drying temperature. The freeze-dried Kashk had the lowest a* and b* values, and in the drying methods using heat (oven and hot air), these indices increased with increasing temperature. The results of the changes in the whiteness index showed that the whiteness of the oven, hot air and solar dried samples decreased compared to the original sample, while it increased in the freeze-dried sample. In the sensory evaluation, the solar dried sample obtained the highest sensory score for odor, texture, mouthfeel, and overall acceptance. With increasing drying temperature in the oven and hot air methods, the sensory score of the samples decreased, so that the samples dried in the oven at 70 ° C obtained the lowest sensory score. In the study of experimental models for drying, the Medley model with the highest correlation coefficient and the lowest root mean square error for the oven, hot air and solar methods and the logarithmic model for the freeze dryer showed the best fit.
Conclusion: According to the results, the freeze-drying method produces better samples in terms of most physicochemical properties, but the investment cost for this method is very high. On the other hand, the samples dried by the solar method obtained the best sensory score. Therefore, using solar-drying method is a suitable method for drying Kashk.