Research subject: Scientific and applied methods in preserving the properties of food materials are of great importance and lead to the creation of a product with excellent quality. Therefore, choosing the right process for producing the product is one of the important parameters. Today, many studies are being conducted on increasing the shelf life of yogurt, one of which is the effect of acoustic treatment in reducing the population of bacteria and eliminating the contaminants created. The explosion of ultrasonic bubbles in the environment is the main mechanism of this process.
Research approach: In this study, the effect of high-intensity ultrasound waves with a frequency of 20 kHz for 5 minutes and heat treatment at 72 °C for 30 minutes was evaluated and compared with the control sample on 2.5% fat yogurt. All samples were stored in a refrigerator at 4 °C for 1, 7 and 14 days and their physicochemical, rheological and microbial properties were evaluated. The yogurt samples were examined for acidity, pH, water content, viscosity and bacterial count. The last item, namely microbial evaluation, was performed by counting yogurt starter bacteria (streptococcus thermophilus and lactobacillus bulgaricus) and counting mold and yeast.
Main results: The results indicate a decrease in pH and an increase in acidity in all samples during storage in the refrigerator, while the sample with ultrasound treatment had the least changes. In the case of waterlogging after fourteen days of storage in the refrigerator, the ultrasound-treated sample exhibited the most significant change (due to cavitation and protein structure breakdown), whereas the control sample showed the least changes. The apparent viscosity of the heated sample was the highest (327.89 cp) and the ultrasonic sample was the lowest (176.55 cp). The results of microbial tests indicated a significant decrease in the number of colonies in the heat-treated sample, and mold and yeast growth were not observed in any of the samples. The results of this research can be cited and used in the food industry.