Effects of ultrasound on milk homogenization and fermentation with yogurt starter

Yogurt production was selected to study the effects of ultrasound on milk homogenization and fermentation. A microscope was used to check the size distribution of fat globules to evaluate ultrasound homogenization. The pH change during fermentation and the water holding capacity, viscosity and syneresis for finished yogurt were evaluated. It was found that ultrasound has a very good homogenization effect at high amplitude levels compared with conventional homogenization. Longer exposure times improved the ultrasound homogenization effect. If enough exposure time and amplitude level are provided, extremely small fat globules can be produced. The fermentation results showed that the method of sonicating bacteria had a significant effect on the pH change. Sonication after inoculation could reduce the total fermentation time by 0.5 hours. Ultrasound also significantly affected the water holding capacity, viscosity and syneresis. In general, increasing the ultrasound amplitude level improved the water holding capacity, and viscosity and reduced syneresis. However, sonication after inoculation did not decrease the syneresis effect. This may have been due to the lower pH of those samples.