Vibrations and ultrasound in fermented milk processing – Adverse effects on yogurt structure, mechanisms, and beneficial applications

Considering the manufacture of yogurt and other fermented milk products, the gelation during fermentation is a mechanically sensitive process. This thesis primarily evaluates the impact of mechanical vibrations including ultrasound (US) during fermentation as a potential cause of texture defects. US is further explored to be used for beneficial structure modifications.

Sonication during yogurt fermentation (US water bath) resulted in the additional formation of large millimeter-scale particles. This sonication-induced particle formation could be counteracted when an exopolysaccharide-producing starter culture was used. Vibration measurements on industrial yogurt production lines showed that pumps and specific operations (e.g., sterilization, CIP) generate vibrations that cause structural excitation of the fermentation tanks. A pilot-scale experimental setup was developed to study the effects of vibrations during fermentation systematically by using a vibration exciter. Vibration treatments (25–1005 Hz) at pH 5.5–5.2 increased syneresis and firmness of the set milk gels as well as the number of large particles of the resultant stirred yogurts. The particle number was related to the vibration amplitude. Yogurts with a high particle number exhibited a reduced water-holding capacity and apparent viscosity. The fluid dynamic effects of vibrations were investigated by particle image velocimetry in order to explain the impact of vibrations on a microstructural level. Manufacturers should consider vibrations as a further cause of graininess and other texture defects. Apart from these adverse effects, applying power US (probe system) during or after the fermentation of milk concentrate (10% protein) resulted in a more homogeneous product with a considerably reduced viscosity and yield stress. Power US is a promising technology to specifically modify the microstructure, particularly in order to modulate the rheological properties of high-protein fermented milk products.