Vibrations as a cause of texture defects during the acid-induced coagulation of milk - Fluid dynamic effects and their impact on physical properties of stirred yogurt

摘要

Vibrations during yogurt fermentation have been reported to disturb the gelation by inducing large, visible particles. Such vibrations are generated by the machinery used for the industrial manufacture and can cause structural excitation of the fermentation tanks. To study the effect of vibrations on yogurt structure, an experimental setup was constructed consisting of a cylindrical tank (height: 600 mm). The bottom of the tank was excited by a shaker, resulting in a vertical vibration propagation. Skim milk was vibrated during fermentation at 30 Hz and different amplitudes. Vibrated gels showed syneresis up to similar to 5%. Samples were taken at different height positions of the fermenter, processed into stirred yogurt, and analyzed. The number of large particles was primarily related to the set vibration amplitude, however, particle formation was more pronounced in the lower and upper area. Yogurts with a high particle number exhibited a reduced water-holding capacity and apparent viscosity. Moreover, the fluid dynamic effects of vibrations were investigated by particle image velocimetry. Vibrations led to uniform to-and-fro motions, whereas chaotic flow was observed at the surface. Finally, the impact of vibrations on the gelation mechanism is discussed.