The effect of ultrasound treatment on the viability of L. bulgaricus(a), S. thermophilus (b) and L. plantarum (c) in stirred yoghurt during cold storage. (P-Y-: probiotic + yoghurt starter cultures, Y-: yoghurt starters, P + Y-: sonicated probiotic + yoghurt starter cultures, Y+: sonicated yoghurt starters, P-Y+: probiotic + sonicated yoghurt starter cultures, P + Y+: sonicated probiotic + sonicated yoghurt starter cultures).