The acidification and reducing capacities of yoghurt bacteria were evaluated in different plant extract-enriched milk samples. The milk samples enriched with thyme and grape seed extracts exhibited the highest values of acidification capacity for Lactobacillus delbrueckii subsp. bulgaricus (LB) (0.0065 pH unit/min) and Streptococcus thermophilus (ST) (0.0068 pH unit/min). The highest values of reducing capacity were observed in thyme (-0.98 mV/min), grape (-1.92 mV/min) and green tea (-0.75 mV/min)-enriched samples for LB, ST and mixed culture of LB + ST, respectively. The fortification of yoghurt with plant extracts modified the acidification and reducing activities of starters, thus changing the fermentation time and quality attributes of the product.