Abstract: The aim of this work was to study the effect of milk protein fortification on the sensory quality, viscosity and water holding capacity (WHC) of stirred yogurt, where reconstituted milk was fortified with skim milk powder (SMP) or whey protein concentrate (WPC-80) to different protein contents of 2.7%, 3.1%, 3.5% and 3.9%, respectively. The results showed that the quality, viscosity and WHC of stirred yogurt were significantly improved after protein fortification with SMP or WPC-80. Stirred yogurts fortified with different levels of SMP showed an obvious difference in texture, a slight difference in flavor, and no difference in color. Identical results were found for fortification with different levels of WPC-80. In addition, the viscosity and WHC of yogurt were both dramatically elevated as the fortification level increased. Protein fortification with SMP at 2.7% provided the best sensory quality of yogurt, and the amount of WPC-80 addition for the best sensory quality of yogurt was 3.5%. WPC-80 was more beneficial to the sensory quality and WHC of yogurt than SMP at the same fortification levels, while SMP fortification provided higher viscosity. Based on a comparison between perceived and instrumentally measured viscosity values, stirred yoghurt with higher viscosity did not necessarily have better quality, and the best viscosity was between 537 mPa·s and 712 mPa·s. It can be concluded from this study that the best protein fortification protocol for stirred yoghurt is adding WPC-80 at 3.5%.

Key words: yogurt, protein fortification, sensory quality, viscosity, water holding capacity