Abstract: The objective of this work was to systematically compare lactose-free yoghurt and plain yoghurt for differences in acidity, viscosity, total viable count (TVC), color, taste, and odor. The yogurts were prepared at 37 or 42 ℃. The trends of acidity, viscosity and TVC during the fermentation and storage period were detected. An electronic nose, an electronic tongue and an electronic eye were used for testing the odor, taste and color, respectively. The results showed that the acidity and viscosity of lactose-free yogurt increased faster than those of plain yogurt during the fermentation period. At the end of fermentation at 37 or 42 ℃, the quantity of lactic acid bacteria in lactose-free yogurt was 3.50 and 2.30 times as high as that in plain yogurt, respectively. The dominant color of yogurt fermented at 37 ℃ was number 4094 compared to number 4095 for yogurt fermented at 42 ℃. After storage for 21 days, the main color number ratio significantly changed (P < 0.05). Comparison between lactose-free yogurt and plain yogurt fermented at 37 ℃ after storage for 1 day showed a significant difference in umami, sweetness, and bitterness (P < 0.05); and in sourness, sweetness, saltiness, umami, and bitterness after storage for 21 days (P < 0.05). Comparison between the yogurts fermented at 42 ℃ after storage for 1 day exhibited a significant difference in sourness, sweetness, saltiness and bitterness (P < 0.05); and in sourness, sweetness, saltiness, umami and bitterness after storage for 21 days (P < 0.05). Furthermore, there was a significant difference between the yogurts fermented at 37 ℃ in the contents of odor components like 2,3-butanedione, 3-hydroxy-2-butanone, ethyl alcohol, and 2-butanone (P < 0.05). A significant difference was found between the yogurts fermented at 42 ℃ in the contents of ethyl alcohol and 2-butanone (P < 0.05). The four yogurts significantly differed in odor (P < 0.05). Therefore, samples of each yogurt fermented at different temperatures were significantly different from each other, and so were lactose-free yogurt and plain yogurt fermented at the same temperature.

Key words: lactose-free yogurt, plain yogurt, sensory quality, comparison of differences