Abstract: The process for production of Sphauericepus gracilis yogurt by fermenting fresh cow milk supplemented with Sphauericepus gracilis pulp was optimized by response surface methodology based on one-factor-at-a-time experiments. The overall sensory score of Sphauericepus gracilis yogurt was studied with respect to four process parameters including fermentation time (x1), Sphauericepus gracilis pulp addition (x2), sucrose addition (x3) and inoculum size (x4). As a result, a regression model equation was fitted as follows: Y = 89.13333－2.39688x12－1.62188x22－1.62188x32－1.99688x42－2.99375x1x3－1.06875x1x4－1.25625x3x4. The optimum fermentation conditions for producing Sphauericepus gracilis yogurt with the highest sensory quality were fermentation time of 3.5 h, Sphauericepus gracilis pulp addition of 18%, sucrose addition of 6% and inoculum size of 4%. In terms of the importance in affecting the overall sensory score of Sphauericepus gracilis yogurt, the four factors ranked in the following order: Sphauericepus gracilis pulp addition, fermentation time, inoculum size and sucrose addition. The Sphauericepus gracilis yogurt obtained under the optimized fermentation conditions exhibited exquisite mouth feeling, homogeneous texture, unique flavor and elegant Sphauericepus gracilis aroma. The optimized process can provide a simple and feasible approach for production of Sphauericepus gracilis yogurt.

Key words: response surface methodology, Sphauericepus gracilis yogurt, production process, optimization