In this experiment, citrus fiber, beet fiber and agar were used to develop a low-sugar, high-fiber room-temperature fermented yogurt, and the texture and sensory quality of yogurt samples made with their combinations in different proportions were evaluated. Lactase and Sweety T-1, a mixed commercial starter culture of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus were used to completely decompose lactose in milk to produce glucose with higher sweetness, thereby improving the sweetness of low-sugar yogurt. The results showed that adding 3% citrus fiber, 0.2% agar and 1% beet fiber imparted good stability and high sensory score to low-sugar, high-fiber room-temperature fermented yogurt. Raw milk was inoculated 200 U/L Sweety T-1 with the addition of 1 600 BLU/L lactase and without the addition of any sugar or sweetener and was fermented at 42 ℃. After 12 h, lactose was completely decomposed, producing 2.5% glucose.

The probiotic Lactobacillus plantarum LIP-1 was cultured in modified MRS medium at high cell density to investigate the effect of changes in medium components (the types and amounts of nitrogen source, carbon source and buffer salt, total amount of carbon and nitrogen, and carbon/nitrogen ratio) on the viable cell count and survival rate after freezedrying of this strain. The optimal medium was found to be composed of carbon source (sucrose) 33.35 g/L, nitrogen source (yeast powder, soy peptone and casein peptone, 3:3:5, m/m) 10.80 g/L, and buffer salt (sodium acetate, potassium dihydrogen phosphate and sodium citrate, 2.5:1:1, m/m) 14.85 g/L (namely 0.13 mol/L). The viable cell count of strain LIP-1 cultured in the optimized medium after freeze-drying was 9.767 (lg(CFU/mL)), which was distinctly higher than that cultured in MRS medium. The modified medium allowed high-cell-density fermentation of Lactobacillus plantarum LIP-1 while maintaining its viability during freeze-drying.