本研究中探討嗜熱鏈球菌菌株MC 與ATCC 19258及保加利亞乳桿菌菌株448 與449 合成葉酸之能力。在此研究中於發酵所用牛乳中添加乳糖與氯化鈣以期提高這些菌株合成葉酸的量。由實驗結果發現，乳糖之添加的確可提高菌株所合成的葉酸，雖然對培養6 小時的菌株只提昇葉酸合成3 至7%，但對培養18 小時的菌株所合成之葉酸可提高12 至198%。而氯化鈣的添加卻導致葉酸合成之降低，只有嗜熱鏈球菌菌株MC 之葉酸合成提高。乳糖的添加同時使培養6 小時與18 小時之菌株其菌數增加，而氯化鈣之添加則對各菌株菌數的增減有不同之效果，但兩株保加利亞桿菌菌數皆增加。以這些菌株發酵之發酵乳探討其葉酸的量與菌數於4°C 下儲存之變化，結果發現，葉酸在第二週時降低9 至28%，菌數在此兩週保存期限內則維持穩定。本研究中同時以細胞株Intestine 407探討葉酸於氧化劑過氧化氫之細胞毒性下保護細胞之效果，實驗結果發現，葉酸可抑制氧化劑過氧化氫之細胞毒性，且當葉酸濃度提高時細胞株Intestine 407 展現較高之存活率。

　　　　　The folate synthesizing ability of yogurt bacteria, including Streptococcusthermophilus MC and ATCC 19258 and Lactobacillus delbrueckii ssp. bulgaricus 448and 449, were evaluated. The major effort of this study focused on increasing thefolate contents of yogurt bacteria fermented milk by adding lactose or calciumchloride. The addition of 2% lactose in reconstituted non-fat dry milk increased folatesynthesis. Although folates increased only about 3 to 7% for yogurt cultures incubatedfor 6 hr, folates increased 12 to 198% for yogurt bacteria grown for 18 hr. The folatecontent decreased for reconstituted milk fermented with S. thermophilus MC or L.bulgaricus 448 or 449 when 0.02% calcium chloride was added. The folate levelincreased only for reconstituted milk fermented with S. thermophilus ATCC 19258when calcium chloride was added. The addition of 2% lactose increased the cellcounts for yogurt bacteria grown for 6 and 18 hr. On the other hand, the results of thecell counts from the addition of 0.02% calcium chloride were not consistent. However,the cell counts increased for both L. bulgaricus strains. The folate levels and cellcounts during refrigerated storage at 4℃ were determined for reconstituted non-fatdry milk fermented with these bacteria. Folate levels decreased about 9 to 28% at theend of the second week. The viable yogurt bacterial counts remained stable for the 2-week shelf life for all four strains tested in this study. To determine the effect of folateon protecting cells from the cytotoxicity of oxidant H2O2, Intestine 407 was used inthis study for evaluation. The cell viability of Intestine 407 was increased due to theinhibition of oxidant H2O2 cytotoxicity by folate and Intestine 407 cells demonstratedhigher viability when treated with higher concentration of folate.