Lactic acid bacteria are able to produce certain vitamins, for in yogurt a higher concentration of folic acid is found than in milk (74). S. thermophilus is known to produce folic acid during growth in milk (75,76). The amount of folic acid found in cows' milk ranges from 20 to 60 Ag L_1, whereas its concentration in yogurt may be increased depending on the strains used for the fermentation and on the storage conditions to values above 200 Ag L_1. This level appears also to be dependent on the strain of Lb. bulgaricus used, because the latter organism has been shown to use and to degrade folic acid during its growth (77). It is therefore of utmost importance to select the optimal combination of S. thermophilus and Lb. bulgaricus strains leading to organoleptically acceptable yogurt
with concomitantly an increased folic acid concentration. The pools of available strains will undoubtedly harbor such strains.
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