Large amounts of crude glycerol produced in the biodiesel industry can be used as a low-cost renewable feedstock to produce chemicals and fuels. Compared to sugars (sucrose, glucose, xylose, etc.), glycerol has a lower reducing level, which is of benefit to the production of reduced chemicals. In this work, glycerol as the sole carbon source in propionic acid fermentation by metabolically engineered Propionibacterium acidipropionici (ACK-Tet) was studied. It was found that the adapted ACK-Tet mutant could use glycerol for its growth and produced propionic acid at a high yield of 0.54-0.71 g/g, which was much higher than that from glucose (similar to 0.35 g/g). In addition, the production of acetic acid in glycerol fermentation was much less than that from glucose. Thus, glycerol fermentation produced a high purity propionic acid with a high propionic acid to acetic acid ratio of 22.4 (vs. similar to 5 for glucose fermentation), facilitating the recovery and purification of propionic acid from the fermentation broth. The highest propionic acid concentration obtained from glycerol fermentation was similar to 106 g/L, which was 2.5 times of the highest concentration (similar to 42 g/L) previously reported in the literature. (C) 2009 Elsevier Ltd. All rights reserved.