Dihydroxyacetone is a three-carbon sugar and a precursor for a number of fine chemicals and pharmaceuticals such as methotrexate. DHA is used in the cosmetics industry for the produc­tion of suntans. Since DHA causes pigmentation of the skin, it is also used in the treatment of vitiligo, an autoimmune disease in which the melanocytes are destroyed and irregular white patches are formed on the skin (Fesq et al. 2001). Gluconobacter oxydans is used industrially for the production of DHA. Glycerol is converted to DHA by the membrane-bound enzyme GldA. At high concentrations, DHA is toxic to microbial cells, and cell viability decreases exponentially with time. Overexpressing GldA gene (sldAB) in G. oxydans, however, increased the concentration of DHA from 18-25 g/L to 30g/L and the inhibitory effect of DHA on cell viability was also reduced (Gatgens et al. 2007).

Propionic Acid

Propionic acid is an important antifungal agent used in industrial production of cellulose — based plastics, solvents, herbicides, perfumes, flavors, thermoplastics, and so on. Propionic acid is formed from succinate via the dicarboxylic acid pathway. Propionic acid is natively produced by Propionibacterium acidipropionici, Propionibacterium acnes. and Clostridium propionicum. These bacteria are capable of fermenting crude glycerol to propionic acid. P. acidipropionici can produce up to 42 g/L of propionic acid, with a yield of 0.844 mol/mol glycerol and a productivity of 0.36 g/L/h (Barbirato et al. 1997).

Citric Acid

Citric acid is used as a flavoring and preservative agent in foods and beverages, as a water softener in detergents, and as a wax and color remover in shampoos. It is generally produced by Aspergillus niger and Yarrowia lipolytica. Y. lipolytica produced 11 g/L of citric acid in a glucose medium (Papanikolaou et al. 2002). When grown in a glycerol medium, this micro­organism produced up to 35 g/L citrate with a yield of 0.42-0.44 g/g glycerol (Papanikolaou et al. 2002 ).