Background and objective: Lactic acid (2-hydroxypropionic acid, CH₃CHOHCOOH) is a naturally occurring, chiral, alpha-hydroxy acid and has L(+) and D(-) optical isomers. It is used in food processing (as an acidulant and preservative), medicine, agriculture, and leather and textile industries. Due to its chiral properties, L-lactic acid is used in the pharmaceutical industry as an intermediate for the synthesis of chiral drugs. Polymers of lactic acid are biodegradable and have found use in manufacturing medical equipment. Although several strains of microorganisms are capable of producing lactic acid, some strains of Lactobacilli are generally recognized as safe (GRAS) in this regard.  In the present study five lactobacillus strains were studied in order to determine the best L(+)lactic acid-producer strain.
Materials and Methods: The lactobacillus strains were obtained from the Persian Type Culture Collection (PTCC), Iranian Scientific and Industrial Research Organization. They were cultured, under sterile conditions, on the slants of de Man, Rogosa and Sharp (MRS) agar, followed by inoculation on MRS broth medium as a preculture. Then 2.5-ml samples of the preculture containing 10⁸ cells/ml were inoculated in 250-ml Erlenmeyer flasks containing 50 ml of a production medium. Fermentation was carried out at 30^oC and 180 rpm on a rotary shaker incubator, and glucose consumption, lactic acid production, and growth of the strains were measured in triplicates every 8 hours.
Results: L. casei subspp. casei produced 95 g/l calcium lactate.  The yield and productivity of this strain on optimized media containing 100 g/l glucose were 95% and 0.98 g/lh, respectively. The other high-lactate producer was L. rhamnosus which produced 81.40 g/l calcium lactate, the yield and productivity being 63% and 0.85 g/lh, respectively.
Conclusion: L. casei subspp. casei PTCC: 1608 and L. rhamnosus PTCC:1637 are the best L(+) lactic acid-producers. They are, thus, the best bacterial strain candidates for pilot plant studies of lactic acid production.