Lactic acid produced by LAB is a useful compound for preserving food because it maintains the acidic conditions of the fermented products, and is antagonistic to food spoilage and food poisoning bacteria. Most species of LAB promotes the decarboxylation of L-malate to lactate and forms CO2 if an energy source such as
glucose is present. A proton is taken up in the reaction, which prevents the decrease of pH in the growth medium caused by lactic acid production from glucose fermentation (Daeschel, 1988). Seven strains of LAB, which showed excellent inhibition zone diameters larger than 30 mm on the growth of S. aureus (Table 1), were chosen for study in lactic acid production and reduction of pH. All of them were cultured in MRS broth including D-glucose (dextrose) 20 mg/ml which was used as the carbon source. The strain LD219 was the best for lactic acid production and pH reducting ability at 37 C for 48 h (Table 2). The homofermentative LAB converted more than 90% of glucose to lactate via the EmdemeMyerhoffeParnas (EMP) pathway under anaerobic conditions. In this pathway, two molecules of pyruvate and NADH are generated from one molecule of glucose. The pyruvate generated acts as the endogenous final electron acceptor for the oxidation of NADH catalyzed by lactate dehydrogenase (LDH).