Evaluation of biofilm reactor solid support for mixed-culture lactic acid production

Abstract

A combination of lactobacilli and biofilm-forming bacteria were evaluated in continuous fermentations for lactic acid production using various supports. Twelve different bacteria, including species of Bacillus, Pseudomonas, Streptomyces, Thermoactinomyces, and Thermomonospora were tested for biofilm-forming capabilities. Solid supports that were evaluated in either batch or continuous fermentations were pea gravels, 3M-macrolite ceramic spheres, and polypropylene mixed with 25% of various agricultural materials (e.g. corn starch, oat hulls) and extruded to form chips (pp-composite). Biofilm formation was evaluated by the extent of clumping of solid supports, weight gain and (in some instances) Gram stains of the supports after drying overnight at 70° C. The supports consistently producing the best biofilm were pp-composite chips followed by 3M-Macrolite spheres then by pea gravels. The best biofilm formation was observed with P. fragi (ATCC 4973), S. viridosporus T7A (ATCC 39115), and Thermoactinomyces vulgaris (NRRL B-5790), grown optimally at 25, 37, and 45° C, respectively, on various pp-composite chips. Lactic acid bacteria used in the fermentations were Lactobacillus amylophilus (NRRL B-4437), L. casei (ATCC 11443), and L. delbrueckii mutant DP3; these grow optimally at 25, 37 and 45° C, respectively. Lactic acid and biofilm bacteria with compatible temperature optima were inoculated into 50-ml reactors (void volume 25 ml) containing sterile pp-composite supports. Lactic acid production and glucose consumption were determined by HPLC at various flow rates from 0.06 to 1.92 ml/min. Generally, mixed-culture biofilm reactors produced higher levels of lactic acid than lactic acid bacteria alone. S. viridosporus T7A and L. casei on pp-composite chips were the best combination of those tested, and produced 13.0 g/l lactic acid in the reactors without pH control. L. casei produced 10.3 g/l lactic acid under similar conditions.