Process Performance of Continuous Inoculation and Acidification of Milk with Immobilized Lactic Acid Bacteria

Abstract

Several parameters of a milk prefermentation process by immobilized lactic acid bacteria in a stirred bioreactor were investigated. This process was designed to acidify and inoculate skim milk continuously. Four strains of mesophilic lactic acid bacteria were entrapped separately in κ-carrageenan and locust bean gum gel beads. We studied the effect of four process parameters pH (5.8, 6.0, and 6.2), temperature (22, 26, and 30°C), stirring speed (90, 120, 150, and 180rpm), and ratio of gel to reactor volume (20, 25, 30, and 40%) on dilution rate, lactic acid productivity, fermentation time, and the composition of the prefermented milk and coagulum obtained after further fermentation to pH 4.7.Dilution rate increased with temperature, pH, gel ratio, and low stirring speed. Under the conditions used, the maximum reactor dilution rate was 30.6/h at pH 6.2, 30°C, 120rpm, and a gel ratio of 25%. The preacidified milk was inoculated with the four strains of lactic acid bacteria at 2 × 108 to 3 × 108 cfu/ml. Fermentation time was reduced by more than 50% with this prefermentation step, and the final coagulum contained about 109 cfu/ml. Citrate was entirely consumed during fermentation, and acetoin and diacetyl concentrations in the coagulum reached 2.8 and .014mM.