PROCESS PERFORMANCE OF CONTINUOUS INOCULATION AND ACIDIFICATION OF MILK WITH IMMOBILIZED LACTIC-ACID BACTERIA

Several parameters of a milk prefermentation process by immobilized la ctic acid bacteria in a stirred bioreactor were investigated. This pro cess was designed to acidify and inoculate skim milk continuously. Fou r strains of mesophilic lactic acid bacteria were entrapped separately in kappa-carrageenan and locust bean gum gel beads. We studied the ef fect of four process parameters [pH (5.8, 6.0, and 6.2), temperature ( 22, 26, and 30 degrees C), stirring speed (90, 120, 150, and 180 rpm), and ratio of gel to reactor volume (20, 25, 30, and 40%)] on dilution rate, lactic acid productivity, fermentation time, and the compositio n of the prefermented milk and coagulum obtained after further ferment ation to pH 4.7. Dilution rate increased with temperature, pH, gel rat io, and low stirring speed. Under the conditions used, the maximum rea ctor dilution rate was 30.6/h at pH 6.2, 30 degrees C, 120 rpm, and a gel ratio of 25%. The preacidified milk was inoculated with the four s trains of lactic acid bacteria at 2 x 10(8) to 3 x 10(8) cfu/ml. Ferme ntation time was reduced by more than 50% with this prefermentation st ep, and the final coagulum contained about 10(9) cfu/ml. Citrate was e ntirely consumed during fermentation, and acetoin and diacetyl concent rations in the coagulum reached 2.8 and .014 mM.