GROWTH-KINETICS OF LACTOBACILLUS-ACIDOPHILUS UNDER OHMIC HEATING

Lactobacillus acidophilus OSU133 was inoculated into MRS broth in a fe rmenter vessel and incubated at 30, 35, or 45 degrees C with agitation . Incubation temperatures were attained by conventional or ohmic heati ng. An electrical current at low (15 V) or high (40 V) voltage was use d to heat the culture directly during fermentations under ohmic heatin g. The growth parameters (lag period, minimum generation time, and max imum growth) and changes in pH were determined during fermentation. Me tabolic activities (consumption of glucose and production of lactic ac id and bacteriocin) were determined during fermentation at 35 degrees C under both heating methods. Lag period for L. acidophilus was affect ed appreciably by the method of heating, but the magnitude of these ch anges depended on the fermentation temperature. When fermentation was done at 30 degrees C, lag period decreased by 94% under low-voltage oh mic, compared with conventional, heating methods. Ohmic heating did no t change the generation time significantly and caused slight, but sign ificant (p < 0.01) decrease in maximum growth. Therefore, the electric current enhances the early stages, but it inhibits the late stages of growth. Ohmic, compared with conventional, heating resulted in higher final pH and lower bacteriocin activity in the fermented medium. Howe ver, ohmic heating at 35 degrees C had minimal effect on glucose utili zation and lactic acid production by L. acidophilus. Results show that measurement of the electric current when ohmic heating is done at a c onstant voltage may be used in monitoring such fermentations. In concl usion, ohmic heating is potentially useful in certain applications rel ated to fermented foods. (C) 1996 John Wiley & Sons, Inc.