Effect of moderate electric field frequency on growth kinetics and metabolic activity of Lactobacillus acidophilus

Moderate electric fields (MEF) have been previously shown to alter the metabolic activity of microbial cells; thus, the effect of frequency and electric field would be of considerable interest. We investigated herein the effects of MEF frequency on microbial growth kinetics and bacteriocin (Lacidin A) production of Lactobacillus acidophilus OSU 133 during fermentation. The following fermentation treatments were compared: conventional (for 40 h), MEF (1 V cm(-1), for 40 h), combination of MEF (1 V cm(-1), for the first 5 h) and conventional (for 35 h) at various frequency levels (45, 60, and 90 Hz) all at 30 degrees C, and control (conventional) fermentation at 37 degrees C. MEF treatments with purely sinusoidal waveforms at all frequencies at 30 degrees C produced a shorter lag phase than conventional fermentation. However, no lag phase reduction was found for a 60 Hz waveform that contained high-frequency harmonics. There was, however, a significant increase in the bacteriocin production under early MEF treatment at 60 Hz with high-frequency harmonics. On the basis of these observations, the fermentation process is accelerated by applying pure sinusoidal MEF at the early stage of growth while a significant increase in the bacteriocin production occurs when sinusoidal field at 60 Hz with harmonics is applied at the early stage of the growth.