The effect of nanosecond pulsed electric field on the production of metabolites from lactic acid bacteria in fermented watermelon juice

Lactic acid fermentation offers a processing alternative for preservation of watermelon juice, which is sensitive to heat, oxygen, and light. In this study, 8.8 × 10⁷ CFU/mL of Lactobacillus plantarum DSM 9843 in MRS broth was inoculated in 9.9 mL of sterilized watermelon juice. Nanosecond-pulsed electric field was applied during the log growth phase of the bacteria. An 19% increase in L-lactic acid, 6.8% increase in D-lactic acid and 15% increase in acetic acid were observed over control. The final pH was 3.8. These increased levels of metabolites were dependent on the applied voltages (L-lactic acid: 5.0 kV 700 pulses, D-lactic acid: 4.5 kV 700 pulses and acetic acid: 4.5 kV 1000 pulses). The nsPEF treatment did not affect the viability of the cells and sufficient numbers remained in the product after fermentation (1.6 × 10⁹ CFU/mL in average). These results suggest that the metabolism of lactic acid bacteria was stimulated by the PEF treatment.