LAUSR

Abstract The effect of different sonication amplitudes (30 kHz, 100 W; 25%, 50% and 75%) as well as the exposure time (5, 10 and 15 min) on the microbial growth and propionic acid production by Propionibacter freudenrechii subsp. shermanii PTCC 1661 and Propionibacterium freudenreichii subsp. freudenreichii PTCC 1674 in the matrix of date syrup was investigated. Results showed the ultrasonication significantly caused reductions around 37% and 47% of lag time (λ) for P. freudenrechii subsp. shermanii and 47% and 52% for P. freudenreichii subsp. freudenreichii at 25% and 50% amplitudes, respectively. However, an increase around 120% in λ was observed at 75% amplitude for 15 min. The growth rate (μmax) had an increase rate with respect to the increase in sonication strength and exposure time except of 75% amplitude for 15 min. For 25% and 50% amplitudes, increment in the exposure time to 15 min resulted in increasing in propionic acid about 25.8% and 41.7% by P. freudenrechii subsp. shermanii and about 22.3–38.7% by P. freudenreichii subsp. freudenreichii while compared with the control, respectively. However, increasing in the exposure time to 15 min at 75% amplitude significantly decreased propionic acid production about 55.5% and 39.1% by P. freudenrechii subsp. shermanii and P. freudenreichii subsp. freudenreichii, respectively. The maximum propionic acid was produced using P. freudenrechii subsp. shermanii (6.43 g/L) at 75% amplitude for 10 min. However, there is no significant different (p