Stimulation of low intensity ultrasound on fermentation of skim milk medium for yield of yoghurt peptides by Lactobacillus paracasei

Herein the effect of low intensity ultrasound on the fermentation of skim milk medium by Lactobacillus paracasei were investigated to obtain optimum ultrasonic conditions for the highest yield of yoghurt peptides. The results showed that the fermented skim milk medium treated with ultrasound with its seed culture without ultrasonic treatment was an optimum scheme. In this scheme with the ultrasonic conditions of 28 kHz, ultrasonic pulsed model of on-time 100 s and off-time 10 s, 100 W/L for the treatment time of 30 min after the fermentation time of 9 h, the peptide content in the fermented skim milk media increased by 49.5% and the viable cells in the same media increased by 43.5% compared with those in the untreated samples. By response surface methodology (RSM) analysis and its verification experiments, a reasonably accurate empirical model was established for investigating and predicting the relationship between skim milk concentration, ultrasonic treatment time, power and the yield of yoghurt peptides. The former two parameters 12.6% w/v and 35 min were taken in the verification experiments in which the peptide content of the fermented media reached 5.9 mg/mL with an increase by 64.23% and the peptide yield was 14.2%, similar to its theoretical value of 14.6% according to the empirical model. The comparison of extracellular enzyme activities in the fermented skim milk media between with and without ultrasonic treatment under the conditions in the optimum scheme indicated that the mechanism of the ultrasound-activated peptide content increment might be the extracellular enzyme activities immediately activated by the ultrasound, effect of which would disappear in the progress of fermentation after the ultrasound was removed.