Effects of ultrasound on the fermentation profile and metabolic activity of lactic acid bacteria in buffalo's (Bubalus bubalis) milk

In this study, the ultrasound was applied at the lag phase and logarithmic phase of the growth of Streptococcus thermophilus subsp. Salivarius (6.76 ± 0.15 log cfu/mL) and Lactobacillus bulgaricus (6.36 ± 0.22 log cfu/mL) in buffalo's milk. The results revealed that S. thermophilus entered into the logarithmic phase after 45 min of incubation. Ultrasound application during lag phase (1685 J mL^?1) and logarithmic phase (561.6 J mL^?1) reduced the fermentation time by 32 min (12.5%) and 40 min (15.7%), respectively. Ultrasound treatment (1684.8 J mL^-1) during the lag phase resulted in fractures (approximately 0.1–0.2 μm) on the cell membrane of S. thermophilus. Application of ultrasound for a 2–6 min period enhanced (P < 0.05) the microbial growth while increasing (P < 0.05) the β-galactosidase activity. The lactose hydrolysis was increased up to 49.2% compared to that of the control. Ultrasound treatment during the lag phase showed higher (P < 0.05) storage modulus, yield point and consistency index compared to the logarithmic phase. It can be concluded that ultrasound has huge potential to improve the fermentation rate of buffalo's milk.Industrial relevanceConventional food fermentation is one of the most time and resources consuming processing steps in the food industry. In this work, it was shown that the appropriate application of ultrasound during different stages of microbial growth reduced the fermentation time of buffalo milk by 13% to 16% by enhancing the metabolic activities of lactic acid bacteria. Therefore, ultrasound has great prospects in the food processing industry to improve process efficiency. Scaling-up of laboratory applications of US in to industrial scale through designing new power transducers with high capacity and greater acoustic filtration is necessary to enhance its potential to be used at large scales.