The promotion effects of flat-form ultrasound on fermentation efficiency, kinetic, and quality characteristic of room-temperature-storage yogurt.

The influence of flat ultrasound-assisted fermentation (FUAF) on the fermentation and quality of room-temperature-storage yogurt (RTSY) was investigated. Six different ultrasonic frequencies (20, 28, 35, 20/28, 28/35, and 20/35 kHz) were used in the treatments. The results showed that FUAF treatments, especially the 28 kHz, observably enhanced the quality of RTSY. Under these optimized conditions, the counts of Lactobacillus bulgaricus and Streptococcus thermophilus in RTSY prior to secondary pasteurization increased by 90.53 % and 94.76 %, respectively. This enhancement significantly shortened the fermentation time from 5 h (control group) to 4 h. Additionally, the water-holding capacity improved by 11.51 %, resulting in superior storage stability. Kinetic model analysis revealed that lactic acid bacteria exhibited faster growth rates, higher lactic acid production, and accelerated glucose consumption. Furthermore, texture and rheological analyses indicated significant enhancements in hardness, adhesiveness, cohesiveness, and apparent viscosity, contributing to an overall improvement in product quality. Electronic nose and headspace solid phase microextraction-gas chromatography-mass spectrometry (HS-SMPE-GC-MS) analysis showed that sonication increased key volatile compounds related to flavor (such as acetaldehyde, 2,3-butanedione, etc.), and thus, enhanced the flavor characteristics. In conclusion, the use of FUAF, especially at 28 kHz, in the fermentation of RTSY was beneficial in enhancing the fermentation efficacy and quality of RTSY.