Optimizing secondary pasteurization induced gel defects of ambient-temperature-storage yogurt: Synergistic effects of low-methoxyl pectin and diffusing/focusing ultrasound

Ambient-temperature-storage yogurt (ATSY) requires secondary pasteurization for extended shelf life but suffers reduced stability, making the pectin-ultrasound (US) combination promising. Consequently, this study investigated the effects of low-methoxyl pectin (LMP) combined with diffusing/focusing ultrasound (DUS/FUS) on the stability and textural properties of ATSY. First, 0.5% LMP was optimized as it maximized water-holding capacity (WHC). The results indicated that the FUS-0.5% LMP treatment increased WHC by 46.71% and improved the apparent viscosity at 50 s^-1 by 38.43%. Notably, the combined treatment enhanced texture characteristic, alongside superior sensory acceptability. Furthermore, a significantly inhibition of whey precipitation was demonstrated over a 180-day storage period. Subsequently, the gelation process of ATSY was simulated using acid-induced LMP-casein complex gels to explore the mechanism of the combined treatment. The combined treatments induced the cleavage of disulfide bonds into free thiol groups, increased the β-sheet content (from 49.13 ± 0.41 to 56.24 ± 0.65%), and enhanced the exposure of hydrophobic residues in casein. This resulted in smaller protein aggregates and a significant reduction in particle size (from 44.62 ± 0.98 to 19.54 ± 0.97 μm), thereby forming a denser and more stable gel network. Collectively, these findings provide scientific insights and basis for the application of FUS and LMP to enhance the textural and sensory attributes of ATSY, and offer potential solutions for production of high-quality ATSY products.