Acoustic biostimulation as a potentially greener non-thermal processing strategy for developing a sugar-free fermented açaí beverage: Impact on microbial kinetics and exopolysaccharide structure

This study investigated the use of low-frequency, high-intensity ultrasound (US) as an acoustic biostimulation strategy to enhance the fermentation of a carbon-restricted unsweetened açaí juice by a water kefir consortium. Water kefir grains were subjected to acoustic processing prior to inoculation into the açaí juice. Microscopic evaluation revealed that US altered the grain's native cauliflower-like architecture, transforming the mass-transfer-limited structure into a partially dispersed substrate. This mechanical disruption was accompanied by increased solubilized EPS during the initial stages of fermentation, followed by a significant decrease over 72 h of unsweetened açaí juice fermentation, suggesting a possible change in EPS accessibility under the carbon-restricted conditions. The observed morphological changes were consistent with physical dispersion of the microbial aggregates within the grain matrix. The highest LAB population peak reached 11.90 Log CFU.mL^-1 in P3, compared with 9.12 Log CFU.mL^-1 in the control. These differences were accompanied by changes in the timing and magnitude of microbial population peaks. FTIR analysis showed that the main absorption bands associated with carbohydrate-related functional groups remained similar between the control and sonicated samples. These findings indicate that targeted acoustic biostimulation can modulate fermentation kinetics and microbial population dynamics in an unsweetened açaí matrix. The results provide evidence for the potential use of ultrasound as a non-thermal processing strategy for modifying water kefir fermentation.