Comparative analysis of heat and ultrasound inactivation of Lactobacillus plantarum NCTC 13644 fermented apple pomace: viability and postbiotic characteristics

The functionality of postbiotics is influenced by inactivation methods used, yet direct comparisons between inactivation methods remain limited. This study evaluates the viability and characteristics of postbiotics by comparing heat and ultrasound treatment for the preparation of postbiotics generated from Lactobacillus plantarum NCTC 13644 fermented in apple pomace. Postbiotics were prepared by applying nine heat treatments and ultrasound treatments to induce fermented bacterial inactivation and extract postbiotic components. Bacterial viability was determined by the plate count, γ-Aminobutyric acid (GABA) and total free amino acids (TFAA) were quantified using LC-MS/Q-TOF. Thermal inactivation and GABA degradation were modelled using Bigelow, Weibull–Mafart, and Arrhenius-based first-order kinetics, antioxidant activity was assessed using an in vitro radical-scavenging assay.

Significant differences (p < 0.05) were observed in bacterial viability and all characteristics assessed across treatment parameters. All heat treatments achieved bacterial inactivation and a significant increase in TFAA concentrations, with the maximum observed at 80 °C for 30 minutes (TFAA = 89,296.84 ± 982.82 mg/100 g). However, heat treatment caused GABA degradation, characterised by a first-order rate constant increasing from k∼d∼ = 0.044 min^-1 at 60 °C to 0.0773 min^-1 at 80 °C (activation energy = 27.62 kJ/mol). In contrast, only six of the nine ultrasound treatments were effective for bacterial inactivation. Ultrasound treatment enhanced GABA production, with the highest yield (5077.43 ± 155.56 mg/100 g) achieved at ultrasound intensity of 180 W·s/mL. Elevated ultrasound intensities also increased TFAA concentrations. Both heat and ultrasound treatments improved antioxidant activity. These findings underline the importance of selecting inactivation strategies tailored to the desired postbiotic function.