Sustainable valorisation of Apple pomace for postbiotic production: optimising GABA and free amino acid yields using Taguchi methodology

Apple pomace poses environmental and economic challenges due to its high biodegradability. Despite existing studies on the recovery of polyphenol and pectin from apple pomace, systematic use of apple pomace for GABA and free amino acid production through lactic acid bacteria fermentation remains underexplored. Apple pomace was valorised by investigating and optimising the production of γ-aminobutyric acid (GABA) and free amino acids in postbiotic mixtures derived from lactic acid bacteria (LAB) fermentation. A Taguchi L16 orthogonal array was employed to systematically evaluate the effects of bacterial strain, fermentation time, pH, temperature, and yeast extract concentration on metabolite production. GABA and free amino acids were quantified using LC-MS/QTOF analysis. LAB fermentation significantly enhanced GABA content, total free amino acids, and antioxidant activity in the resulting postbiotic mixtures. Optimal conditions for GABA production were identified as Lactobacillus plantarum, 48 h, pH 4, 35 °C, and 3.75 g yeast extract. In contrast, Lactobacillus casei at 72 h, pH 4, 40 °C, and 3.75 g yeast extract maximised total free amino acid yield. Bacterial strain and fermentation temperature were the most influential parameters governing both GABA and free amino acid production. Validation experiments conducted under the optimised conditions yielded 4396.19 ± 98.21 mg/100 g of GABA and 86559.28 ± 2000.2 mg/100 g of total free amino acids, exceeding the values obtained in all other experimental conditions investigated. The close correlation between predicted and experimental values confirms the robustness of the Taguchi optimisation approach. These findings demonstrate a scalable, systematic biotechnological process for converting apple pomace into value-added postbiotic ingredients, advancing sustainable waste management strategies within circular food systems.