Probiotic metabolites, known as postbiotics, have received attention due to their wide variety of promoting health effects. One of the most exciting postbiotic is gamma-aminobutyric acid (GABA), widely produced by lactic acid bacteria, due to its benefits in health. In addition, the performance of the biosynthesis of GABA by Lactiplantibacillus plantarum could be modulated through the modification of fermentation parameters. Due to their high nutritional value, agri-food by-products could be considered a useful fermentation source for microorganisms. Therefore, these by-products were proposed as fermentation substrates to produce GABA in this study. Previously, several experiments in Man Rogosa Sharpe (MRS) broth were performed to identify the most critical parameters to produce GABA using the strain Lactiplantibacillus plantarum K16. The percentage of inoculum, the initial pH, and the concentration of nutrients, such as monosodium glutamate or glucose, significantly affected the biosynthetic pathway of GABA. The highest GABA yield was obtained with 500 mM of monosodium glutamate and 25 g/L of glucose, and an initial pH of 5.5 and 1.2% inoculum. Furthermore, these investigated parameters were used to evaluate the possibility of using tomato, green pepper, apple, or orange by-products to get GABA-enriched fermented media, which is an excellent way to revalorise them.
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