Screening of γ-Aminobutyric Acid-Producing Lactic Acid Bacteria and Fermentation Process Optimization of Aronia melanocarpa Juice

To develop a functional fermented beverage of Aronia melanocarpa rich in γ-aminobutyric acid (GABA), a GABA-producing lactic acid bacterium (LAB) suitable for fermenting Aronia melanocarpa juice was screened, and the fermentation process was optimized. Six GABA-producing strains were screened based on L-MRS medium from 64 LAB isolates derived from traditional Chinese fermented foods by thin-layer chromatography and high-performance liquid chromatography. These six strains were further used to ferment Aronia melanocarpa juice, and the mass concentration of GABA in the fermented juice was determined. Morphological observation and 16S rDNA molecular identification were performed on the six strains to select one excellent fermentation strain. For this strain, the growth curve, acid production curve, acid tolerance, and glutamate decarboxylase (GAD) activity were measured. Single-factor experiments, response surface methodology, and an artificial neural network (ANN) model were applied to optimize the fermentation process and determine the optimal conditions. Subjective sensory evaluation and electronic tongue analysis were used to evaluate the fermented Aronia melanocarpa juice produced under the optimal conditions and to assess the impact of fermentation on juice flavor. The results showed that among the fermented juices prepared by the six GABA-producing LAB strains, the highest GABA mass concentration reached 2.07 mg/mL. The strain yielding the highest GABA concentration in the fermented juice was selected as the excellent strain (designated Lj16). Based on morphological observation and 16S rDNA molecular biological identification, Lj16 was identified as Levilactobacillus brevis. The GABA production of Lj16 in fermented Aronia melanocarpa juice was significantly higher than that of the other strains (P < 0.05). Characterization of the growth curve, acid production curve, acid tolerance, and GAD enzymatic activity of Lj16 revealed that Lj16 exhibited a long logarithmic phase, indicating sustained and robust biomass accumulation. After treatment at pH 2.5 for 4 h, the survival rate of Lj16 reached 65.79% ± 0.84%. The GAD enzymatic activity was as high as (2.020 ± 0.182)U/mL, and the acid production was mild (final pH 5.0), which facilitated sustained GAD function. Single-factor experiments, response surface methodology, and an artificial neural network-genetic algorithm (ANN-GA) coupled model were used to optimize the fermentation process. The optimal conditions were determined as follows: sodium L-glutamate mass fraction 2.30%, yeast extract powder mass fraction 2.40%, fermentation temperature 36.5 ℃, fermentation time 120 h and glucose mass fraction 1.5%. Under these optimal conditions, the GABA mass concentration in the fermented juice reached (8.77 ± 0.16) mg/mL. Subjective sensory evaluation and electronic tongue analysis of the fermented juice prepared under the optimal conditions showed that the sour and astringent tastes were significantly reduced, and the flavor harmony was markedly improved. This study screened a LAB strain suitable for GABA enrichment during Aronia melanocarpa juice fermentation, optimized the fermentation conditions, and improved the juice flavor. These findings could provide technical guidance for the development of functional fermented beverages with high GABA content and for the flavor regulation of Aronia melanocarpa juice.