Inga paterno produces seeds (Se) covered by a sarcotesta (Sa), both of which remain understudied. Meanwhile, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus (L-S) are commonly used in fermented products. This study investigated the potential of Se and Sa as substrates for probiotic fermentation using L-S.

Three formulations were prepared: 5%(w/v) Sa, 5%(w/v) Se, and a 1:1 mixture (Ae). These were sterilized, inoculated with L-S (7.67×10⁶ CFU/ml), fermented (72 hours at 35°C), and stored at refrigeration temperature (5°C). Antioxidant activity (AA), total phenolic content (TPC), condensed tannins (CT), flavonoids (FC), and phytic acid content (PAC) were measured at 0, 4, 11, and 25 days of storage. One-way ANOVA and Tukey's test (p < 0.05) were used to analyze the data.

L-S counts ranged from 6.69 to 9.41 log CFU/ml, with higher stability in Se (8.39 to 7.62 log CFU/ml) and Sa (8.44 to 8.01 log CFU/ml). AA ranged from 27.50 to 115.92 μg Trolox equivalents/ml, with Sa showing the highest and stable levels (84.07 to 113.46 log CFU/ml). PAC increased in Sa (from 34.22 to 42.54 μg of phytic acid /ml) but decreased in Se (from 40.13 to 37.73 μg of phytic acid /ml). CT decreased in all formulations (ranging from 1.53 to 0.70 mg of catechin equivalents/ml), and Sa presented a higher amount of FC (from 0.53 to 1.04 mg of quercetin equivalents/ml) than Se (from 0.03 to 0.08 mg of quercetin equivalents/ml) and Ea (from 0.09 to 0.21 mg of quercetin equivalents/ml). AA was mainly correlated with FC (R² = 0.6). L-S remained viable throughout storage in all formulations, while Sa provided a rich source of TPC and FC. These findings suggest that I. paterno sarcotesta can serve as promising substrates to produce a functional fermented beverage with enhanced antioxidant properties.