Production of Bioactive Peptides in Tomato Seed Protein Isolate Fermented by Water Kefir Culture: Optimization of the Fermentation Conditions

The aim of this work was to explore the use of protein isolate from tomato seed as a medium for the growth of water kefir mixture culture. Response surface methodology (RSM) and a central composite rotatable design were used to optimize the fermentation conditions of tomato seed isolate by water kefir mixture culture, including inoculum level, sucrose concentration and ascorbic acid concentrations, in order to improve cell growth and exopolysaccharides production. The models established showed that the values of R2 were high and the p-values < 0.0001 were also suitable for this experiment, which confirmed the effectiveness of these models. The evaluation of the radical scavenging activity of the isolate after 24 h of fermentation showed an improvement of about 74%. HPLC analysis showed a significant decrease of the concentration of total amino-acids exceeding 155%, especially for glutamic acid and aspartic acid. After 24 h of fermentation, the protein isolate contains respectively by about 41.27 and 20.29 mg/100 g of glutamic acid and aspartic acid. FTIR results showed that the fermentation favors the production of new amides and aromatic compounds. The results of these experiments indicated that RSM design is promising approach for the optimization of tomato isolate fermentation conditions, and that water kefir culture could degrade and convert the proteins into bioactive peptides that contribute positively in the improvement of antioxidant activity.