Production of gamma-polyglutamic acid with tunable molecular weight via electrofermentation using soybean protein concentrate as a feedstock

Gamma-polyglutamic acid (γ-PGA) is a biopolymer with applications in cosmetics, food, and pharmaceutical industries. Its conventional production through submerged fermentation is limited by low yield and inadequate molecular weight (MW) control. The soybean industry generates high-protein products that can be repurposed as feedstock to produce high-value chemicals, including γ-PGA. This study introduces a cheaper alternative to refined nitrogen sources by evaluating for the first time soybean protein concentrate (SPC) as electrofermentation (EF) feedstock for γ-PGA production by Bacillus subtilis PB5760 biofilms. Polarized electrode-induced metabolism of B. subtilis PB5760 enabled enhanced biofilm formation and a four-fold increase in γ-PGA titre (from 0.83 ± 0.08 to 3.12 ± 1.04 mg cm⁻²). The MW of γ-PGA decreased with the potential applied during EF, from 6747 ± 486 kDa at open circuit potential (OCP) to 3344 ± 271 kDa at 0.2 V, and further to 1532 ± 49 kDa at 0.4 V. At optimal concentration, SPC supported γ-PGA production better than ammonium sulfate, particularly at 0.4 V (13.84 ± 0.23 mg cm⁻² versus 3.12 ± 1.04 mg cm⁻²). SPC also served as a glutamic acid source, with incremental replacement of pure glutamic acid enhancing γ-PGA titres. The optimal glutamic acid replacement (6.56 g L⁻¹ SPC) gave the highest titre (17.31 ± 0.72 mg cm⁻²) at 0.2 V. Higher SPC concentrations further increase the titre, while reducing the tunability of γ-PGA MW. This study demonstrates that food-grade, protein-rich feedstock can enhance γ-PGA production in EF and reduce cost, while preserving the MW control of EF process.