Prolyl endopeptidase from Aspergillus niger immobilized on a food-grade carrier for the production of gluten-reduced beer

The recently growing demand of gluten-reduced beer is leading to the development of diverse approaches to be applied in brewing. The current work focuses on the development of an innovative and sustainable biocatalytic tool for the continuous production of gluten-reduced beer, based on the application of immobilized prolyl endopeptidase from Aspergillus niger (AN-PEP). This food-grade protease has been immobilized on A. niger chitosan beads and applied, for the first time, for the reduction of gluten in a commercial beer from barley malt. The immobilization procedure was optimized for maximizing the specific activity of the biocatalyst (0.016 I.U./mgBSAeq) and the best performance was reached using an immobilization solution at an initial protein concentration of 0.3 mgBSAeq/mL. The immobilization increased the thermal stability of the protease, which showed similar catalytic properties in synthetic beer (toward the synthetic substrate Z-Gly-Pro-pNA) when it was applied at 20 °C or at 50 °C. The continuous treatment in fluidized bed reactor (FBR), containing 10 g of immobilized AN-PEP (corresponding to 0.0036 gBSAeq), was optimized varying the flow rate (Qv). The suitable conditions to achieve reduction of the intact gluten of authentic beer was Qv of 728 mL/min. The continuous treatment in FBR allowed us to reduce the initial gluten content (65 mg/kg) in the commercial beer from barley malt, reaching the concentration of 19 mg/kg after 9 h and 15 mg/kg after 10 h of treatment.