Direct utilization of bacterial fermentation broth as wet strength agent and flocculant on straw fiber composites

Environmental concerns associated with synthetic additives have driven the development of sustainable alternatives. This study investigates the direct application of Bacillus velezensis RL7 fermentation broth as an eco-friendly wet strength agent and flocculant, with CaCl₂ as a coagulant aid. The fermentation broth contains extracellular polysaccharides, comprising eight monosaccharides. These polysaccharides, rich in carboxyl, aldehyde, and hydroxyl groups, constitute 75.60 wt% of the broth and contribute 97.59 % of its flocculation activity. At optimal addition levels (10 wt% broth and 1 wt% CaCl₂), filler retention increased to 34.45 %, the wet tensile index improved by 260.48 %, and air permeability decreased by 24.23 %. The enhancement is attributed to aldehyde-mediated bonding with fiber hydroxyl groups. In addition, microbial cells imparted antifungal activity. These results demonstrate that B. velezensis RL7 fermentation broth is a promising sustainable alternative to conventional synthetic additives.