Screening of protease-producing microorganisms and optimization of their fermentation processes for the efficient preparation of Broussonetia papyrifera feed

With the rapid development of China’s livestock industry, the demand for protein feed has been surged, underscoring the importance of developing and utilizing new types of feed. As a novel and unconventional protein source, fermented Broussonetia papyrifera feed has become a promising alternative to traditional feeds due to its high nutritional value, biologically active ingredients and wide range of applications. However, the efficient utilization of crude proteins in B. papyrifera has been hindered by currently used microorganisms and fermentation processes. In the present study, nine proteolytic bacterial strains and eight yeast and fungal strains were isolated from the root zone soil of B. papyrifera by qualitative and quantitative methods. Among them, the protease activities of strains Paenibacillus sp. AY and Rhodotorula mucilaginosa FG were determined to be 21.95 and 55.16 U/mL, respectively. As compared with the control group, the contents of acid-soluble proteins and ammonia nitrogen in the fermented feed were significantly increased by 26.7% and 12.2% (P < 0.05) by Paenibacillus sp. AY, respectively, while they were respectively enhanced by 43.3% and 24.5% by R. mucilaginosa FG, and their effects were not lower than those of type strains. Finally, to increase the quality of fermented B. papyrifera feed, the cultivation conditions were further optimized by single-factor experiments and orthogonal design. Under optimum conditions, the content of acid-soluble proteins reached 7.63, which was 27.2% higher than that of the control. Our results lay a solid foundation for the establishment of a novel process for the efficient utilization of crude proteins in B. papyrifera feed and can accelerate the application of B. papyrifera feed in animal production.