Adaptive evolution of Aspergillus oryzae 3.042 strain and process optimization to reduce the formation of tyrosine crystals in broad bean paste

Tyrosine crystals occasionally appeared on the broad bean paste surface, which will cause economic losses. This study aimed to eliminate the tyrosine crystals in broad bean paste through decreasing the activities of proteolytic enzymes produced by Aspergillus oryzae and process optimization. Broad bean pastes containing no more than 6.16 mg/g dry material tyrosine showed low possibility to form tyrosine crystals. Using tyrosine as substrate, the A. oryzae 3.042 was adaptively evolved and the tyrosine content in the broad bean paste fermented by the evolved A. oryzae was reduced from 6.49 mg/g dry material to 6.14 mg/g dry material (p < 0.05). When the production process was optimized, the tyrosine content in broad bean paste was further reduced to 5.67 mg/g dry material (p < 0.05). In this condition, no tyrosine crystals were formed in broad bean paste after the 12-month storage while the product quality was not influenced. PRACTICAL APPLICATIONS: Tyrosine crystals were one of the most important factors which negatively influence the quality of traditionally fermented food, including broad bean paste, soybean paste, and sausages. The appearance of tyrosine crystals in these foods will cause economic losses to manufacturers. This study tried to eliminate the appearance of tyrosine crystals through decreasing the activities of proteolytic enzymes produced by Aspergillus oryzae 3.042 and fermentation process optimization. The adoption of modified A. oryzae and optimized fermentation process successfully guaranteed the elimination of tyrosine crystals formation in the production and storage period of broad bean paste. This will not only benefit the broad bean paste manufacturers but also provide guidance for other fermented food producers to deal with tyrosine crystals problem.