Exploring the potential of katsuobushi grounds as a source of bioactive peptides through fermentation with Aspergillus sydowii

Dried bonito broth, made from katsuobushi flakes, is a commonly used seasoning in Japanese cuisine. While extracting the broth, high-quality proteins are left behind in the katsuobushi grounds, which could be a valuable source of bioactive substances via a hydrolysis process. The xerophilic Aspergillus sydowii showed potential for protein hydrolysis and production of amino acids and soluble peptides through solid-state fermentation. Several peptidases involved in katsuobushi protein hydrolysis were purified from the crude enzyme preparations and identified as aminopeptidase, trypsin-like serine protease, and elastinolytic serine protease. The hydrolysates from the culture contained glutamic acid, leucine, and lysine and markedly increased soluble peptides with molecular weights ranging from 0.2 to 17 kDa. Electronic tongue analysis indicated that the hydrolysates had substantially stronger salty and umami tastes. Moreover, the hydrolysates exhibited radical-scavenging activities, ferric ion reducing antioxidant power, and angiotensin converting enzyme inhibitory activity, as well as promoting growth in lactic acid bacteria. In addition, de novo peptide sequencing was performed for peptides fractionated from the hydrolysates, and eight peptides were identified. The hydrolysates produced by A. sydowii fermentation could have potential applications in the food industry as natural flavor enhancers and functional ingredients with several bioactivities.