Citrus pomace co-fermentation improved its protein and amino acids by Bacillus amyloliquefaciens and Candida utilis

High-value utilization of citrus pomace (CP) is of great significance to the sustainable development of the citrus industry. Co-fermentation of CP was investigated using the autochthonous probiotic bacteria, Bacillus amyloliquefaciens BF2 and Candida utilis GIM 2.9. Crude protein, soluble protein, and small peptide content of CP increased by 40.01%, 923.53%, and 626.67% after fermentation, respectively. Essential amino acids increased from 28.51% to 47.04% of total free amino acids. Leucine level reached 4.04 mg/g DW, increasing by 1293.10%, and produced histidine, valine, cysteine, and tyrosine. Scanning electron microscopy results showed that BF2 and GIM 2.9 could effectively degrade and utilize the CP fibers. Enzymolysis co-fermentation (ECF) system secreted higher levels of non-starch polysaccharide enzymes and proteases than co-fermentation (CF) system. Acid protease activity was positively correlated with soluble protein and free amino acid contents. BF2 possessed 63 essential amino acid-biosynthesis genes, nearly twice as many as that of non-essential amino acids. Eight key enzyme genes were identified for leucine biosynthesis in the BF2 genome, suggesting that BF2 played a key role in leucine biosynthesis. Our results provide a potential method for the high-value utilization of CP as a protein feed additive or dietary supplement.