Changing the nutrient composition and enhancing the hydrolytic enzyme activity of citrus pulp residue by cofermentation with Candida utilis and Bacillus subtilis

Abstract In this study, citrus pulp residues were treated to improve their nutritional compositions through solid-state fermentation using Candida utilis and Bacillus subtilis. The crude and soluble protein contents increased by 54.83% and 866.50%, reaching 242.50 ± 6.50 mg/g and 39.53 ± 1.77 mg/g after fermentation, respectively. The contents of essential amino acids and nonessential amino acids significantly increased by 230% and 402%, especially leucine and phenylalanine (8.3% and 5.1%, based on total protein). Moreover, antinutritional factors, such as pectin and crude fiber, were reduced by 43.10% and 20.21%, respectively. Correlation analysis showed that the activities of four enzymes, including acid protease, neutral protease, pectinase and xylanase, were significantly correlated with the variation in the contents of soluble protein, total sugars and reducing sugars (p