PEER-REVIEWED ARTICLE bioresources.com De Lucio et al. (2020). “Degradation by P. ostreatus,” BioResources 15(4), 8849-8861. 8849 Protein Fraction, Mineral Profile, and Chemical Compositions of Various Fiber-based Substrates Degraded by Pleurotus ostreatus Brianda Susana Velázquez-De Lucio, a,b Edna María Hernández-Domínguez, a Alejandro Téllez-Jurado, a Maricela Ayala-Martínez, c Sergio Soto-Simental, c and Jorge Álvarez- Cervantes a, * The objective of this study was to characterize the substrates after their degradation by P. ostreatus within 60 days of cultivation in four Mexican mushroom-producing companies, in order to use it as a complement feed for ruminants. The acid detergent fiber (ADF), neutral detergent fiber (NDF), protein fraction, and mineral profile were analyzed after degradation. Crude protein (CP) of companies 1 and 4 increased by 6.1% and 6.8%, respectively (p ≤ 0.05). Soluble nitrogen reached 60% relative to un-degraded substrate with 43.8%. Proteins A, B1, and B3 fractions increased compared to controls (p ≤ 0.05) at some companies. The B2 fraction was decreased in all the degraded substrates (p ≤ 0.05), but the C fraction at companies 1, 2, and 3 presented no significant differences with respect to their controls (p ≤ 0.05). Crude fiber (CF), ADF, NDF, and hemicellulose (HC) decreased while dry matter digestibility (DMD) increased to 55.5-58% on degrade substrates. The mineral composition increased disproportionately. The substrate degraded by P. ostreatus by improving its digestibility and soluble protein content may be a low-cost food supplement. However, due to its mineral imbalance, it is not recommended as the sole food source for ruminants. Keywords: Protein fractions; Bromatological profiles; Microelements; Residues degraded; Fungi; Animal feed Contact information: a: Universidad Politécnica de Pachuca UPP, Zempoala, Hidalgo, México b: Posgrado en Biotecnología, Universidad Politécnica de Pachuca UPP, Zempoala, Hidalgo, México; c: Instituto de Ciencias Agropecuarias, Universidad Autónoma del Estado de Hidalgo UAEH, Tulancingo, Hidalgo, México; *Corresponding author: jorge_ac85@upp.edu.mx INTRODUCTION Agricultural residues such as straw and husks have been used as feed for ruminants despite their high fiber content. However, the strong bonds between cellulose, hemicellulose, and lignin impede the microorganisms in the rumen from fully utilizing the carbohydrate content (cellulose and hemicellulose) (Van Kuijk et al. 2015). In principle, enhancing the digestion of these food sources can be achieved by delignifying the fiber content by physical and/or chemical means. Nonetheless, this involves two disadvantages, as it increases costs, and affects the environment. Biological processes using fungi such as Pleurotus ostreatus, in contrast, offer benefits: the production of fructiferous bodies apt for human consumption and, simultaneously, degraded materials with high enzyme and metabolite contents. The changes that substrates undergo after fungal growth, are due to the excretion of enzymes, radicals, and other chemical compounds that degrade the lignocellulose into polysaccharides.