## 4 Discussion
Finally, [PERSON], [PERSON], and [PERSON] [27] reported that an augmented crude protein concentration may be due to the colonization and growth of the microbial biomass in the substrate. Our results also agree with these literature findings, because the crude protein content was increased for all substrates examined. The degradation of the lignocellulosic materials is considered as a complex procedure. Specifically, cellulose and hemicellulose modifications are attributed to the action of the hydrolytic enzymes, whereas lignin's degradation is associated with the activity of the oxidative enzymes [4]. White rot fungi such as _P. ostreatus_ secrete extracellular lignin modifying enzymes, of which lacase, lignin peroxidase, and manganese peroxididases are considered as the most efficient. CFS reduction could be caused by the action of the cellulase enzymes that are excreted by the cellulolytic fungi [25]. Additionally, their valorization and transformation by fermentation microorganisms could potentially be one more reason for their depletion, because CFS form an energy source for their biological and other cellular activities [26]. Our results are in line with the reported literature findings because, in our case, the amounts of CFS were determined as slightly decreased at the end of the fermentation. As we have already highlighted, the degradation of cellulose and hemicellulose is performed through the secretion of hydrolytic enzymes, whereas the action of oxidative enzymes are more effective for lignin modification. Herein, _P. ostreatus_ has the efficacy to degrade lignin and hemicellulose rather than cellulose, and consequently can be characterized as a selective degrader [4]. Thus, in our study, the cellulose content remained practically unaffected because only a slight increase was observed, while lignin presence displayed an increment exceeding 2-fold. This finding can be interpreted in respect of the variation of digestion rate for the examined substrates and its ability to provide the microorganism with the necessary nutrients, namely N, P, and K, that result in a more extensive degradation [28]. Additionally, fungus has the ability to produce new organic compounds such as \(\beta\)-glucans and vitamins, enriching the substrate's biomass. In respect of the observed increased lignin content, a possible explanation may be associated with the reduction of hemicellulose level [27]. Ash content was notably increased at the end of the process, presumably as a result of the contribution of the microorganisms in the fermentation process [26]. Additionally, [PERSON] et al. [29] stated that ash's increment could be explained by the reduction in organic matter that is performed during the SSF procedure. It must be noted that ash content is considered as a critical parameter for mineral content evaluation, which could contribute in the sector of animal feeding [30]. Finally, in respect of the TSS and RSS contents, a similar pattern was observed because both values were reduced at the end of fermentation (Day 11). This can be rationalized by considering the fact that, during the fermentation process, _P. ostreatus_ produced some specific enzymes in order to degrade the lignocellulosic material efficiently. The observed reduction in TSS content could be a result of microorganism activity, which hydrolyzed starch into glucose and utilized them in order obtain energy and to fulfill some other cellular activities [30]. Additionally, another interpretation could possibly be associated with the lignocellulosic nature of the examined substrate. On the other hand, RSS depletion could be explained by considering that _P. ostreatus_ probably consumed the fermentable sugars, which served as an energy source for its growth. In the present study, \(\beta\)-glucan content was increased by 32.51% at the end of the fermentation. This is a result of _P. ostreatus_ activity, which is able to enlarge \(\beta\)-glucan content by utilizing the nutrients of the studied substrates.