Study herein aimed the utilization of BSG as substrate for the solid-state fermentation process initiated by Pleurotus ostreatus. Final goal of the study was the evaluation of fermentation products as proteinaceous animal feed. In this respect, BSG’s moisture content constitutes a crucial factor for fungi growth, since the increased moisture content inhibits the oxygen transfer, generating concurrently a suitable environment for contamination. On the contrary, low moisture levels prevents the fungi/microbial growth, enzyme production and confines the nutrition availability (He et al. 2019). In our experiments the moisture content always maintained around 76%, a value that is in line with previous literature reports (Khidzir et al. 2020; Musstato et al. 2006; Wang et al. 2001; Xiros and Christakopoulos 2012). On the other hand, the most crucial variable for BSG upgrade is the protein content of the fermentation outcome. Previous studies have demonstrated the ability of fungi initiated SSF procedure to increase the protein content. In this respect, Xiros and Christakopoulos (2012) have reported that the initial protein content of unfermented BSG which varied between 10–30%, was significantly increase upon SSF initiated by Pleurotus ostreatus, Trichoderma pseudokoningii and Rhizopus sp respectively (Wang et al. 2001, Bayitse et al. 2015 and Ibarruri et al. 2019). These findings are in accordance with our results indicating that the protein content was gradually increased during incubation to conclusively result a statistically significant increase at the end of fermentation (Day 12). In similar studies, according to Akinfemi et al. (2010) proteins’ content was increased probably because of the excretion of certain extra cellular enzymes which are proteinaceous in nature into the waste during their breakdown and its subsequent metabolism. Additionally, proteins’ increase could probably be explained by the intake of nitrogen excess via aerobic fermentation. Darwish et al. (2012) and Terassan and Carmona (2015) have reported that proteins’ content increase is probably due the fungal biomass accumulation.
BSG is considered that constitutes a source of lower quality protein content as compared to other proteinaceous supplements such as soybean meal, fishmeal and milk (Westendorf and Wohlt 2002). This upgrade through SSF could probably improve its amino acid profile by upgrading the protein content, thus recommending the treated BSG as suitable and enriched with proteins substrate for livestock. On the other hand, the content of crude fiber substances varied between Days 0 and 12, recording a slight increase at the end of fermentation (Day 12), possibly as a result of a parallel Lignin increase.
Ritota and Manzi (2019) and Darwish et al. (2012) noted that Pleurotus spp. excrete hydrolytic enzymes (lignin peroxidase and manganese peroxidase) displaying the potential of degrading lignocellulosic raw materials, resulting to their improved digestibility (Akinfemi et al. 2010). Herein, the cellulose content was diminished without significance, whereas the lignin content showed a slight increase at the end of fermentation. It is notable that Fibers’ concentration affects the feed conversion rate. According to Lao et al. (2020) growing/finishing pigs that were fed with BSG up to 23% didn’t result to a significant gain reduction or a decline in quality’s carcass.
The augmented BSG’s ratio, over more than 6%, initially increased fibers content but concurrently caused a decreased conversion rate of feed in pigs. Thus, a decreased performance was observed, with the authors concluding that in order to have an essential management, BSG could be absorbed at a rate up to 50%, fulfilling the additional protein needs without a decrease in performance. (Westendorf and Wohlt 2002). Thus, the present study highlights the importance of BSG’s biotransformation by SSF where the entire BSG’s proportion can be used since a reduction of cellulose is observed.
Both, Total and Reducing Soluble Sugars concentrations varied until Day 6 and then they were gradually diminished until the end of fermentation. This may be rationalized considering that Pleurotus ostreatus consumed fermentable sugars as an energy source for its growth.
Barley as well as mushrooms, are composed by many bioactive compounds such as β-glucans, which constitute polysaccharides composed by D-glucose monomers linked through β-glycosidic bonds. β-glucans from barley consist of 1,3 − 1,4 bonds, while β-glucans originated from mushrooms consist of 1,3 − 1,6 linkages (Jin et al. 2004; Zhu et al. 2015). Previous studies have displayed the β-glucans beneficial impacts on human and animal health, which vary due to the differences between their linkage bonds. Barley’s β-glucans have been displayed on the list of the European Food Safety Authority (EFSA) by claiming that they can exhibit positive health effects under certain circumstances. Furthermore, the consumption of (1,3 − 1,4) β-glucans is associated with a decrease of blood cholesterol (Steiner et al. 2015; Zhu et al. 2015). Additionally, β-glucans derived from mushrooms are known for their antitumor and immune-stimulating properties that are responsible for health promotion and welfare. β-glucans derived from barley, are the predominant constituent (70%) of barley endosperm cell walls. According to previous brewing literature, barley’s β-glucans content ranged from 2–6% (w/w) (Jin et al. 2004), which is also in line with the results of the present work. Additionally, the content of 1,3 − 1,4 β-glucans derived herein from BSG were metabolized and therefore resulted in a significant reduction between Days 0 and 12, while the content of 1.3–1.6 β-glucans was significantly increased. This notable increase of 1,3 − 1,6 β-glucans content was due to the fungal growth. Finally, the fungal growth was achieved by breaking the cell walls, utilizing enzymes such as cellulases and glucosidases which resulted to reducing sugars release. Pleurotus ostreatus consumed these fermentable sugars as an energy source as well as structural components. Overall, according to Ibarruri et al. 2019, BSG proteins increased by 54% ranging from 20.5–31.7% via SSF with Rhizopus. Nowithstanding BSG used in this study, had lower initial protein concentration probably due to its variety or cultivation practices, a similar pattern was observed, via SSF initiated by Pleurotus ostreatus demonstrated a notable increase of 50% ranging from 16.73–25.01%. Also, BSG is consisted of 1,3 − 1,4 β-glucans which contribute to the cholesterol and blood glucose regulation. SSF initiated by Pleurotus ostreatus enriched BSG with 1,3 − 1,6 β-glucans, which impact to the immunostimulation and the animals’ good health and welfare. This procedure generates a novel fermentation outcome as a proteinaceous animal feed consisted of 1,3 − 1,4 and 1,3 − 1,6 β-glucans contributing to the economic and environmental impact in the context of the circular economy.