# The Influence of Substrate and Strain on Protein Quality of _Pleurotus ostreatus_
## 3 Results and Discussion
### Functional Properties  
Figure 4 and Table S5 present the functional properties of the _P. ostreatus_ flours and PCs. The water absorption capacity (WAC), also referred to as the water-binding capacity, is related to the hydrogen bonding and the hydration of ionic group abilities of protein molecules [36]. Overall, the WAC of the flours was significantly higher than that of the corresponding PCs. Among the flours, the highest and lowest WAC values were observed for the strain AMRL144 when it was cultivated on BOS (503.1 \(\pm\) 1.5%) and WS (437.1 \(\pm\) 2.9%), respectively. The WAC of the PCs ranged from 244.0 \(\pm\) 3.6% for AMRL150 to 330.6 \(\pm\) 9.2% for AMRL144 when they were cultivated on WS. The observed differences between the mushroom flours and PCs could be attributed to the higher content of soluble carbohydrates in the flours [26]. It was found that the total carbohydrate content decreased by 60% in protein concentrates due to the harsh conditions applied during the protein extraction process [18]. The WAC for flours from three strains of _P. ostreatus_ and their corresponding protein concentrates was also studied by [PERSON] et al. (2018) [34] who reported similar results to ours. Plant protein concentrates are also characterized by lower WAC values than their corresponding flours and this was attributed to the extraction procedure as well as to the protein conformation and the availability of polar amino acids for protein-water interactions [26; 34]. The oil absorption capacity (OAC) of the _P. ostreatus_ PCs was significantly higher than that of the corresponding flour. The highest OAC was observed for the PCs of AMRL150 when cultivated on either WS or BOS (540.9 \(\pm\) 11.3% and 539.3 \(\pm\) 4.2%, respectively). Among the flours, the flour from AMRL144 cultivated on BOS presented the lowest OAC (414.7 \(\pm\) 5.0%) value. Similar findings were obtained when the WAC and OAC of flours and PCs from three strains of _P. ostreattus_[34] and _P. tuberegium_ sclerotia [58] were examined. Obviously, the ability of PCs to bind with oil is directly attributed to their high concentration of non-polar amino acids. The ability of proteins to form and stabilize foams is critical for food acceptability. A higher protein concentration enables higher foam stiffness and stability. The foam capacity of the _P. ostreatus_ PCs was significantly higher than that of the corresponding flour. Among the PCs, the highest FC value (57.3 \(\pm\) 1.5%) was observed for AMRL144 cultivated on WS (Table S5) and the lowest for AMRL150, independent of the cultivation substrate (BOS: 51.2 \(\pm\) 1.3%; WS: 51.3 \(\pm\) 1.8%). Among the flours, AMRL150 cultivated on BOS presented the lowest FC value (39.2 \(\pm\) 0.7%). [PERSON] et al. (2014) [59] found that the foam capacity of lentil protein concentrates was influenced by the environmental pH (pH of the solution: 3, 5 and 7) regardless of the pH used for the protein extraction (pH 8, 9 and 10). The dependence of FC on pH was studied by [PERSON] et al. (2018) [34]. They found that the PCs of three _P. ostreatus_ strains presented the highest FC values (220-276%) at pH 8, which decreased (by 116-200%) with the increase of the pH to 10. The lower FC values reported in our study could be attributed to the fact that we did not adjust the pH of the suspension. The observed difference between the flours and PCs may also be attributed to differences in flexibility and charge density of the protein molecules under the testing conditions. The foam stability (FS) of the PCs was significantly higher than that of the flours except for AMRL144 cultivated on BOS, which did not present a significant difference. Foam stability is dictated by a low charge density of the protein molecule, which allows for less intermolecular repulsion and thus occurs around the pI. Unlike FC, the FS of lentil protein concentrates was not influenced by the pH of the solution nor the extraction pH (pH range: 8-10) and the authors concluded that the interfacial film properties were not pH-dependent [59].