# The Influence of Substrate and Strain on Protein Quality of _Pleurotus ostreatus_
## 4 Conclusions  
Our findings support the high nutritional value of _P. ostreatus_ protein from the two strains tested in the present study. All the essential amino acids studied were found in the proteins, with the basic amino acids prevailing over the acidic amino acids. In all the samples, the limiting amino acid was methionine, whereas the samples were a rich source of lysine and threonine. The EAAI values indicate that both strains can produce high quality protein especially when they are cultivated on a wheat straw substrate. The produced protein concentrates were characterized by a low protein solubility index (PSI), indicating that they possess better nutritional properties compared to the mushroom flours. The wheat straw substrate enhanced the protein quality of the produced PCs according to the PSI results. The lowest PSI was noted for AMRL144 cultivated on wheat straw, and its protein concentrate had the highest foam capacity. As already stated in the literature and based on the results of the present study, the correlation of PSI with the functional properties of PCs is worth investigating. In addition, the spectroscopic analysis of the mushroom flours revealed the presence of polysaccharides and salts of uronic acids, particularly glucuronic acid and phospholipids. The flours and PCs differed based on their functional properties. The WAC was significantly higher in the flours and the OAC, FC, FS and ES were higher in the PCs. The functional properties were positively correlated with  
Figure 5: Principal component analysis of the functional properties and protein content of _P. ostreatus_ flours (F) and protein concentrates (PC). (**a**) Score plot (the shaded areas represent the 95% confidence ellipses) and (**b**) biplot of scores and loadings of the first two principal components. the strain, indicating differences in protein surface hydrophobicity. The foam stability was positively correlated with the _P. ostreatus_ strain AMRL150, whereas the foam capacity was positively correlated with AMRL144. Since the solubility of proteins is essential for their foaming and emulsifying properties, partial hydrolysis, the addition of salt and adjustment of the pH of the protein concentrates of P. _ostreatus_ strains may improve their function in new food formulations. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14104040/s1](https://www.mdpi.com/article/10.3390/app14104040/s1)]([https://www.mdpi.com/article/10.3390/app14104040/s1](https://www.mdpi.com/article/10.3390/app14104040/s1)), Table S1: The influence of pH on _P. ostreatus_ flour (F) protein solubility (strains AMRL144 and 150) cultivated on different substrates. Results are given as mean \(\pm\) standard deviation. Different superscript letters between lines correspond to significantly different solubilities (\(p\leq 0.05\)); Table S2: Amino acid score (\(\%\)) \({}^{\star}\) of _P. ostreatus_ flours from strains AMRL150 and 144 cultivated on different substrates (WS: wheat straw; BOS: barley and oat straw). Data are expressed as mean \(\pm\) standard deviation (n = 3); Table S3: The main peaks of the ATR-FTIR spectra of _P. ostreatus_ flours (F) from strains AMRL150 and 144 cultivated on different substrates (WS: wheat straw; BOS: barley and oat straw); Table S4: The assignments of the ATR-FTIR spectra peaks; Table S5: Functional properties (WAC, OAC, EC, ES, FC and FS) of flours (F) and protein concentrates (PC) from _P. ostreatus_ cultivated on different substrates (WS: wheat straw; BOS: barley and oat straw). Results are given as mean \(\pm\) standard deviation (n = 3). Different superscript letters between lines correspond to significantly different contents (\(p\leq 0.05\)). Conceptualization, C.G. ; methodology, C.G. and A.M.; software, C.G., C.P. and A.M.; investigation, A.M., C.G., N.M., A.K., M.D. and E.K. ; writing--original draft preparation, C.G., A.M., M.D., P.D. and C.P. ; writing--review and editing, C.G. and A.M.; supervision, C.G.