# The Influence of Substrate and Strain on Protein Quality of _Pleurotus ostreatus_
## 3 Results and Discussion
### Protein Solubility Index (PSI)  
Protein solubility is a critical feature that determines the uses and applications of protein concentrates and isolates in the food industry. It is one of the most important functional properties of proteins that is governed by their water-binding capacity as well as by the degree of protein molecule denaturation. For this reason, protein solubility is used to estimate the overall protein quality [36; 38]. Moreover, protein solubility mostly affects the protein concentrate's foaming, emulsifying and gelation capacities. Several methods have been developed to evaluate protein solubility as a measure of protein quality based on diverse criteria such as the nitrogen solubility index (NSI), protein dispersibility index (PDI), protein solubility in 0.2% KOH (PS-KOH), urease activity, trypsin inhibitor activity and protein solubility index (PSI) [26; 29]. In the present study, the PSI was used to estimate the protein quality of the _P. ostreatus_ flours and PCs. This choice was based on the small sample size that the PSI procedure requires compared to the others found in the literature, as well as on its good reproducibility. Figure 2 presents the PSIs of the _P. ostreatus_ flours and PCs, which were calculated using Equation (2). In all cases, the PCs presented lower PSI values than their corresponding flour. The highest PSI value was noted for the mushroom flour from AMRL150 cultivated on both substrates and the lowest for AMRL144 cultivated on WS. A profound effect of substrate was observed, especially in the case of the PCs. The produced PCs of both mushroom strains when cultivated on WS were characterized by the smallest PSI values. Given that the PSI value is inversely proportional to the actual solubility, it can be inferred that the PCs should possess better functional properties than the flours. Moreover, a better protein quality is achieved when both strains were cultivated on WS. In the literature [29], a series of soy products were evaluated using the PSI procedure. It was found that for full-fat soy flour, low-fat soy flour and soy protein concentrate, the PSI values were equal to 86.0%, 55.5% and 13.0%, respectively. Thus, the  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**Protein Content (\% _mlm_ on a Dry Mass Basis)**} \\ \cline{2-3}  & **Mushroom flour (F)** & **Protein Concentrate (PC)** \\ \hline
150 WS & 17.2 \(\pm\) 0.1 \({}^{\text{a}}\) & 37.2 \(\pm\) 0.0 \({}^{\text{a}}\) \\
150 BOS & 24.6 \(\pm\) 0.4 \({}^{\text{b}}\) & 43.1 \(\pm\) 0.4 \({}^{\text{b}}\) \\
144 WS & 18.7 \(\pm\) 0.5 \({}^{\text{c}}\) & 39.8 \(\pm\) 0.6 \({}^{\text{c}}\) \\
144 BOS & 22.7 \(\pm\) 0.2 \({}^{\text{d}}\) & 43.5 \(\pm\) 0.1 \({}^{\text{b}}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Protein content (mean \(\pm\) standard deviation, n = 3) of _P. ostreatus_ flours and protein concentrates. Different superscript letters in each column denote significantly different contents (\(p\leq\) 0.05) between samples.  
quality of _P. ostreatus_ flours (mean PSI: 71.2%) and PCs (mean PSI: 17.7%) is close to that of soy protein, supporting the possibility for many applications in new food formulations and meat alternative foods [21].