# The Influence of Substrate and Strain on Protein Quality of _Pleurotus ostreatus_
## 3 Results and Discussion
### Amino Acid Composition  
The AA composition of the _P. ostreatus_ flours is shown in Table 2. The total AA content ranged from 266.20 \(\pm\) 1.90 mg/g (on a dry mass basis) for AMRL150 cultivated on BOS to 202.45 \(\pm\) 1.67 mg/g for the same strain cultivated on WS. The highest concentration was found for both strains when cultivated on the BOS substrate, showing a similar trend as the protein content (Table 1). Glu, Arg, Asp and Lys were the predominant amino acids in all cases. In the literature, the sum of the acidic amino acids Glu and Asp predominates in the composition of mushroom protein flours [17] and usually dominates the composition of plant proteins [26]; in the present study, a higher content of the basic amino acids Lys, His and Arg was noted. The percentage of the total essential AAs (EAAs) constituted more than 50% of the amino acid composition of the mushroom samples under investigation. As can be observed in Table 2, the EAAs His, Thr, Arg, Val, Phe and Met together with the non-essential amino acids Glu + Gln, Ser, Ala, and Tyr were present in significantly higher amounts when both strains were cultivated on the BOS substrate. Similarly, on the same substrate, the amount of the EAAs, the sum of the acidic amino acids (aspartic and glutamic acid), the sum of the basic amino acids (histidine, arginine and lysine), as well as the amount of amino acids with an aromatic side chain (phenylalanine and tyrosine) were enhanced. On the contrary, in the literature [39], it was found that when maize or pumpkin straw was used for the growth of _P. ostreatus_, the nitrogen content and the amino acid content were not affected by the substrate. Figure 2: Protein solubility index (PSI, %) of _P. ostreatus_ flours (F) and protein concentrates (PC). Error lines denote standard deviation (n = 3). Different lower- and upper case letters correspond to significantly (\(p\leq\) 0.05) different contents among _P. ostreatus_ flours and protein concentrates, respectively. WS and BOS denote wheat straw and barley and oat straw substrate, respectively. To assess the protein quality of the _P. ostreatus_ flours, the amino acid score (AAS) (Table S2) and the essential amino acid index (EAAI) were calculated based on [PERSON] et al. (2022) and [PERSON] et al. (2018) [40,41] using whole egg protein as a reference. The amino acid with the lowest AAS is the first-limiting AA. According to Table S2, the first-limiting AA in all samples was Met, whereas the highest AASs (>100) were observed for Thr and Lys. The latter indicates that both amino acids fit the ideal protein standard proposed by the FAO/WHO/UNU [42]. The EAAI values for AMRL150 ranged from 86.3 \(\pm\) 4.5% to 93.4 \(\pm\) 6.2% when grown on the BOS and WS substrates, respectively. The corresponding values of EAAI for AMRL144 were 90.0 \(\pm\) 4.0% and 99.5 \(\pm\) 6.8% when grown on BOS and WS, respectively. According to the literature [43], EAAI values greater than 90% indicate high quality protein, whereas 70-89% indicates moderate quality. Obviously, both strains can produce high quality protein, especially when cultivated on WS. The findings in the literature support the higher nutritional value of mushroom protein compared to that of plant proteins [16]. [PERSON] et al. (2011) [44] analyzed the amino acid composition of eleven wild species in Turkey and reported a high abundance of Glu, Asp and Leu [44]. [PERSON] et al. (2022) [45] found that the powder of the edible fungus _Pleurotus citrinopilients_ had the highest hydrolyzed amino acid content, reaching 14.656 g/100 g, compared to that of _Agrocybe chazingsu_, _Flammulina veltipes_, _Lentius edodes_ and _Hericium erinaceus_.