# The Influence of Substrate and Strain on Protein Quality of _Pleurotus ostreatus_
## 1 Introduction  
Global mushroom cultivation is continuously increasing due to their nutritional value and culinary properties [1]. _Agaricus bisporus_, _Lentiula edodes_ and _Pleurotus_ spp. are the most cultivated mushrooms worldwide [2, 3]. Edible mushrooms constitute a rich source of carbohydrates and protein [4, 5]. The high molecular mass carbohydrates primarily consist of polysaccharides that have been shown to exert multiple benefits for human health [6]. Among them, \(\beta\)-gluucans are the most important, with various biological functions [7]. Moreover, _Pleurotus_ spp. contain all nine essential amino acids, making them a viable alternative source instead of animal protein, which is ideal for vegetarians [8]. The fat fraction of edible mushrooms, although quantitatively low, contains valuable unsaturated (oleic acid) and polyunsaturated (linolenic acid) fatty acids [4, 9]. Several bioactive compounds have been also detected in mushrooms such as phenolics, tocopherols, ergosterol, vitamins B1, B2 and C, and minerals [10, 11, 12]. Among mushrooms, _Pleurotus_ spp. are the most suitable higher fungi that can be used to colonize and fructify on several types of agro-industrial residues and wastes [5, 13]. Apart from wheat straw, which is the most common cultivation substrate, other materials, such as barley and oat straw, been wood shavings, coffee residue, rice bark, etc., have been examined as alternative substrates [5; 13; 14; 15]. Compared to plant proteins, which lack some essential amino acids, mushrooms are considered to cover the dietetic requirements [16]. Moreover, their high glutamic acid, aspartic acid and sulfur amino acid (methionine and cystine) contents contribute to the meaty taste of processed foods and thus may be served as flavor enhancers [17]. _Pleurotus_ spp. have been widely studied for their role in the development of functional foods [18; 19; 20; 21]. [PERSON] et al. (2022) [21] summarized the uses of _P. ostreatus_ in the food industry. Among them, mushroom powder has been used as a meat replacement in chicken nuggets and beef salami; as a partial wheat flour replacement in biscuits; and as a substrate to produce novelty alcoholic drinks with unique flavors. The manufacturing of PCs has been studied to enhance the uses of _P. ostreatus_. The species, strain and growth substrate of the mushrooms, together with the growing environment and maturity stage, all have a significant impact on PC quality [18; 21]. Moreover, the protein extraction conditions directly affect the functionality of the produced PCs due to the molecular changes that may occur and are linked with the disruption or stabilization of chemical bonds, mainly non-covalent interactions [22]. The most commonly used approach for protein isolation is based on alkaline extraction followed by isoelectric precipitation (AE-IP) of the protein fraction. Due to the severe alkaline conditions (pH 8.0-12.0), protein denaturation and exposure of hydrophobic bonds may occur, which result in decreased solubility of the extracted proteins. Moreover, partial protein degradation may occur, resulting in increased proportions of low molecular weight proteins and free amino acids that are susceptible to degradation, thereby influencing the amino acid composition of the PCs [23; 24]. It has been found [25] that by applying a nanoparticulation strategy of the protein fraction received after AE-IP, the protein dispersibility in water and the emulsion capacity can be improved. It was also found that a combination of different extraction conditions (i.e., type of solvent, salt addition and pH) could effectively separate plant proteins based on their solubility characteristics, whereas enzymatic hydrolysis is mostly preferred due to the gentle pH and temperature conditions, which enhance the solubility of the protein isolates [16; 26]. The increasing demand for alternative protein sources to replace animal proteins has brought mushrooms to be the top choice for the food industry and consumers.