## 1 Introduction  
Mushrooms have been considered a delicacy for centuries, due to their distinct texture, aroma and exceptional flavor, while pertinent research evidenced unique nutritional and health-promoting benefits [1, 2, 3]. Mushrooms' popularity steadily increased during the last decades, leading to a 30-fold increment of the world production of edible mushrooms, with _Lentinus edodes_ and _Pleurotus_ (\"oyster\") spp. accounting for over 40% of the respective total supply [4]. _Pleurotus_ (oyster) mushrooms are low energy, low fat, low sodium and cholesterol-free food items, being at the same time rich in proteins, minerals, functional polysaccharides like chitin and \(\beta\)-glucucas, and water-soluble vitamins. In addition, they contain health-promoting bioactive microconstituents like ergosterol (provitamin D2), phenolic acids, the antioxidant amino acid ergothionein and lovastatin [5, 6]. Currently, several species of _Pleurotus_ are produced worldwide (e.g., _P. ostreatus_, _P. pulnomarius_, _P. rgrngii_, _P. djamor_ and _P. citrinopileatus_), by exploiting a wide range of lignocellulosic residues as substrates [7]. Of particular interest is their cultivation on olive mills and wineries by-products (i.e., materials that are difficult to manage due to their high content in organic compounds, notably lipids and polyphenolics), which is in line with the concepts of waste biodegradation and valorization [5; 8].  
Volatile compounds emitted by fungi are not only associated with key-functions related to growth physiology and mycelium interactions [9; 10], but they also add to the quality of edible mushrooms [11; 12]. They are additionally formed as a result of post-harvest processing, especially after thermal treatment [13; 14]. The characteristic odor of mushrooms is the combined result of several groups of compounds, such as alcohols, aldehydes, ketones, acids, hydrocarbons, esters, and heterocyclic, aromatic and sulfur compounds [12; 15; 16; 17]; their relative abundance may vary among species, strains, stage of growth and part of mushroom examined (i.e., cap, gills or stipe) [18; 19; 20].  
More than 110 volatile compounds have been identified so far in mushrooms; among them, eight carbon (C8) compounds account for 44-97% of volatiles fraction, with 1-octen-3-ol, 3-octanol, 3-octanone, 1-octanol and 1-octen-3-one being detected in most cases [2; 15; 21; 22]. In addition, of those identified only 3% are odor-active and can contribute to the aroma or flavor of foods [12; 23].  
Common laboratory methods for the isolation of aroma compounds include supercritical extraction, simultaneous distillation-extraction, dynamic headspace, and headspace solid phase microextraction (HS-SPME); the latter is an easy, rapid, relatively simple technique that allows the isolation and concentration of volatile analytes in a solvent free, environmentally friendly, manner [2; 21; 22; 24]. Coupled with gas chromatography-mass spectrometry (GC-MS), HS-SPME is considered a powerful tool with high reproducibility and sensitivity [21; 24; 25; 26; 27].  
In the present study, we investigated the influence of genetic (species, strain) and environmental (substrate) factors on the profile of volatile compounds produced by eight _P. ostreatus_ and _P. arggini_ strains (four per species), which were cultivated on substrates composed of various agricultural and agro industrial by-products. The main objective was to verify whether a discrimination based on volatile aroma compounds was possible among species, strains and/or substrates. Volatiles were extracted from fresh fruit bodies with HS-SPME and analyzed by GC-MS. The same analytical scheme was applied to selected samples to assess the influence of cooking (roasting) on volatiles' profiles.