# The Flavor Profiles of Highland Barley Fermented with Different Mushroom Mycelium
## 1 Introduction  
Highland barley (_Hordeum vulgare_ L. _var. nudum Hookf._) grows at high altitude and in cold regions. It is mainly distributed in Tibet, Qinghai and other places in China and is an important highland cereal crop. It is resistant to barrenness and cold, but with a high yield and wide adaptability [1]. Highland barley has a high protein (10-17%), dietary fiber (11%-34%) and vitamin (1.5-2.5%) content, but with low fat (2-3%) and carbohydrate (65-68%) contents [2]. The \"three high and two low\" nutritional characteristics of highland barley give it potential to be an ingredient in healthy foods [3]. Currently, highland barley mainly has been used as a main ingredient in noodles, cookies and beverages [4]. However, due to its grainy texture and poor processing properties, the application of highland barley in foods is still limited. Compared with other grains, the high levels of volatile compounds, such as hexanal and decanal, provide a strong grassy flavor in highland barley [5]. As a result, consumers' acceptance of highland barley products is low [6]. Therefore, it is necessary to find appropriate processing methods to modify highland barley's sensory characteristics and broaden its application.  
Currently, grain processing property modification can be achieved by physical, chemical and biological methods [7]. Physical modifications are generally divided into thermal and non-thermal modification. Thermal modification includes pre-paste and hydrothermal treatments (heat and moisture treatment (HMT) and annealing (ANN)) [8; 9], while non-thermal modifications include high-pressure treatment (HPP), micromization, ultrasonic, pulsed electric field (PEF), etc. [10; 11]. Chemical methods use derivatization reactions(etherification, esterification, cross-linking) or hydrolysis and oxidation reactions to modify grain chemical structures [12]. Compared with other modification methods, fermentation has the advantages of lower cost and high yield. Beneficial metabolites can be produced during fermentation as well [13]. Large molecules are degraded to small molecules after fermentation, which improves flavor characteristics [14]. Strains commonly used in the food industry are _Lactobacillus plantarum_ and yeast. They can provide strong sour and winey flavors to fermented foods, respectively [15].  
Mushroom mycelium is easy to grow and has aromatic flavors during its growth and development. Solid fermentation of grains with mushroom mycelium brings out the sweetness of flowers and herbs [16]. It has been reported that the fermentation of soybean residues with _Stenotrophomonas_ edible fungi was able to reduce the legume flavor, and the flavor profile of the fermented soybean residue were described as floral and sweet [15].The fermentation of wheat bran using _Fomitopsis pinicola_ resulted in decreases in aldehyde and lipid contents and an increase in ketones and phenols [17]. It has also been found that fermentation of bran with mushroom mycelium can improve its nutritional values and form aromatic components [18]. This evidence provides the basis for the fermentation of mushroom mycelium to improve the flavor of grains. However, there is limited research on the flavor modification of highland barley with mushroom mycelium.  
Differences in metabolism among mushroom strains could lead to different flavor profiles in the final product [19]. Therefore, strain selection is considered to be a key step. Screening the suitability of mushroom strains would be of great importance. The aims of this study were to modify highland barley with a solid-state fermentation by using five different edible mushroom strains (_C. millitaris, S. rugoso-annulata, S. commune, T. sanguinea_ and _M. esculenta_). The aroma characteristics of different types of fermented highland barley were investigated. The volatile profiles of fermented highland barley were analyzed by E-nose and headspace solid-phase microextraction (HS-SPME) with GC-MS. A sensory flavor evaluation was also used to analyze the aroma profiles.