## 2 Materials and Methods  
### Mushroom Pretreatments and Fermentation  
Fresh oyster mushrooms (Sylvan HK 35 grown on straw substrate; moisture content: 89.78 \(\pm\) 0.77%; protein: 144.34 \(\pm\) 1.82 mg/g in dry mushroom) were obtained from Magyar Gombakertesz Kft. (3395 Demjen 0173/7/A/1 Hungary). The mushrooms were harvested on the 36 th day after plantation and stored at 4 \({}^{\circ}\)C until their utilization. Cultivation method of Sylvan HK 35 was as follows. First, there is an intercropping period of 17-21 days (heat-treated straw with mycelium of the fungus). The core temperature of the blocks is 30-32 \({}^{\circ}\)C. Post-planting humidity is 92-95%. After interweaving, the temperature is reduced from 30 \({}^{\circ}\)C to 16 \({}^{\circ}\)C in 6 days. Temperature of the substrate is reduced from 20 \({}^{\circ}\)C to 13 \({}^{\circ}\)C, and relative humidity is reduced from 95% to 83%.  
The pretreatment steps were carried out at the pilot facility of the Department of Live-stock Products and Food Preservation Technology, Hungarian University of Agriculture and Life Sciences. After discarding any damaged and unwanted parts of the mushrooms, the remaining were thoroughly cleaned and longitudinally sliced (5-10 mm thickness) to be utilized for pretreatments and fermentation. For each pretreatment group, 2 kg of fresh oyster mushrooms was used. In addition to fresh oyster mushrooms (Fresh), six pretreatment methods were applied prior to mushroom fermentation: blanching in water (Blanch), steaming (Steam), oven cooking (Oven), microwaving (MW), high hydrostatic pressure (HHP), and ultraviolet light treatment (UV) (Figure 1). Pretreated mushrooms were allowed to cool down on perforated stainless-steel trays until further processing. The experiment was repeated two times.  
_Blanching in water_: Blanching was performed by immersing oyster mushrooms in boiling water (V = 7 L, mushroom:water ratio was 1:3) at 100 \({}^{\circ}\)C for 3 min.  
_Steaming:_ Steaming was performed at 100 \({}^{\circ}\)C for 3 min in a multifunctional oven (Lainox VE051P, Lainox, Vittorio Veneto, Italy) using the steam function. Mushroom monolayer on the stainless-steel tray was 10-15 mm thick.  
_Oven pretreatment_: Oven pretreatment was performed at 100 \({}^{\circ}\)C for 3 min in a multifunctional oven (Lainox VE051P, Lainox, Vittorio Veneto, Italy) using the oven cooking function without steam. Mushroom monolayer on the perforated stainless-steel tray was 10-15 mm thick.  
_Microwave pretreatment_: For the microwave pretreatment, mushroom samples were divided into 300 g portions and were subjected to 85 \({}^{\circ}\)C at 100 W for 3 min using the vegetable program (A3) setting in a microwave oven (SHARP R722 STWE, Sharp Electronics Europe Ltd., Middlesex, UK).  
_HHP pretreatment_: High-hydrostatic-pressure (HHP) treatment was applied at 20 \({}^{\circ}\)C and 300 MPa, with a holding time of 3 min (RESATO PU-100-2000, Resato International B.V., Assen, The Netherlands), in sealed plastic pouches (90 \(\upmu\)m PA/PE poach, (20 \(\upmu\)m PA-70 \(\upmu\)m PE, AMCO Kft., Budapest, Hungary).  
_UV light pretreatment_: During UV light pretreatment, mushrooms were irradiated with a 30 W UV light at 312 nm (VL-115.M, Vilber Lourmat, Marne La Vallee, France) for 15 min at 20 \({}^{\circ}\)C. To ensure even irradiation, 0.5-1 cm mushroom layer was placed evenly on an open shelf, 20 cm below the light source.