## 4 Discussion  
The feasibility of integrating advanced technologies, such as HHP, microwave, and UV light, alongside traditional methods like steaming, oven cooking, and water blanching prior to mushroom fermentation, was investigated. A total of 22 free amino acids were identified in the oyster mushroom samples. Three types of biogenic amines--spermidine, cadaverine, and tyramine--were detected in the pretreated control samples, while fermentation additionally produced histamine, putrescine, spermine, and agmatine. The total and essential amino acid contents of the pretreated samples were significantly higher than that of the fresh oyster mushrooms, with the microwave-pretreated samples exhibiting the most substantial improvement. All essential amino acids, except tryptophan, were detected in the mushroom samples. Essential amino acids, including arginine, comprised 35-44% of the total amino acids in the pretreated mushroom samples, indicating that the oyster mushrooms maintained high protein quality regardless of the pretreatment applied. The blanching and microwave pretreatments reduced the free amino acid content, whereas the steaming, oven, HHP, and UV light pretreatments increased it compared to fresh oyster mushrooms (34.07 mg/g). When the pretreatments were combined with fermentation, a decrease in free amino acid content was observed only in the blanched samples. The measured values are determined from the dry matter content; therefore, the loss of weight (yield) during treatments has a significant influence and may explain the reduction in TAA content after fermentation in the steamed, microwaved, HHP-, and UV-treated samples. In the case of UV treatment (that has almost no weight loss at the pretreatment)  
Figure 12: PCA plot (PC 1 vs. PC 2) of biogenic amine profile of mushroom samples. retained its proteins. This combined with the degradation of peptides and proteins due to the UV treatment may explain the high biogenic amine content after fermentation. The amino acid content, as measured on a dry matter basis, is higher in the fermented samples. The yield values resulting from the various pretreatment methods were 91.00% in the blanched, 92.33% in the steamed, 94.12% in the oven-pretreated, 80.00% in the microwaved, 98.65% in the HHP-treated, and 99.71% the UV-light-pretreated samples (the fresh untreated samples have a value of 100% as a reference). Following fermentation (the pretreated mushroom samples were the reference), the highest yield was observed in the microwaved samples (95.46%), followed by the blanched (92.32%) and steamed (89.59%) samples. The HHP and UV samples had the lowest yields post-fermentation, with 68.70% and 69.41%, respectively. These have a major effect on the results and also underscore the significance of pretreatment methods in influencing the yield during fermentation. The increased values of the BA content measured in this study may have multiple explanations. In the fresh fermented samples (i), there was no pretreatment and, thus, the cell count was higher. In the case of the HHP-pretreated samples, there was a perforated membrane structure (ii), leading to the higher accessibility of the substrates. The UV-light-pretreated fermented samples showed a considerably higher BA content. The bactericidal effect is limited to the surface, so between the gills, its lethality is minimal, which allows for (iii) the survival of a higher active cell count in the sample. As the UV light accelerates protein degradation, resulting in a higher FAA content, (iv) it also promotes the degradation of essential amino acids, which are the precursors of BA formation. There are only limited reports of similar research in the literature. The biogenic amine content of putrescine and histidine after the fermentation of white and brown button mushrooms with individual lactobacillus species showed similar negative, undetectable results [46]. Cadaverine was detectable in our fresh, steamed, and high-pressure-pretreated laska samples.