## 3 Results and Discussion
### Colour Measurements
In turn, according to [PERSON] [80], the caps of fresh oyster mushroom fruiting bodies are darker (L* = 75.94 and 78.09), while the other parameters range from \(-0.04\) to \(-0.21\) and from 12.29 to 13.05 for parameter a* and b*, respectively. According to other researchers, the color parameters of freeze-dried button mushrooms are as follows: parameter L* from 73.39 to 86.19, parameter a* from 0.89 to 1.66, and parameter b* from 10.22 to 15.29 [14; 81]. In the case of freeze dried oyster mushrooms, the whiteness parameter L* ranged from 82.42 to 84.72, while the value of the chromatic coordinate a* ranged from \(-0.58\) to 1.49, and for the chromatic coordinate b* it was 12.42 and 15.47 [82; 83]. The color of dried mushrooms is influenced by many factors, including preliminary processing (e.g., washing, cutting) [84] and the drying method [85], as well as the parameters used [86]. During pre-treatment, the chemical composition (phenolic compounds, free amino acids) and the activity of oxidizing enzymes (e.g., peroxidase, polyphenol oxidase) contributes significantly to changing the color of the raw material [87]. _A. bisporus_ are mushrooms with a high polyphenol content and high enzymatic activity [88]. The drying method also determines the color of the raw materials. When convective drying is used, the main role in the darkening of dried products is played primarily by Maillard reactions [89], while in freeze drying, the enzymatic activity of raw materials [90] has a significant impact. According to [PERSON] and [PERSON] [83], the L* value of _P. ostreatus_ increased after freeze-drying. This is confirmed by the results obtained, where the mushroom powder from white button mushrooms is clearly darker than that obtained from oyster mushrooms. The effects of partial replacement of wheat [21; 22], whole grain [91], barley [72] and sorghum [24] flour with mushroom powder on the color change of cookies have been observed by other researchers. Regardless of whether the basic flour was replaced by an amount of 1-5% [22] or even up to 100% [21] mushroom powder, the value of the L* parameter was lower compared with control cookies. Only cookies made from whole grain flour where 10% and 30% of the flour was replaced with dried oyster mushrooms showed a similar L* value [91], and cookies in which sorghum flour was replaced by 5%, 10%, or 15% silver ear mushroom powder had L* values that were significantly higher [24]. In the case of the chromatic coordinate a*, there was no significant effect of supplementation with dried _Pleurotus albidus_[21], and there was increase in the value of this parameter for cookies supplemented with _Cordyceps militaria_ powder [22], while there was a decrease in the redness of cookies with the addition of _P. ostreatus_[91], shiitake, or black ear mushrooms [24] compared with the control. For the chromatic b* coordinate, an increase in the value was observed in the case of cookies supplemented with _C. militaria_[22], _A. bisporus_[72], or silver ear mushrooms [24] and a decrease in the yellowish of cookies supplemented with _P. albidus_[21], _P. ostreatus_[91], shiitake, or black ear mushrooms [24]. The change in the color of the surface of bakery products is the result of non-enzymatic browning reactions. i.e., Maillard reactions and caramelization, which result in the formation of colorful high-molecular compounds, caramels, and melanoidins [92]. Maillard reactions depend on the presence of reducing sugars, free amino acids, proteins, and nitrogen-containing compounds, while caramelization occurs as a result of the reaction of reducing sugars at appropriately high temperatures. These reactions occur more intensively with lower water content, i.e., mainly on the surface of bakery products, where water evaporates intensively [49].