## 3 Results and Discussion  
### Antioxidant Properties of Extracts Obtained from Walnut Husks  
The antioxidant properties of walnut husk extracts determined using the ABTS, DPPH and \(\beta\)-carotene methods are presented in Figure 1 whereas using FRAP and CUPRAC methods are shown in Figure 2. As can be seen from the presented data, the tested extracts exhibit different antioxidant properties depending on the research method and extraction techniques used, as well as the type of extraction solvent. These relationships are known from the literature [27]. As shown previously [9], higher temperatures enhance the extraction yield of compounds and increase the biological activity of extracts. Changing the type of extractant results in a variation of the extraction selectivity and likely modifies the biological activity. When comparing the antioxidant properties of the extracts, it can be noticed, however, that the higher the extraction temperature, the greater the antioxidant properties [19]. This fact is rarely raised in the literature because an increase in temperature generally favors the thermal degradation of compounds, resulting in a decrease in antioxidant properties. According to the presented data, the highest antioxidant properties are obtained for extracts obtained using PLE, and the weakest for maceration. This fact is known from the literature because the PLE technique is characterized by a higher analyte mass transfer rate compared to classical approaches [28]. As for the influence of the method of assessing antioxidant properties, as can be seen in Figure 1, the weakest antioxidant properties (lowest % inhibition values, lowest bars) are demonstrated by the DPPH method, while the ABTS and \(\beta\)-carotene methods are more sensitive, resulting inhigher % inhibition values (higher bars). Differences in antioxidant properties between methods result from the fact that a different type of antioxidant activity is determined in them. In the ABTS and DPPH methods, the ability of the sample to neutralize colored radicals (ABTS cation radical and DPPH radical, respectively) is measured, while in the \(\beta\)-carotene method, antioxidant properties are demonstrated by substances that have the ability to delay the oxidation of the standard molecule (\(\beta\)-carotene) [29] Hence, there are differences in the percentage of inhibition obtained between the \(\beta\)-carotene method and the two methods mentioned above. Figure 1: Comparison of the antioxidant properties of walnut husk extracts obtained by extraction in the Soxhlet apparatus (blue bars), maceration (red bars) and pressurized liquid extraction (PLE) (green bars) using different extractants (chloroform, acetone, ethyl acetate and methanol) and methods for assessing antioxidant properties (ABTS, DPPH and \(\beta\)-carotene). Figure 2: Comparison of the antioxidant properties of walnut husk extracts obtained by extraction in the Soxhlet apparatus (blue bars), maceration (red bars) and pressurized liquid extraction (PLE)(green bars) using different extractants (chloroform, acetone, ethyl acetate and methanol) and FRAP and CUPRAC methods for assessing antioxidant properties. In turn, the differences in antioxidant properties between the DPPH and ABTS methods result from differences in the availability of unpaired electrons in radicals. In the former, the unpaired electron is less accessible due to spherical hindrance, hence the lower inhibition percent in the DPPH method [30]. Figure 2 shows the antioxidant properties, expressed as the absorbance value obtained for the examined extracts, assessed using the FRAP and CUPRAC methods. As can be seen from the presented data, the values obtained for the FRAP method exceed those obtained using the CUPRAC method. Most likely, despite the same nature of the measurement (both methods determine the ability to reduce metal ions, Fe\({}^{3+}\) and Cu\({}^{2+}\), respectively), they are not the same.