## 3 Results and Discussion
### Determination of Antioxidant Properties by DPPH and FRAP Methods  
Antioxidant activity is a complex procedure and is influenced by many factors that cannot be described with only one single method. Therefore, it is important to perform more than one type of antioxidant capacity measurement to take into consideration various mechanisms of antioxidant action. In the study of these high-protein fat-free dairy desserts, antioxidant activity was tested using different methods (DPPH and FRAP). Applying the DPPH method, it was possible to measure the ability of the antioxidants to donate an electron or hydrogen radical to the stable DPPH free radical. Meanwhile, the FRAP method made a comparison of antioxidants based on their ability to reduce ferric (Fe\({}^{3+}\)) to ferrous (Fe\({}^{2+}\)) ions through the donation of an electron [47]. The results of our experiments are presented in Figure 4 (DPPH measurements) and Figure 5 (FRAP measurements). Figure 3: Effect of different WPC80 contents on water activity of high-protein fat-free dairy desserts. Letters (a–d) indicate significant differences at \(p<0.05\) ([PERSON]’s HSD test). Both selected spectrophotometric methods for determining the antioxidant properties follow the same reaction mechanism, i.e., electron transfer from antioxidant to oxidant (Fe\({}^{3+}\) and DPPH radical). All tested samples could reduce the stable, purple-colored radical DPPH into the yellow-colored DPPH-H. An increase in the value of measurement (\(\mu\)mol Trolox/g) along with the increase in value of WPC80 content (5-9%) was observed (Figure 4). The highest value of the measured parameter was observed in the sample with 13% WPC80 (0.38 \(\mu\)mol Trolox/g) and the lowest values were obtained in the product with 5% WPC80 (0.027 \(\mu\)mol Trolox/g). [PERSON] and [PERSON] [48] tested the antioxidant activity of yogurt fortified with sodium and calcium caseinate and 2 or 4% whey-protein concentrate. Their results suggested that the product fortified with 4% WPC80 had the highest (\(p<0.05\)) scavenging activity. They did not report significant differences among yogurt samples with the addition of lower amounts of this ingredient (\(p>0.05\)). They concluded that the scavenging effect of whey proteins could be attributed to lactoferrin, which has been reported as an important component for high scavenging activity [48]. The FRAP method examined the ability of the sample to reduce the ferric ion, which is a criterion of the antioxidative capacity [49]. This method has already been used to characterize the antioxidative capacity of a dairy product, e.g., whey proteins [50]. An analysis of the antioxidant properties of the high-protein fat-free dairy desserts was presented in Figure 5. In general, the values described in individual products remained at a similar level. However, the lowest values were obtained in the product with 7% WPC80 (0.33 \(\mu\)mol Trolox/g) and the highest in the sample with 13% WPC80 (0.58 \(\mu\)mol Trolox/g). Figure 4: Effect of different WPC80 contents on antioxidant properties (DPPH) of high-protein fat-free dairy desserts. Letters (a–d) indicate significant differences at \(p<0.05\) ([PERSON]’s HSD test). Figure 5: Effect of different WPC80 contents on ferric reducing antioxidant power (FRAP) of high-protein fat-free dairy desserts. Letters (a–b) indicate significant differences at \(p<0.05\) ([PERSON]’s HSD test). Literature describes lactoferrin as a whey compound with a strong antioxidative capacity [50; 51; 52]. Different studies suggest that the addition of whey-protein concentrate to dairy products significantly increases the antioxidative properties of the tested samples [51]. It is worth mentioning that the FRAP and DPPH methods are in vitro assays conducted without alternative targets and obtained data cannot be readily translated to more complex systems. Despite this, researchers suggest that whey proteins have the potential to survive in the gastrointestinal tract and the bloodstream post-consumption [52].