## 4 Discussion
### Oxidative Stability  
The ORP parameter has been used in this study as an indicator of a biochemical system's ability to receive or donate electrons, and thus can serve as an indicator of factors with antioxidant properties in a product that naturally protects against changes in fat. The higher the antioxidant effect (the greater the possibility of electron donation and the elimination of free radicals), the lower the redox potential. The ORP value in this study ranged from 235.9 mV (SU after 42 days) from 333.5 mV (C after seven days), and the ORP values obtained were within the ranges previously reported in the literature [9; 20; 23]. The changes of this parameter with time and treatment were detected. These differences may indicate the proportion of compounds released from the protein sequence, such as peptides of low molecular weight or free amino acids with antioxidant properties [22].  
An indirect method for assessing the ability of antioxidant compounds present in the meat tissue is to assess the rate of lipid peroxidation by TBARS as major oxidation products of polyunsaturated fatty acids. The results are related to the amounts of secondary fat oxidation products and were expressed in milligrams of MDA in one kg of the product. Similar values of TBARS obtained in this study were observed by [PERSON] et al. [24] in Pastirma (dry-cured beef), which were airtight packed and stored for 60 days, e.g., within 0.86 and 1.54 mg MDA kg\({}^{-1}\). An increase in TBARS was observed with prolonged storage time, especially between 7 and 21 days, which indicates intensified oxidation processes in the product. The lowest TBARS values were recorded in the C samples after 7 days of storage, while the highest values were recorded in the SW samples (Table 5). These results are probably indicative of the involvement of nitrogen compounds as a strong factor inhibiting lipid oxidation. In contrast, the higher content of TBARS in the whey variants is probably due to the presence of H\({}_{2}\)O\({}_{2}\) in acid whey resulting from the action of LAB, which may act as a catalyst for oxidation reactions. In addition, a decrease in TBARS (between 21 and 42 days) was observed in batches, probably when MDA covalently binds to amino acids from proteolytic reactions [25] or residual nitrite [26]. Generally, no effect of ultrasound treatment on the TBARS index was noted. In the literature, only a few studies have evaluated the impact of ultrasound on lipid and protein oxidation in meat and meat products. However, the results are not conclusive. Generally, it is believed that the initial application of ultrasound leads to changes in the microstructures of objects due to the mechanical effects of cavitation (strong shear forces), but they can also form free radicals, resulting in increased lipid and protein oxidation. These assumptions contradict the obtained results, which may be due to the application of different ultrasound treatment regimens. [PERSON] et al. [27] found that ultrasound speeds up the biochemical response rate, as evidenced by the increased content of TBARS in cobia (_Rachycentron canadum_) sashimi tenderized with ultrasonic water bath. Analysis of the TBARS content in beef showed that ultrasound treatment significantly increased lipid oxidation compared to the nonsonicated sample, which was salted with 6% NaCl (for comparison, in this study it was 2.8%) and exposed to ultrasound power [28]. [PERSON] et al. [29] found similar results, where TBARS increase in Italian salami was accelerated by ultrasound (25 kHz, 500 W, 6-9 min in water). However, [PERSON] [30] and [PERSON] et al. [31] did not find oxidative changes in ultrasound-treated beef and pork, respectively. Nevertheless, [PERSON] et al. [32] found a reduction of lipid oxidation in ultrasound-treated sausages.