# Comprehensive Evaluation of the Nutritional Quality of Stored Watermelon Seed Oils
## 3 Results and Discussion
### Determination of Acid Value  
The acid value (AV) indicates the degree of freshness of the fats. It describes the degree of hydrolysis of the tested oils. It is also a measure of the free fatty acids content. The Codex Alimentarius specifies the requirement that the AV of cold-pressed vegetable oils does not exceed 4 mg KOH/g fat, while for refined vegetable oils the limit is 0.6 mg KOH/g fat [41]. The average AVs of the studied oils, obtained during the testing period, met the requirements specified by Codex Alimentarius (Figure 2). It should also be noted that the lowest average AV (0.29 mg KOH/g) was reported for refined watermelon seed oil (WO\({}_{\text{ref}}\)). This oil exhibited the highest hydrolytic stability compared to the other oils. Moreover, its AV was not affected by the storage process. The low AV for WO\({}_{\text{ref}}\) indicates the freshness of the tested oil. The presented results are consistent with the literature data for refined oils, as both free fatty acids and primary oxidation products are removed from the oil when subjected to the refining process, as reflected in its low AV values [42,43]. The quality of cold-pressed oil depends mainly on the quality of the seeds. The most important factors influencing the quality of seeds include, among others, the degree of their maturity, the content of damaged seeds, the content of other impurities, the moisture content of the seeds, and the storage conditions [44]. The quality of cold-pressed oil is also influenced by the hygienic conditions during processing, the conditions and parameters of the pressing, the filtration process, and the storage conditions of the finished product. Therefore, the quality of such oils can be quite diverse [45]. This is confirmed by the AVs obtained for the cold-pressed oils tested in this study. WO\({}_{1}\) and WO\({}_{2}\) consistently showed low AVs across all storage dates. The greatest degree of hydrolytic changes was observed in the case of WO\({}_{3}\) which exhibited AVs more than twice as high at every stage than those of other cold-pressed oils. The higher AV of WO\({}_{3}\) (1.96 mg KOH/g) may indicate that the oil was obtained from partially damaged seeds or that the pressing took place in undesired conditions. Oil extracted from damaged seeds undergoes hydrolysis. A greater degree of damage stimulates the activity of lipolytic enzymes, which is reflected in an increase in the AV [46]. An effect of storage time on the AV of cold-pressed oil was also observed, with an increase in the average AV for all tested samples following three months of storage after opening. An increase in the average AV was noted for all tested cold-pressed oils following three months of storage after opening, particularly in the case of WO3. [PERSON] et al. [21] extracted watermelon seed oil through cold-pressing, reporting an AV of 2.80 mg KOH/g fat. [PERSON] et al. [47] observed a significantly lower AV of 1.63 mg KOH/g fat for cold-pressed watermelon seed oil. [PERSON] et al. [35] analyzed cold-pressed oils available in grocery stores, noting an AV of 1.15 mg KOH/g fat for watermelon seed oil. In the present study, unrefined cold-pressed watermelon seed oils exhibited AVs ranging from 0.84 to 1.96 mg KOH/g fat at the beginning of the testing period. These results are consistent with the findings of other researchers, with slight variations, likely due to differences in seed quality, processing hygiene, or pressing parameters. The WOext had a higher AV than cold-pressed oils, which is probably related to the hydrolytic changes that may have occurred as a result of heating the oil during the evaporation of the solvent used in the extraction process. The degree of hydrolysis of this oil may also have been influenced by the quality of the seeds used for oil extraction [46]. However, it is worth noting that the AV value did not depend on the storage time. [PERSON] et al. [48] extracted watermelon seed oil using a Soxhlet apparatus with the same solvent as used in the present study, resulting in AV of 2.88 mg KOH/g fat.