## 3 Results and Discussion  
### Acid Value, Peroxide Value, and Oxidative Stability  
The results of acid value testing ranged from 0.11 (walnut oil) to 2.49 (milk thistle seed oil) mg KOH/g (Table 1). All these values were in accordance with the limit provided by AOCS (2004) for oils produced with cold-pressing technology (4 mg/g). Milk thistle seed oil showed the highest value, primarily because this kind of oil is rich in unsaturated fatty acids, which are extremely susceptible to hydrolysis in the presence of light and moisture. This results in higher concentrations of free fatty acids and a greater acid value [18]. The acid value is an important indicator of the quality of oils, especially from the point of view of storage. Higher values indicate oxidation, which decreases the quality of oil. This parameter reflects the abundance of acidic substances that are produced through the oxidation of hydrocarbons. Thus, the acid value directly represents the degree of oil degradation [19]. Eating rancid edible oil is unlikely to have an immediate negative effect on human health [20]. Such fats, however, have drastically decreased nutritional value because of the degradation of nutrients and vital fatty acids [20]. Similar results were reported by [PERSON] et al. [21], who analysed soybean, palm, mustard, and bran oil. The acid value was the lowest in soybean oil--0.61 mg KOH/g and the highest in mustard oil--6.29 mg KOH/g. The flax seed oil samples in our study showed an acid value of 0.15 mg KOH/g, which was lower than that reported in a study of [PERSON] et al. [22], where a value of 0.89 mg KOH/g was given for oil originating from Canada. [PERSON] et al. [20] confirmed that a high moisture content in oil negatively impacts the acid value and causes rancidity, which makes storage conditions very important in shaping the quality of oil. Preferably, the storage should be done in a cold, dry, and dark place. The degree of oxidation of lipids, fats, and oils is determined by analysing the concentration of peroxides. The amount of total peroxides in a sample is represented by the peroxide value, which is associated with rancidity in lipid-rich food products. Oxidation is one of the primary causes behind the development of off-flavours, quality deterioration, and the formation of toxins in oils [23]. In our study, peroxide values ranged from 2.27 (milk thistle oil) to 8.51 (flax seed oil) mmol O\({}_{2}\)/kg. A high susceptibility to oxidation limits the use of flax seed oil in the food industry. During storage, lipids undergo hydrolysis and oxidation, which leads to the formation of polymers and volatile substances rich in oxygen. Oxidation of flax seed oil, rich in \(\alpha\)-linolenic acid, is accompanied by significant colour changes. Flax seeds contain many cyclic peptides and cyclolinopeptides. Some of these peptides are coextracted during cold pressing, which usually gives the product a mild nutty flavour when fresh [21]. However, a persistent bitter aftertaste develops during storage of the oil at room temperature. It is caused by methionine sulfoxide originating from the conjugated linoleic acid present in flax seed oil in amounts of 485 to 925 mg/kg [23]. Higher peroxide values were also determined in grape seed and poppy seed oil, but they were still in accordance with AOCS [16]. Generally, the peroxide value should not be higher than 20 mmolO\({}_{2}\)/kg in cold-pressed oil [16]. This requirement was fulfilled in all the samples tested in our study. [PERSON] et al. [24] determined the peroxide value of