## 1 Introduction
The resulting inflammation may be a fundamental factor leading to the aggravation of complications of many chronic diseases. The most common disease associated with oxysterols is atherosclerosis. Probably the factor that starts the formation of atherosclerotic plaque is damage to the vascular endothelium, which causes monocytes to accumulate, then move to the vessel walls and transform into macrophages. Further reactions caused by excited monocytes and macrophages lead to the production of foam cells that form atherosclerotic plaque. 7\(\alpha\) and 7\(\beta\)-hydroxycholesterol and 7-ketocholesterol cause an inflammatory phenotype in endothelial cells; additionally, 7-ketocholesterol induces the formation of foam cells [14]. Many neurodegenerative diseases, such as Parkinson's disease, Huntington's disease, multiple sclerosis, Alzheimer's disease, and amyotrophic lateral sclerosis, have been associated with oxidative stress. It has been proven that 7-ketocholesterol has a strong cytotoxic effect on neuronal cells; its presence in the brain can cause significant damage and contribute to the aggravation of the above-mentioned diseases [18]. Regrettably, no safe dose of sterol oxidation products has been determined so far, possibly because these compounds can also form within the human body, for example, during digestion or enzyme interaction. Figure 1: Chemical structure of \(\beta\)-sitosterol. Among the methods used for the analysis of sterol oxidation products, gas chromatography with a flame ionization detector can be mentioned. Another group of methods used for this application are high-performance liquid chromatography with UV (ultraviolet detection) or ELSD (Evaporative Light Scattering Detection) detectors. The above-mentioned separation methods coupled with a mass spectrometer (GC-MS, LC-MS) or a tandem mass spectrometer (GC-MS/MS, LC-MS/MS) are the most common techniques for the detection and quantitative analysis of sterol oxidation products, e.g., 7-ketocholesterol, hydroxysterols, epoxycholesterol, etc. [19]. Such techniques require the alkaline, acidic, or enzymatic hydrolysis of the fat fraction. The next step of the sample preparation before GC or LC analysis is the saponification or transesterification of the samples. Often, sterols and oxysterols are derivatized before injection into a gas or liquid chromatograph (trimethylsilyl derivatives) to obtain more stable derivatives than free sterols/oxysterols. The application of mass spectrometry coupled with GC or LC provides better qualitative analysis of sterols and their derivatives. Different methods of molecular ionization, e.g., electron ionization, can be used to obtain a specific mass spectrum, which can be compared with the mass spectrum of standards or compared with the mass spectrum from libraries [19]. Such techniques requires specific reagents, e.g., for hydrolysis and derivatization, and advanced measuring equipment and trained personnel. Currently, the interest of scientists in testing the amount of oxysterols in Polish food products has decreased significantly. One of the most recent research studies describing the content of oxysterols in Polish dairy products was published by [PERSON] et al. (2024). The authors claim that the following COPs were identified: 7\(\alpha\)-hydroxycholesterol, 78-hydroxycholesterol, 5,6\(\beta\)-epoxycholesterol, 5,6\(\alpha\)-epoxycholesterol, 5,6\(\beta\)-epoxycholesterol, and 7-ketocholesterol. Cheese with internal mold and cheddar contained 13.8 and 11.7 mg/kg, respectively. On the other hand, yogurt and kefir contained 0.94 and 0.57 mg/kg, respectively. The authors reported that the concentration of total oxysterols in dairy products represented 1.7% of cholesterol content [20]. Another study performed by [PERSON] et al. (2024) aimed to detect oxysterols in eight different animal origin products.