## 1 Introduction
Interest in red bell pepper consumption is motivated mainly by the presence of bioactive compounds that act as essential antioxidants in the diet. Red bell pepper is an excellent source of vitamin C, carotenoids, and polyphenols. The phytochemical composition of peppers can be affected by various factors, including maturity, genotype, cultivation practices, processing methods, and storage conditions [20]. Red bell pepper fruits typically contain between 70 to 400 mg of vitamin C per 100 g of fresh weight, with the highest concentrations found in colored fruits and those that are overripe, although levels decrease significantly in overripe specimens [21]. In a study conducted by [PERSON] and [PERSON] [20], it was observed that the levels of ascorbic acid, carotenoids, and phenols varied among red, yellow, and green peppers at full fruit maturity. Red bell peppers exhibited higher levels of ascorbic acid than green and yellow peppers. Additionally, red and yellow peppers showed higher carotenoid content than green peppers. Among the varieties, the red sweet red bell pepper, _Slonovo uvo_, exhibited the highest vitamin C and polyphenol content, with 204 mg/100 g and 202.6 mg GAE/100 g, respectively. The characteristic flavor of peppers is attributed to capsaicin, which is present in both sweet and hot varieties, although hot varieties contain higher levels of it. The red coloration of ripe peppers is due to carotenoid pigments such as capsanthin, capsorubin, cryptoxanthin, zeaxanthin, or \(\beta\)-carotene. The intensity of the red color primarily depends on the amount of the pigments capsanthin and capsorubin, which account for 30-60% and 6-18% of the total carotenoid content, respectively. Research shows that most vegetables can undergo lacto-fermentation due to their natural sugar content. Red bell peppers, recognized for their outstanding nutritional benefits, present an up-and-coming candidate for this process [22; 23]. This study aimed to assess the effects of the non-thermal pre-treatment method on the red bell pepper fermentation process. Two different types of ultrasound methods were used: ultrasound by immersion and ultrasound with sonotrode. Different times for the application of US were tested too. The research examined the physical and chemical properties of red bell peppers, including dry matter content, color, polyphenol content, antioxidant activity, carotenoid content, texture, and structure, both before and after the ultrasound pre-treatment. In the final phase, this study included fermentation with the lactic acid bacterial strain _Lactiplantiobacillus plantarum_ ATCC 4080, assessing the properties, pH, and lactic acid bacteria counts in the fermented product. The following research hypotheses were tested:  
* The application of ultrasound pre-treatment, varying in duration and method, significantly affects the lactic acid fermentation process of red bell peppers inoculated with a single bacterial strain. * Ultrasound pre-treatment induces measurable changes in red bell peppers' physical and chemical properties, including carotenoid and polyphenol content, texture, dry matter, pH, color, and internal structure.