## 3 Results and Discussion
### Structure of the Bell Pepper Samples  
The internal structure of plant material is a critical factor influencing tissue appearance and structural properties, such as shrinkage and porosity. These characteristics significantly affect processes like water diffusion during various biological activities. The structure is shaped by processing methods, including freezing, drying, ultrasound treatment, pulsed electric fields, and fermentation [30; 46; 53]. The microstructure of pepper samples was analyzed using scanning electron microscopy (SEM) and \(\mu\)CT microtomography, revealing notable differences in internal structures. Cross-sectional images obtained via SEM and two- and three-dimensional \(\mu\)CT microtomography (Table 3) illustrated significant structural changes. Fresh, untreated peppers displayed a closed, homogeneous texture with few small pores. In contrast, peppers subjected to ultrasound treatment exhibited damaged tissue structures, including more shrinkage and numerous large, irregularly shaped pores forming open spaces. The extent of the tissue damage increased with longer sonication times for both immersion and contact methods, particularly in sample S_5. Control samples showed reduced uniformity, with larger pores forming open spaces. Pores were also visible on the peel, and the tissue had a shredded appearance. Fermented samples with pre-treatment showed greater damage than the control fermented sample, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **SEM** & **Micro-Tomography** & **SEM** & **Micro-Tomography** \\ \cline{2-3}  & & **Non-Femented** & **Fermented** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Structure of the samples. Photography from scanning microscope and micro-tomography. characterized by ruptured pore boundaries and larger open spaces. The degree of tissue damage correlated with pre-treatment methods and parameters. Longer sonication times or higher pulsed electric field energy exacerbated tissue damage. Similar findings were reported by [PERSON] and [PERSON] [46], where carrot tissue subjected to ultrasound showed greater damage than untreated samples. Larger pores formed, and damage increased with longer sonication times, regardless of ultrasound frequency. The cavitation effect likely caused these changes, creating microscopic channels that evolved into interconnected pores. Ultrasound-induced structural changes could influence polyphenol content, carotenoids, and color. [PERSON] et al. [54] observed that fermentation relaxed the tissue structure in vegetables such as beetroot, carrot, and red pepper, irrespective of raw material type. Fermentation also caused discrepancies in cell size, possibly due to saltwater migration into tissues. Large voids were observed within the tissue post-fermentation, reflecting significant structural alterations. Thermal properties  
Thermogravimetric analysis (TGA) is a method used to evaluate the thermal stability of materials and the integrity of cell wall-forming polymers in plant tissues [55]. TGA measures the weight loss of a sample during heating, which is depicted on a TGA curve. In contrast, the differential thermogravimetric (DTG) curve illustrates the rate of mass change as the temperature varies. Analyzing TGA and DTG curves reveals three distinct stages for both non-fermented and post-fermented samples, each corresponding to specific chemical reactions [53]. A slight mass loss was observed in the initial stage (30-100 \({}^{\circ}\)C), ranging from 2-4.7% for fermented samples and 2.5-2.7% for non-fermented samples (Table 4). This mass loss was primarily attributed to the evaporation of water and the release of minimal volatile compounds following freeze-drying. The second stage of decomposition (100-480 \({}^{\circ}\)C) demonstrated the most pronounced weight loss, averaging 63.7% for non-fermented samples and 64.6% for fermented samples.