Effect of Ultrasonic Pre-Treatment on the Textural, Structural, and Chemical Properties of Fermented Red Bell Peppers

This study examined the influence of ultrasound pre-treatment and its parameters on the physical and chemical properties of bell peppers undergoing lactic acid fermentation. Two ultrasound methods were employed: immersion (for 15 and 30 min) and contact (for 1, 3, and 5 min). The fermentation process was carried out with the addition of Lactiplantibacillus plantarum ATTC 4080. The physicochemical and structural properties of both fresh and fermented bell peppers, including pre-treated and untreated samples, were analyzed, with untreated fresh and fermented peppers serving as control samples. The findings revealed that ultrasonic pre-treatment significantly affected the peppers’ physical, chemical, and structural characteristics. Specifically, ultrasound reduced dry matter content, with fresh bell peppers exhibiting the highest dry matter content (10.58%). Dry matter content decreased by 16–24% after ultrasound pre-treatment and by 0–14% after fermentation. The immersion method and longer sonication times had the most pronounced effects on dry matter reduction. In addition, ultrasound caused changes in color and texture, while fermentation influenced attributes such as hardness, elasticity, cohesiveness, and chewiness. The most significant changes occurred with 30 min of ultrasound treatment, leading to a 25.6% increase in red color, whereas the contact method increased yellow color, with a 30.3% increase between 15 and 30 min of immersion sonication. Fermentation generally reduced the proportion of red and yellow color compared to fresh samples, except for samples treated with contact ultrasound for 5 min, which retained higher red and yellow coloration after fermentation. Fermentation significantly softened the peppers, reducing hardness by up to 85% compared to fresh untreated samples. Chewiness followed a similar trend, decreasing from 17% to 80% in pre-treated samples and up to 90.65% after fermentation. Elasticity and cohesiveness also decreased with increasing sonication time and fermentation, but not significantly. Ultrasound pre-treatment had no significant effect on total polyphenol and carotenoid contents in fresh samples, which ranged from 43.55 mg β-carotene/100 g dry matter to 147.89 mg β-carotene/100 g dry matter. However, fermentation significantly increased carotenoid levels, with a 40% increase observed in samples treated with 5 min of contact ultrasound. In addition, pre-treatment methods helped to preserve polyphenols in fermented peppers. Levels of lactic acid bacteria (LAB) in fermented samples varied according to pre-treatment, with the highest LAB levels found in peppers exposed to immersion ultrasound for 30 min (10.55 log CFU/g). Despite ultrasound-induced tissue damage, fermentation proceeded effectively, resulting in consistent pH levels (ranging from 3.01 to 3.06) across all samples. Structural analysis showed that tissue damage increased with longer sonication times. In conclusion, ultrasonic pre-treatment significantly affected the physicochemical, structural, and bioactive properties of sweet peppers. While it reduced dry matter content and altered texture and color, it also helped to preserve polyphenols in fermented samples and increased carotenoid content after fermentation. The immersion method and longer sonication times had the most pronounced effects, highlighting ultrasound as a promising tool for modifying the properties of fermented vegetables.