Thermal Processing Shapes Phenolic Profiles in Iodine-Biofortified Kale: Cultivar- and Iodine-Treatment-Associated Patterns Revealed by LC–HRMS

The impact of culinary processing on the phenolic profile of iodine-biofortified kale remains insufficiently resolved at the compound level. Green- and red-leaved kale grown without iodine supplementation or biofortified with KIO3 or 5,7-diiodo-8-quinolinol (DIQ) was subjected to blanching, boiling, or steaming. LC–HRMS profiling resulted in 135 phenolic annotations, including three compounds confirmed using authentic standards, while selected major phenolics were additionally quantified or semi-quantified by PDA detection. Multivariate analysis revealed pronounced cultivar-related differences. Among processed samples, thermal processing accounted for the largest proportion of phenolic variation (61.6% in green and 68.5% in red kale), while iodine treatment accounted for 9.3% and 7.2%, and the Processing × Iodine component for 24.2% and 19.6%, respectively. Compound-level analysis revealed pronounced differences between steamed and boiled samples, with predominantly higher relative LC–HRMS signals after steaming, especially in red-leaved kale. Processing-related phenolic patterns also differed among Control, KIO3, and DIQ sample pools. Because each cultivar × iodine treatment combination represented a single pooled biological source, iodine-associated differences were interpreted descriptively rather than as inferential treatment effects. Overall, thermal processing was a major determinant of phenolic composition, while processing-related patterns varied among iodine-treatment pools.