Apple pomace, a by-product of apple juice production, is typically discarded as waste. Recent approaches have focused on utilizing apple pomace by extracting beneficial bioactive compounds, such as antioxidant phenolic compounds (PCs). Before these PC-rich extracts can be used in food products, they must undergo food preservation and processing methods. However, the effects of these processes on the composition, stability, and properties of the PC remain insufficiently understood. The present study aimed at investigating the effects of a thermal treatment (TT), a high-pressure thermal treatment (HPTT), and a pulsed electric field treatment (PEF) on the composition and antioxidant activity of PC-rich apple pomace extracts (APEs). Major PCs, including phloridzin, chlorogenic acid, and epicatechin, as well as minor compounds, were identified by liquid chromatography–tandem mass spectrometry (LC-MS/MS) and high-performance thin-layer chromatography (HPTLC). As a stability indicative property, the antioxidant activity was analyzed by a Trolox equivalent antioxidant capacity assay (TEAC), electron paramagnetic resonance spectroscopy, and the Folin–Ciocalteu reagent assay. The results showed that TT at 80 °C increased phloridzin content, likely due to the hydrolysis of bound forms, while higher temperatures and HPTT resulted in a substantial PC conversion. The PEF treatment also caused notable PC conversion, but generally, it had a milder effect compared to TT and HPTT. Hence, low temperatures with and without high pressure and PEF seem to be the most promising treatments for preserving the highest content of major PC in APE. Antioxidant activity varied among the analytical methods, with HPTT showing minor changes despite PC loss compared to the untreated APE. This suggests that other antioxidant compounds in the extracts may contribute to the overall antioxidant activity. This study demonstrates that apple pomace contains valuable PC, highlighting its potential as a health-promoting food additive and the impact of conventional preservation and processing methods on PC stability.

A common strategy for a protein’s functionality modification is the covalent binding of phenolic compounds (PCs) under alkaline conditions. Whether intentionally applied or arising during food processing and storage, such reactions are highly relevant, as alkaline pH promotes oxidation, covalent adduct formation, and polymerization, thereby altering both PC and protein properties. However, the interplay of these reactions and the impact of PC structure remain insufficiently understood. This study aimed at characterizing covalent binding products of structurally related PCs with tryptic peptides of the model protein β-lactoglobulin (β-Lg) at pH 9. Emphasis was given on substitution patterns and steric effects influencing polymerization and peptide adduct building. Hydroxycinnamic acid and flavonoid derivatives differing in hydroxyl substitution and carrying polar (glycosidic) groups were selected. Incubation products were characterized by HPLC–DAD and high-resolution mass spectrometry. Results showed that both mono- and dihydroxy PC undergo oxidation under alkaline conditions, but with distinct reactivity. Monohydroxy PCs form only limited peptide adducts due to resonance stabilization and steric hindrance. In contrast, dihydroxy PCs displayed a higher reactivity, producing more polymerization products and covalent adducts. Their enhanced reactivity is linked to the ability of quinone formation with reduced electrostatic repulsion, while additional polar substituents promote interactions with polar amino acids. At the same time, these substituents impose steric constraints on PC polymerization, modulating oligomer size and thereby influencing peptide binding. Overall, the findings highlight structural determinants of PC reactivity and provide mechanistic insight into the balance between polymerization and covalent peptide modification under alkaline conditions.