Acid-free microwave extraction of citrus pectins: Linking molecular structure to functional properties

Exploring greener and more controlled processes remains a key challenge in food ingredient development. In this study, pectins from orange, lemon, and lime peels were extracted by acid-free hydrothermal microwave-assisted extraction (MAE) and characterized for their structural and functional properties. Competitive yields were achieved without external acid addition, reaching up to 26.38% at 160 °C for 5 min. Extraction conditions and citrus source significantly affected the degree of esterification, monosaccharide composition, and molar mass distribution. Orange pectins were classified as high-methoxyl pectins (HMP), whereas lemon and lime pectins remained HMP only below 180 °C for 10 min and 160 °C for 10 min, respectively. More severe conditions produced low-methoxyl pectins (LMP). Enzymatic de-esterification using pectin methylesterase (PME) further reduced the degree of methylation by 77.8-91.8%, yielding LMP with enhanced water-holding capacity, improved emulsion stability, and shear-thinning behavior. Compared with commercial pectins, MAE- and PME-derived pectins exhibited higher antioxidant activity, likely due to co-extracted phenolic compounds. FTIR and thermogravimetric analyses confirmed the structural modifications induced by MAE and PME. Overall, acid-free MAE combined with enzymatic de-esterification offers a sustainable strategy to produce citrus pectins with tunable structures and functional properties for diverse food applications.