Pectin Extraction from Citrus paradisi Using Pressurized CO2 in Combination with Citric Acid and Citrate Buffer

Abstract

The objective of this study was to investigate the efficiency of extracting pectin from grapefruit residues using pressurized CO₂ (PCO₂) in a green solvent under mild conditions, aiming to minimize the use of harsh chemicals. The extraction was performed with 1 g of grapefruit residue powder in 30 mL of citric acid and citrate buffer solution at pH 4–6, utilizing PCO₂ at 2.5 MPa. The pectin produced in the citrate buffer (CB-pectin) yielded higher than that produced in citric acid (CA-pectin) at the same pH. Notably, pH 5 was optimal, yielding 11.7–14.3% (w/w) pectin in both solvents. Molecular analysis showed that CA-pectin exhibited higher methyl-esterification intensity compared to CB-pectin. Additionally, increasing the pH in both the citric acid solution and citrate buffer resulted in a decreased degree of esterification (DE). CA-pectin displayed a DE of 92% and a molecular weight (MW) of 70 kDa, figures comparable to those obtained using water and higher than those of CB-pectin. The rhamnogalacturonan-I region was more prevalent in CA-pectin, while the homogalacturonan structure dominated in CB-pectin, as indicated by their monosaccharide compositions. The viscosity of CA-pectin at pH 3 in the presence of 45% sucrose increased significantly more than under neutral conditions. However, this trend was less pronounced in CB-pectin. This study proposes a novel method for producing high-methoxyl pectin using PCO₂-assisted citric acid and citrate buffers.