Optimization of Total Polyphenol Content in Rambutan (<em>Nephelium lappaceum </em>L.) Peel Extracts from Sulawesi Island Using FTIR Spectrophotometry and Multivariate Calibration Analysis

Nephelium lappaceum is a tropical fruit reported for various bioactivities, including antidiabetic, analgesic, anti-inflammatory, immunomodulatory, antioxidant, anticancer, and antimicrobial properties. This study aimed to determine the total polyphenol content in different types of rambutan fruit peel extracts (RFPE) from Sulawesi Island and combine it with multivariate calibration analysis. Four types of RFPE, namely rambutan aceh (RA), rambutan rapiah (RR), rambutan garuda (RG), and rambutan lengkeng (RL), were extracted using the maceration method with variations in solvents (70% ethanol and 96% ethanol) and the amount of ethanol solvent (500 ml, 1000 ml, and 1500 ml), resulting in 24 extracts. The wavelength measurements were conducted using FTIR in the 400–4000 cm-1 range, combined with the partial least squares (PLS) method with leave-one-out validation. The research revealed that the highest total polyphenol content was found in RA RFPE with 70% ethanol solvent (1000 ml), reached 12.474 mg equivalent rutin/g extract. Based on the variations, the total polyphenol content followed the RA &gt; RR &gt; RL &gt; RG, 70% ethanol solvent &gt; 96% ethanol solvent, and 1000 ml solvent &gt; 1500 ml &gt; 500 ml. In the wavelength range of 341.40-3429.43 cm-1, variations in RFPE extracts were detected containing functional groups such as hydroxyl groups, carboxylic acids, aromatic rings, and carbonyl groups as polyphenols. Validation of the rutin compound in RFPE extract variations showed SD values ranging from 0.0000 to 0.5686, % CV from 0.01 to 0.25, and using 2 PLS components resulted in optimal values with X variance 0.999870, R-Sq 0.994407, and R-Sq (pred) 0.991514. The FTIR spectrum and multivariate calibration analysis provide an accurate model for determining compounds in various natural extracts.