Extraction of Pectin from Lime Peel Using Ultrasound: Modeling and Optimization of Process Parameters

Abstract
Background and Objective:
In recent years, the sustainable utilization and valorization of agricultural residues within the framework of green chemistry principles have received increasing attention. Pectin, as an efficient biopolymer with notable gelling ability, biocompatibility, and biodegradability, represents a promising candidate for developing edible and bio-based packaging materials and for replacing petroleum-derived synthetic plastics. Lemon peel, a low-value by-product of the juice industry, serves as a rich and economical source of high-quality pectin. Conventional pectin extraction methods based on heat and mineral acids not only pose environmental and operational challenges but also degrade the molecular structure of pectin, thereby reducing its functional performance in packaging applications. Accordingly, this study aimed to optimize pectin extraction from lemon peel using ultrasound-assisted technology and response surface methodology (RSM) to determine the optimal processing conditions for achieving maximum yield and suitability for bio-based packaging applications.

Materials and Methods:
Fresh lime peel was washed, separated, dried, and ground to prepare it for pectin extraction. The extraction was carried out in a citric acid solution under the influence of three ultrasound amplitude levels (60, 80, and 100%), three extraction times (4, 8, and 12 minutes), and three pH levels (1, 2, and 3). The solid-to-liquid ratio was maintained at 1:15 (w/v) for all treatments. The extracted pectin was precipitated using ethanol, then dried and milled into a uniform powder, and its yield was determined gravimetrically. Experimental design was performed using a Box–Behnken Design within the framework of response surface methodology. Data analysis was conducted using Design Expert V13 software, and a second-order polynomial model was developed to describe the main, quadratic, and interactive effects of the extraction variables. Model adequacy was evaluated through a set of statistical tests, including analysis of variance, coefficient of determination, adequate precision, and residual analysis.

Results:
The proposed model demonstrated an excellent fit to the experimental data (R² = 0.9957, Adjusted R² = 0.9878, Predicted R² = 0.9306, CV = 5.02%). The lack-of-fit test was non-significant (P > 0.05), confirming the validity and adequacy of the model. Scatter plots and normal probability plots of the residuals further indicated normally distributed errors, absence of outliers, and satisfactory homogeneity. According to the ANOVA results, pH was identified as the most influential factor governing pectin extraction yield (P < 0.0001, β = +23.39), while ultrasound amplitude and extraction time exhibited positive but less pronounced effects. Using Derringer’s desirability function, the optimal extraction conditions were determined as 88.42% amplitude, 10.48 min extraction time, and pH 1.04. Under these conditions, the predicted pectin yield was 48.13% with an overall desirability of 1.00. A confirmatory experiment conducted under the same conditions produced an actual yield of 46.16%, and the small deviation between predicted and experimental values confirmed the high accuracy and reliability of the optimization model.

Conclusion:
Ultrasound technology, as an efficient method aligned with green chemistry principles, exhibits high effectiveness in extracting pectin from plant-based sources. Integrating this technique with statistical modeling via response surface methodology (RSM) enables accurate prediction of process behavior, simultaneous optimization of influential factors, and substantial reduction in the number and cost of experiments. The findings of this study showed that acidic conditions and high ultrasonic amplitudes significantly enhance pectin extraction yield. The pectin obtained through this green method also exhibits desirable properties for application in edible film production and bio-based packaging. Overall, the results provide a scientific and practical framework for the industrial valorization of citrus waste and the production of high value-added biopolymers in green packaging. These outcomes can support the development of sustainable technologies based on pectin–cellulose systems as alternatives to conventional plastic packaging.