Optimization of pectin extraction from fruit peels by response surface method: Conventional versus microwave-assisted heating

Fruit waste has significant valorization potential as a source of pectin for use in numerous applications. The initial objective was to determine the optimal parameters of conventional and microwave-assisted heating to achieve maximum yield for pectin extracted from peels of red-flesh (Hylocereus polyrhizus) dragon fruit, white-flesh dragon fruit (Hylocereus undatus) peels (DFP) and passion fruit (Passiflora edulis) peels (PFP). Extraction times and temperatures in the conventional method were optimized by using response surface designed by Minitab (R) while for microwave-assisted heating the variables were microwave power, pH, extraction time and liquid:solid ratio. The extraction conditions and the type of peels had significant effects on yield; pectin yields were from 5.81% to 13.11% for the white-flesh DFP, 8.99%-15.12% for the red-flesh DFP, and 8.56%-13.95% for PFP when using conventional heating. When using microwave-assisted heating, yield of pectin from red flesh DFP was affected by time, microwave power and liquid:solid ratio, while all four variables influenced pectin yield from white-flesh DFP, at a square level. Microwave power, pH, and liquid:solid ratio were shown to be determining factors of yield from PFP, with one of the highest reported yield at 18.73% while also offering the flexibility in the degree of esterification. Structural assessment by Fourier Transform Infrared spectroscopy confirmed that the pectin isolated was very similar to commercially available citrus pectin. These results suggest that these fruit peels that would otherwise be discarded as waste have the potential to be valuable sources of pectin that can be extracted by quick and simple methods, while maintaining similar quality to current commercial sources.