Valorization of walnut processing waste as a novel resource: Production and characterization of pectin

Microwave-assisted extraction of pectin from walnut green husk (WGH) was optimized using Box-Behnken design. The effects of microwave power, pH, irradiation time, and liquid/solid ratio on the extraction yield were investigated. The characteristics of WGH-pectin extracted at the optimal condition were evaluated through physicochemical, structural, and functional analysis. Under optimal condition (microwave power of 500 W, pH of 1.5, irradiation time of 3 min, and LS ratio of 20 ml/g), WGH-pectin had an extraction yield of 18.0 g/100 g, galacturonic acid content of 68.45 g/100 g, and a molecular weight of 260.289 kDa. It was also high-methoxyl pectin (54.07%) which was confirmed by FTIR and NMR spectroscopy. Furthermore, it showed significant antioxidant activity, good value for water and oil-holding capacity, and emulsifying properties. The XRD pattern of WGH-pectin showed an amorphous nature with several crystalline portions. Practical applications Walnut green husk is a potential byproduct of walnut processing sectors and a great resource of valuable compounds such as pectin, which is generally discarded into the landfill in the form of waste. However, it can be used as an inexpensive high-potential resource of pectin. For this purpose, microwave-assisted extraction was optimized to achieve the highest production rate. Then, the isolated pectin was characterized to study the physicochemical, structural, and functional properties in order to show its capability as a natural food ingredient.