Pectin is a complex carbohydrate compound commonly used in food products as a gelling agent, thickener, stabilizer, and fat replacer. It is commercially obtained from citrus or apple by-products. However, there is a growing interest in discovering new sources of pectin, particularly agricultural waste and developing new extraction methods to improve industrial pectin production efficiency. This research aimed to investigate the feasibility of enhancing extraction efficiency in both conventional and microwave-assisted methods using ultrasonic pretreatment (at two power levels and two process times) and electric discharge (at three levels). The focus of this study was on the extraction kinetics, structural characteristics, functional properties, and rheological behavior of pectin derived from watermelon rind, which represents a type of food waste. The findings indicated that both pretreatments induced changes in the surface morphology of the raw powder. Notably, the specific surface area of watermelon rind powder experienced a significant increase when subjected to ultrasound or electrical discharge pretreatment. Moreover, pretreatment resulted in elevated levels of swelling index and electrical conductivity in the sample suspensions. Furthermore, the results demonstrated that, in the conventional extraction method, the maximum extraction efficiency was achieved at 17.4% over a 90-minute extraction period. Nevertheless, when electrical discharge and ultrasonic pretreatment were applied, the extraction efficiency increased to 18.46% and 18.92%, respectively. Within the microwave-assisted extraction method, extraction efficiency rose from 14.39% to 17.97% and 19.03% in 5 minutes, respectively, through electric discharge (20 kV/cm, 10 minutes) and ultrasound pretreatment (300 W). Kinetic studies suggested that all extraction treatments followed the second-order kinetic model, with the effective diffusion coefficient of extraction showing an increase due to pretreatment, particularly with ultrasound presenting greater effectiveness compared to electrical discharge. Subsequently, the research shifted focus towards eva‎luating the properties of the extracted pectin. The findings revealed that the degree of esterification (DE) in the samples was less than 50%, whereas the galacturonic acid content in all samples exceeded 65%. Ultrasound pretreatment was more effective in reducing DE and galacturonic acid percentages. Both pretreatments enhanced the side chain of pectin molecules by increasing the presence of rhamnose and other monosaccharides. Analysis of the infrared spectrum revealed that the pretreatments neither introduced new peaks nor removed existing peaks in the absorption pattern of the control sample. However, there were variations in absorption rates in certain areas. Unlike ultrasound pretreatment, electrical discharge pretreatment did not significantly decrease the molecular weight of pectin samples. In the study, all sample solutions exhibited shear thinning behavior, characterized by decreased viscosity as the shear rate increased. Ultrasonic pretreatment successfully reduced viscosity, whereas electrical discharge pretreatment at 20 kV/cm resulted in increased viscosity. Lastly, the oscillatory rheology analysis of the pectin gel demonstrated that employing these pretreatments maintained the gel's elastic structure and led to an extended gel formation time.

ناصر همدمي

جواد كرامت

هاجر شكرچي زاده , محمد ديناري , بابك قنبرزاده