Microwave assisted extraction with three modifications on structural and functional properties of soluble dietary fibers from grapefruit peel

Microwave-sodium hydroxide treatment (MST), microwave-enzymatic treatment (MET) and microwave-ultrasonic treatment (MUT) were used to treat grapefruit peel, then the structural and functional properties of extracted soluble dietary fibers (SDF), named as MST-SDF, MET-SDF and MUT-SDF, were investigated. The scanning electron microscopy, molecular weight, X-ray diffraction, FT-IR, thermal properties and monosaccharide composition were used to determine the structural properties. Compared to the grapefruit peel SDF without treatment (WT-SDF) and pure microwave treatment (PMT-SDF), MST-SDF, MET-SDF and MUT-SDF showed a more complex and loose structure. Moreover, they possessed higher molecular weight, crystallinity and thermal stability, and more diverse monosaccharide composition. In addition, functional properties, including water holding capacity (WHC), oil holding capacity (OHC), cholesterol adsorption capacity (CAC), glucose adsorption capacity (GAC) and nitrite ion adsorption capacity (NIAC), were enhanced by the three different modification methods. Especially, MUT-SDF showed the highest WHC, OHC, CAC, GAC and NIAC as compared with the other samples. In summary, the present study suggested that the MUT could be used as the ideal modification method for grapefruit peel SDF, and MUT-SDF has great potential for the application in functional food industry.