Effects of ultrasound, freeze-thaw pretreatments and drying methods on structure and functional properties of pectin during the processing of okra

Abstract

Although, okra has features of high moisture content, rich nutrients, and pectin, it has extremely short shelf life, so deteriorate easily. Simple drying can prolong okra's shelf life but takes considerable time and is not cost-effective. Therefore, suitable pretreatments are usually employed to improve the drying efficiency and reduce the costs. This study showed that ultrasound (US) and freeze-thawing (FT) pretreatments, as well as three drying methods including vacuum pulsed drying (VPD), vacuum freeze-drying (VFD) and hot air drying (HD) have effects on structure and functional characteristics of okra pectin. The results indicated that drying methods affected differently the structure and chain conformation of the pectin molecular main chain. The thermal drying processes reduced molecular weight and increased viscosity, oxidation resistance, thermal stability, cohesion, and resilience. Particularly, the VPD pectin samples had lower molecular weight, higher antioxidant activity, better retention of the homogalacturonan (HG) main chain, thermal stability, emulsification activity, cohesiveness, resilience and weaker hardness and gel strength. The pretreatments had little effect on molecular weight, improved the antioxidant activity and emulsification activity of samples, and increased the viscosity of cold processed samples. US pretreatment retained pectin molecular chains, and FT pretreatment aggravated molecular chain debranching. In summary, VPD and US/FT pretreatments are beneficial to the development and utilization of pectin products.