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Abstract

This study was conducted to compare the total phenolics content and antioxidant activity of extracts from fresh (FS), freeze-dried (FD) and hot air-dried (HD) water caltrop hulls. The antioxidant properties, including 2,2-diphenyl-1-picrylhydrazyl (DPPH) free-radical scavenging activity, reducing power, and cupric ion chelating ability were evaluated along with alpha-tocopherol and butylated hydroxytoluene (BHT) in this study. The result showed that total phenolics content of methanolic extracts ranged from 5.21-8.59 g GAE/100 g, and the extracts of FS exhibited a substantial total phenolics content (8.59 g GAE/100 g) compared with other dried samples. It was found that the antioxidative effect of water caltrop hulls was strongly concentration dependent. The dose response study indicated that before reaching a threshold level, there was a positive correlation between DPPH free-radical scavenging ability and concentration of methanolic extracts. At the point where the scavenging ability of DPPH free-radical was 79.3%, the dose of the methanolic extracts was 0.2 mg/mL for FS, 1.5 mg/mL for HD, 1.8 mg/mL for FD, and 1.3 mg/mL for BHT. The chelating ability of three different water caltrop hull extracts reached a threshold level of only 28% at a dose of 2 mg/mL. The total phenolics contents were higher, but IC50 was lower in FS extracts than in other antioxidants. Compared with other treatments with antioxidants, the reducing power of FS extracts was found to be significantly more pronounced than that of HD and FD extracts, BHT and α-tocopherol, at dose ranges of 0-1.0 mg/mL. This finding demonstrated that FS extracts is comparable to BHT in terms of its antioxidant properties in food application.

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