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Changes in phenolic composition, antioxidant and antidiabetic properties of turmeric as affected by cooking time
Stephen A. Adefegha, Ganiyu Oboh, Tosin A. Olasehinde, Kolawole Osunmo
This study sought to assess the effects of cooking time on the phenolic composition, antioxidant and antidiabetic properties of turmeric extracts. Steam cooked turmeric extracts were prepared by cooking 10 g of turmeric for 10 (SCT10) and 20 (SCT20) min, respectively, while the raw sample (RWT) was prepared by soaking 10 g of turmeric in 200 ml of distilled water for 1 h. The total phenol and flavonoid contents of the turmeric extracts were determined and phenolic composition was assessed using high performance liquid chromatography coupled with diode array detection (HPLC-DAD). The ferric reducing antioxidant property, radical [1,1-diphenyl-2 picrylhydrazyl (DPPH), nitric oxide (NO) and hydroxyl (OH)] scavenging abilities and interaction of the extracts with α-amylase and α-glucosidase activities were also investigated. SCT20 (4.26 mg/g and 3.96 mg/g) had significantly higher total phenol and flavonoid contents than SCT10 (3.58 mg GAE/g and 3.38 mg QE/g) and RWT (2.44 mg/g and 2.38 mg/g), respectively. SCT20 (76.8 mmol/100 g) had the highest reducing property, while RWT (68.2 mmol AAE/100 g) had the lowest. Furthermore, SCT20 had significantly (p<0.05) higher DPPH, NO and OH radical scavenging ability than SCT10 and RWT. The extracts inhibited α-amylase and α-glucosidase in a dose-dependent manner. While RWT had the lowest inhibitory effects, SCT20 had the highest. The HPLC analysis revealed the presence of phenolic compounds such as gallic acid, caffeic acid, catechin, quercetin, rutin, luteolin and circumin. The phenolic constituents of the steam cooked extracts were significantly higher than those of the raw samples except for caffeic acid which decreased with increased cooking time. Therefore, steam cooking increased the phenolic composition and antioxidant properties as well as the α-amylase and α-glucosidase inhibitory activities of turmeric.