Effects of Different Drying Methods on Polyphenol Content and Antioxidant Activity of Noni Fruit

In this study, the effects of different drying methods on the polyphenol content and antioxidant activity of noni fruit were compared. The total phenol content and the content of seven phenolic compounds in noni samples after natural sun-drying, vacuum freeze-drying, and hot-air drying were determined using the forint-phenol colorimetric method and high-performance liquid chromatography (HPLC). The antioxidant capacity of each sample was assessed using the free radical scavenging ability and iron ion reducing ability experiment. The results showed that the total phenolic content of noni fruit after natural sun-drying, vacuum freeze-drying and hot air-drying were 7.78, 12.42 and 6.38 mg/g, respectively. The scavenging ability against DPPH radicals were 17.57, 35.98 and 13.63 μmol Trolox/g, and the scavenging ability against ABTS⁺• were 17.57, 36.09, 13.63 μmol Trolox/g. The FRAP antioxidant values were 43.12, 73.49, 39.12 μmol Trolox/g, respectively. There was no significant difference (P>0.05) in total polyphenol content and antioxidant properties between the noni fruits after naturally sun-drying and hot air-drying. However, these properties were significantly different (P<0.05) from vacuum freeze-dried samples. HPLC analysis revealed that the total value of seven phenolic compounds in noni fruit was the highest after vacuum freeze-drying, reaching 1207.42 μg/g. This indicates that vacuum freeze-drying had a lesser effect on the content and antioxidant capacity of noni fruit polyphenols compared to the other two drying methods. Therefore, it was deemed more suitable for the industrial production of higher quality and high antioxidant activity of noni fruit products.