The Opuntia ficus-indica and Opuntia humifusa , native to Mexico and grow in dry areas, has gained popularity all over the world. These are also locally grown and adapted in Jeju Island, Republic of Korea. The fruits of this cactus are particularly infamous as raw material which are processed into various food and medicinal items, such as chocolates and vitamins, for human consumption and are sold as food delicacies and items for tourists to take home as souvenirs or as gifts. Because this crop has been tagged as one of Jeju Island’s iconic agricultural crops, research studies dealing with its medicinal properties, like that of antioxidant activity, of this plant and its cultivated species are very valuable. Nowadays, consumers do not buy products for consumption alone, but as well as added benefits to health and wellness. Modern consumers expect products with antioxidant properties from plants would produce positive benefits, such as such as anti-aging, anti-cancer, with various nutrients, and immune-friendly ingredients.
Reactive oxygen is one of the causes of diseases in adults, obesity, metabolic syndrome, inflammation, and cancer in the human body, thus experiments on antioxidant components that would combat reactive function from functional plants are actively being conducted in plant research. Artificial antioxidants are cheap, although there are reported side effects. Hence, research on natural antioxidants from plant components is found to be essential, and natural antioxidants are contained in many secondary metabolites of plants. Opuntia ficus-indica and Opuntia humifusa fruits, also commonly known as fan cactus, pad cactus, and palm cactus, are reported to have high antioxidant content in previous studies. In order to develop Korea''s natural antioxidant from functional plants, it is necessary to evaluate the sensitivity of the antioxidant level by prickly pear cactus variety, plant part, and harvesting period. Therefore, the present study aimed to determine the effect of the above factors particularly focused on two species of prickly pear cactus that are most proliferative cultivated and used in Jeju Island, Republic of Korea. These species are Opuntia ficus-indica (‘Baeknyeoncho’) and Opuntia humifusa (‘Cheonnyeoncho’). From these species, specific plant parts including their flowers, mature and young stems, roots, and fruits were subjected to antioxidant analysis. Five major antioxidant analyses, total polyphenol content, total flavonoid content, DPPH radical and ABTS radical scavenging activity, and total anthocyanin content, were analyzed for each of the two varieties. Results revealed that between species, Opuntia ficus-indica had a much higher antioxidant level with a total polyphenol content (7.04 mg/g), a total anthocyanin content (0.26 mg/g), and ABTS radical scavenging activity (RC 50 = 6.18 mg/mL). For DPPH radical scavenging activity, the Opuntia ficus-indica (RC50 = 3.14 mg/mL) was also higher than that of the Opuntia humifusa species. However, the total flavonoid content (7.34 mg/g) of Opuntia humifusa was superior to that of ‘Baeknyeoncho’. In comparing the variety, we also found that Opuntia ficus-indica has a high antioxidant polyphenol level (7.04 mg/g), while Opuntia humifusa has a high flavonoid level (7.34 mg/g). Upon evaluation by plant part, the total polyphenol content (8.91 mg/g), flavonoid content (7.32 mg/g), and anthocyanin content (0.36 mg/g) were higher from those of the flowers compared to other parts of the plant. In addition, the antioxidant levels were higher in root and young stems in terms of the DPPH radical (3.28 mg/mL, 3.32 mg/mL) and ABTS Radical scavenging activity (6.22 mg/mL, 6.24 mg/mL) than in other plant parts. Fruits were also found to be higher in DPPH radical scavenging activity (3.30 mg/mL). The reason for the high number of polyphenols, flavonoids, and anthocyanins in flowers was thought to be due to the high number of polyphenols in the Opuntia ficus-indica variety, and the abundance of flavonoids in the Opuntia humifusa variety, and the concentration of these antioxidants were widely distributed in flowers. In comparison, the roots, young stems, and fruits were found to be excellent in the scavenging activity of DPPH and ABTS. Although Opuntia ficus-indica and Opuntia humifusa varieties are widely used as food, the less-used flowers, roots, and young stems were thought to be highly likely to be a new source of natural antioxidants. In terms of season, summer had much higher antioxidant content than winter. The total polyphenol content (7.11 mg/g), the total flavonoid content (6.67 mg/g), and the total anthocyanin content (0.25 mg/g) were much higher in summer than in winter. However, DPPH radical scavenging activity was much higher in winter than in summer (3.15 mg/mL). ABTS radical scavenging activity also had higher levels during winters. The reason for this was thought to be that fruits, roots, and young stems were not present during winter, while the roots accumulated many nutrients underneath the soil surface in winter. In this study, Opuntia ficus-indica was thought to have a high content of polyphenol antioxidants, Opuntia humifusa had a high content of flavonoid antioxidants, and these secondary metabolites were included in the prickly pear cactus flowers, hence they had a high content of polyphenols, flavonoids and anthocyanin antioxidants during the summer season. Fruits, roots, and young stems were scarcer with phenolic antioxidant content and were thought to be the reason why prickly pear cactus had a higher level of radical scavenging activity in winter. Thus, in this study, we could present the possibility of new natural antioxidants of flowers, roots, and young stems as well as fruits, and we could see the differences in antioxidants characteristics by variety and season of prickly pear cactus Opuntia ficus-indica and Opuntia humifusa. This study would be a standard of reference in terms of the nutritional value of future antioxidant studies of prickly pear cactus.