The effects of UV-C on the total polyphenol contents and antioxidant activity of New Zealand grown green asparagus : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Human Nutrition at Massey University, Manawatū, New Zealand
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Date
2022
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Massey University
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Abstract
Background: Asparagus is favoured by global consumers for its exceptional nutritional value. New Zealand has a long history of asparagus cultivation and a considerable domestic market. Recently, several technologies such as UV and gamma irradiation have been used to increase the bioactive compounds in plants, resulting in improved plant quality and human health. The main aim of the present study was to study the hoemetic effects of UV-C (0.45- 2.27 kJ/m²) on the total polyphenol contents and antioxidant activity of New Zealand grown asparagus. Methodology: A three-factor-three-level Box Behnken Design (BBD) was used to determine the optimum condition for ultrasonication-assisted extraction (UAE). Three doses of UV-C radiation (0.45, 1.13, and 2.27 kJ/m²) were applied to study the effects of UV-C light on the antioxidant activity of asparagus. The antioxidant assays were evaluated by total polyphenol contents (TPC), 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging ability, and peroxidase (POD) activity. The leading flavonoid of asparagus extracts was analyzed by High-Performance Chromatography (liquid chromatography and mass spectrometry; HPLC-MS). Results: The optimum conditions for extracting asparagus TPC were 60% ethanol (v/v), 20 min extraction time, and a solid to liquid ratio of 1:30 (w/v). Overall, the UV-C irradiated samples showed significantly higher TPC than unirradiated samples (p< 0.05). The highest TPC value (131.37 ± 2.05 mg GAE/g; dry weight) was obtained at 1.13 kJ/m² UV-C exposure, and the TPC started to decrease as UV-C irradiation surpassed 1.13 kJ/m². The DPPH activity showed a similar trend as the TPC assay. The lowest IC50 of DPPH assay was found in the 1.13 kJ/m² group (36.84 µM), followed by the 0.45 kJ/m² group (49.20 µM), 2.27 kJ/m² group (99.21 µM), and unirradiated group (115.01 µM). The effects of UV-C irradiation on POD activity exhibited an opposite trend as that of TPC. The HPLC-MS results showed that rutin was the main flavonoid in asparagus with a content of 14.095 mg/100 g (dry weight). Conclusion: Ultrasonication-assisted extraction (UAE) was more effective than traditional maceration on extracting asparagus TPC. The hormetic effects of UV-C were seen in this study as promoting the plant bioactivity at an optimum UV dose and decreasing such activity when the dose continued to increase. Rutin contains o‐diphenol, benzene ring, and keto functional group (-C=O) and might be the reason for the high antioxidant activity of asparagus.
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New Zealand green asparagus, UV-C, total polyphenols, antioxidant activity, UAE, RSM