UCC Library and UCC researchers have made this item openly available. Please let us know how this has helped you. Thanks! Title Effects of pre and post-harvest treatments on the phenolic compounds and antioxidant activity of different onion varieties Author(s) Ren, Feiyue Publication date 2019 Original citation Ren, F. 2019. Effects of pre and post-harvest treatments on the phenolic compounds and antioxidant activity of different onion varieties. PhD Thesis, University College Cork. Type of publication Doctoral thesis Rights © 2019, Feiyue Ren . http://creativecommons.org/licenses/by-nc-nd/3.0/ Embargo information Not applicable Item downloaded http://hdl.handle.net/10468/8360 from Downloaded on 2021-10-07T14:57:07Z Ollscoil na hEireann National University of Ireland Coláiste na hOllscoile, Corcaigh School of Food and Nutritional Sciences Effects of pre and post-harvest treatments on the phenolic compounds and antioxidant activity of different onion varieties Thesis presented by Feiyue Ren, BSc, MSc Under the supervision of Prof. Dr. Joseph Kerry Dr. Brijesh Tiwari To obtain the degree of Doctor of Philosophy – PhD in Food Science and Technology June 2019 Table of Contents Table of Contents .................................................................................................................... i Acknowledgements ............................................................................................................. viii Declaration of Authenticity .................................................................................................. ix Work Flow .............................................................................................................................. x Publications and Presentations ............................................................................................ xi Abstract ................................................................................................................................ xiii Chapter 1 Literature Review ................................................................................................ 1 1.1 Introduction ................................................................................................................ 2 1.2 Phenolic compounds in onions .................................................................................. 5 1.2.1 Flavonoids in onions ....................................................................................... 5 1.2.1.1 Quercetins in onions ............................................................................. 7 1.3 Variation of phenolic contents among different crops and onion cultivars ............... 8 1.3.1 Flavonoids distribution in onion tissue ........................................................... 9 1.4 Bioactivity of phenolic compounds in onions .......................................................... 10 1.4.1 Antimicrobial activity of onions ................................................................... 12 1.4.2 Antioxidant activity of onions ....................................................................... 13 1.4.3 Anticarcinogenic and antimutagenic activities ............................................. 15 1.4.4 Hypotensive and bradycardic effects ............................................................ 15 1.4.5 Anti-hyperglycemic and anti-diabetic potential ............................................ 16 1.4.6 Anti-platelet effect ........................................................................................ 16 1.5 Factors and treatments influencing the phenolic compounds in onion production .. 16 1.5.1 Light .............................................................................................................. 19 i 1.5.2 Soil status ...................................................................................................... 21 1.5.3 Agronomic conditions ................................................................................... 21 1.6 Changes in contents of the phenolic compounds in onions after cultivation ........... 22 1.6.1 Effect of curing on the phenolic compounds ................................................ 22 1.6.2 Evolution of the phenolic compounds when sprouting ................................. 24 1.6.3 Evolution of the phenolic compounds during storage ................................... 24 1.6.4 Effect of temperature during storage on phenolic compounds ..................... 26 1.6.5 Other storage technologies ............................................................................ 28 1.7 Variation of phenolic compounds during onion processing..................................... 30 1.7.1 Minimal food processing .............................................................................. 30 1.7.2 Thermal processing ....................................................................................... 31 1.7.3 Non-thermal treatments ................................................................................. 34 1.7.3.1 Ultraviolet (UV) ................................................................................. 34 1.7.3.2 High pressure ..................................................................................... 34 1.7.3.3 Chemical treatment of onions ............................................................ 35 1.8 Extraction of phenolic compounds from onions ...................................................... 35 1.9 Conclusions .............................................................................................................. 40 Chapter 2 Evaluation of Phenolic Content and Antioxidant Activity in Two Onion Varieties Grown under Organic and Conventional Production Systems ....................... 42 Abstract .......................................................................................................................... 43 2.1 Introduction .............................................................................................................. 44 2.2 Materials and methods ............................................................................................. 45 2.2.1 Field trial experiments .................................................................................. 45 2.2.2 Extraction and analysis of phenolic compounds ........................................... 48 ii 2.2.3 Analysis of total phenolics ............................................................................ 49 2.2.4 Analysis of total flavonoids .......................................................................... 49 2.2.5 Analysis of antioxidant activity .................................................................... 50 2.2.5.1 Ferric reducing antioxidant power (FRAP) assay .............................. 50 2.2.5.2 DPPH antioxidant power assay .......................................................... 50 2.2.6 Statistical analysis ......................................................................................... 51 2.3 Results and discussion ............................................................................................. 51 2.3.1 Total phenolic and total flavonoid content .................................................... 51 2.3.2 Antioxidant activity....................................................................................... 58 2.4 Conclusions .............................................................................................................. 62 Chapter 3 Effect of Storage on Chemical Composition, Antioxidant Activity and Quality in Organically and Conventionally Managed Systems on Fresh and Dried Onions (Allium cepa L.) ....................................................................................................... 63 Abstract .......................................................................................................................... 64 3.1 Introduction .............................................................................................................. 65 3.2 Materials and methods ............................................................................................. 66 3.2.1 Sampling ....................................................................................................... 66 3.2.2 Dry matter content ........................................................................................ 67 3.2.3 Preparation of extracts from dried onions ..................................................... 67 3.2.4 Total phenolic content (TPC) ........................................................................ 67 3.2.5 Total flavonoid content (TFC) ...................................................................... 67 3.2.6 Analysis of antioxidant activity .................................................................... 67 3.2.6.1 Ferric reducing antioxidant power (FRAP) assay .............................. 67 3.2.6.2 DPPH antioxidant power assay .......................................................... 67 3.2.7 Quercetin and its glycosides ......................................................................... 67 iii 3.2.8 Colour ........................................................................................................... 68 3.2.9 Statistical analysis ......................................................................................... 69 3.3 Results and discussion ............................................................................................. 69 3.3.1 Weight loss and dry matter (DM) during storage of onion bulbs ................