Effects of pre and post-harvest treatments on the phenolic compounds and antioxidant activity of different onion varieties

Onion (Allium cepa L.) is an important vegetable crop with an annual production of around 66 million tonnes worldwide. Onion and its by-products have the highest amount of quercetin content compared to other fruits and vegetables, which makes it a good free radical scavenging ability, contributing to its antioxidant capacity. Phytochemicals and antioxidants of onion are predominantly important for human health since phytochemicals have multiple compounds, including phenolics, flavonoids, anthocyanins, and quercetin. The total phenolic content of onions is not only higher than its fellow allium vegetables, such as garlic and leeks, but also higher than that of other common vegetables, such as carrots and red bell pepper. The main objective of this study was to investigate phenolic contents and antioxidant activity of different onion varieties by using different pre-harvest (soil, sunlight, climate, and fertiliser) and post-harvest (processing and storage) methods. In order to achieve this, several experiments have been designed and carried out among different onion varieties, particularly between organic and conventional ones. The first experimental chapter aimed to compare the total phenolic contents, total flavonoid contents, and antioxidant activity in onions grown under organic, conventional, and mixed cultivation practices in a multi-year experiment. ‘Hyskin’ and ‘Red Baron’ varieties had significantly higher phenolic contents and antioxidant properties in organic than in conventional production in most years. This study further investigated storage potential of organic and conventional onions at different storage conditions (-20 oC, 5 oC, and 25 oC with 60–75% relative humidity) for 10 weeks in the second experimental chapter. The findings suggested the fresh conventional onions were stored at -20 oC and 5 oC showed no significant quality (total phenolics, total flavonoids, flavonol, and antioxidant activity) loss. Meanwhile, the quality of dried organic onions remained stable during 10 weeks at -20 oC and 5 oC. Moreover, the third and fourth experimental chapters aimed to evaluate the effects of food thermal processing technologies on organic onions nutritional (the levels of phenolic compounds, colour and antioxidant properties). More specifically, this study firstly investigated the quality change in onions dried by different drying methods (freeze-drying, hot-air drying, vacuum oven drying, and oven drying) in comparison with fresh samples. Results indicated that the dried onion showed significantly higher total phenolic contents and antioxidant activity than those of undried onions. In order to reduce the energy involved and reducing the drying time in onion drying methods, blanching as a pre-treatment method optimizes the onion drying process. Therefore, the combination of a fast blanching and hot-air oven drying (60 oC) as a pre-treatment may be favorable since the application of heat treatment is the most common strategy for stabilising foods due to its capacity of destroying microorganisms and inactive enzymes. Since thermal processing, particularly blanching for a long time can negatively affect levels of phytochemicals in onions by experiencing a thermal breakdown or leakage of components, there is a growing interest in identifying new non-thermal strategies for the food industry. Finally, the last part of this thesis aimed to investigate the effects of ultrasonic pre-treatment (non-thermal) and blanching prior to hot-air or freeze-drying of onions on the retention of phenolic compounds. These results showed that ultrasound pre-treatment is a potential alternative to conventional blanching pre-treatment in the different drying onion slices. It may be used in a combination with other processing techniques to obtain high nutritive dehydrated onions compared to that of the products dried without the pre-treatment. In general, the research conducted in this thesis makes a notable contribution to the existing knowledge because it gives insights into pre and post- harvest conditions that contribute to high phenolic content and antioxidant activity in onions. The pre-harvest treatments and the novel approach to the post-harvest processing methods (blanching and ultrasound) can serve as ‘recommendations and guidelines’ for the industry or the agriculture authorities at a national level