Studies of bioactive compounds in brown and red onions (skin and flesh) : A dissertation submitted in partial fulfilment of the requirements for the Degree of Master of Science in Food Innovation at Lincoln University

Abstract

Onions (Allium cepa L.) have been cultivated and consumed for thousands of years around the world. The bioactive compounds in onions such as phenolic compounds, are beneficial for the health of humans and can potentially reduce the risk of some diseases, which will lead to longevity of the population. Onion skin is non-edible and is always discarded by consumers and food producers. However, onion waste, mainly consisting of onion skin, is still a rich source of bioactive compounds. Red and brown onion samples in this dissertation were divided into two parts (skin and flesh) and two different extraction methods using two different solvents were tested. Total phenolic content (TPC) and total flavonoid content (TFC) were conducted to assess the bioactivity of onion extracts. The TPC, TFC values were significantly higher in the skin for both kinds of onions (p < 0.05) and 70% ethanol also had a significantly positive effect on the yields of total phenolic compounds and total flavonoids (p < 0.05) compared with reverse osmosis (RO) water. Red onions (both skin and flesh) had higher values than brown onions. Three antioxidant assays (ABTS, DPPH and FRAP assays) were conducted and onion skin also showed higher antioxidant activity than flesh. High performance liquid chromatography (HPLC) and ultra-high performance liquid chromatography (UPLC) were employed to identify and quantity quercetin in onion skin and flesh. The quercetin content had a similar distribution to TPC and TFC.