Effect of Culinary Practices on Phenolic Compound Content and Antioxidant Activity of Organically and Conventionally Grown Onion Bulbs

Abstract

Onions are a rich source of bioactive compounds, particularly phenolic compounds, which can help to prevent and treat chronic diseases. However, if not properly applied, culinary practices can compromise this nutraceutical potential. The objective of this study was to determine the influence of cutting and cooking versus immersion on the phenolic compounds and antioxidant activity of onions from organic and conventional farming. Standard spectrophotometric methods were employed to determine the levels of phenolic compounds and antioxidant activity in onion samples of various sizes (thin, medium, large and diced) from both types of cultivation. Analysis of these data showed that total phenolic levels ranged from 108.08 ± 7.4 to 507.21 ± 31 µg GAE/g during immersion in water, with the maximum level obtained after one hour in the conventional sample with a medium cut. The highest flavonoid content (75.46 ± 6.6 µg EQ/g) was obtained after one hour's immersion of the thin-cut organic onions, whereas the conventionally grown onions showed an increase in flavonoid content over the course of the experiment for all cut types. Conversely, when boiled, phenolic compound levels decreased substantially. Therefore, the optimal boiling time would be 10 minutes to ensure the maximum total phenolic and flavonoid content. DPPH inhibition decreased during boiling and immersion. However, the FRAP antioxidant activity increased, reaching its highest value in organic onion bulbs. Therefore, in addition to protecting the environment, organic onion bulbs are more effective at protecting consumers' health.