Red-skinned onion phenolic compounds stability and bioaccessibility: A comparative study between deep-frying and air-frying

In this study, the effect of deep-frying and air-frying on the stability and gastro-intestinal release of red-skinned onion phenolic compounds have been assessed by high-resolution mass spectrometry. Forty-four phenolic compounds were identified and quantified in raw onion. Both the frying treatments caused an increase in total phenolic compounds, which was more evident after air-frying (47.4% vs 18.6% increase). This increase was mainly a consequence of the water loss observed during frying. Quercetin was the most important phenolic compound detected both in raw and air-fried samples (99.5 ± 1.7 and 133 ± 2 μmol/100 g, respectively) while deep-fried onion was richer in quercetin-4’-O-glucoside and quercetin-3-O-glucoside-4'-O-glucoside (48.7 ± 0.9 and 41.8 ± 0.3 μmol/100 g, respectively). After digestion, 38.6%, 60.5% and 89.6% of total phenolic compounds were bioaccessible in raw, deep-fried and air-fried onion. Air-fried onion exhibited the highest concentration of bioaccessible phenolic compounds (206 ± 1 μmol/100 g). Oxidative degradation and hydrolysis reactions of quercetin-hexosides occurred during in vitro digestion. Hence, air-frying can be considered a healthy cooking method able to preserve and release after digestion most of the health-promoting compounds found in raw onion and with a lower amount of fats and polar toxic compounds compared to deep-frying.