Question corner Science

Why does the onion lose its colour when fried in oil?

Onion rings. Photo: S. Siva Saravanan   | Photo Credit: S_SIVA SARAVANAN

Why does onion lose its colour (become translucent) when fried in oil?

Vagadheeswari, Chennai

Onion ( Allium cepa) is a monocot belonging to Alliaceae family. It is richly endowed with nutrients such as sulphur, selenium, fibre, polysaccharides and antioxidants. It comes in various colours ranging from white to pale yellow, orange, pink and dark purple. The colour of the onion is due to the presence of anthocyanins and anthoxanthins which belong to the class of Flavonoids. Anthocyanins impart orange, yellow or purple colouration while anthoxanthins impart pale yellow or white colour. These pigments are present in the cell sap and are water soluble.

These pigments change their colour with the pH of the medium. Cooking vegetables and fruits brings about many changes in their texture, colouration and flavour Frying the materials using oil makes them undergo certain chemical changes such as caramelisation (conversion of sugars to carbon, water), Maillard reaction (reaction of aminoacids and reducing sugars bringing about suitable flavour) that result in the browning of the food material and also imparting characteristic flavour due to the release of certain chemicals such as diacetyl.

On frying in oil, onions initially lose moisture and their size is reduced. As the pigments anthoxanthins and anthocyanins are temperature-sensitive, pigment loss will occur on frying onions in oil. This results in translucent onions. Pigment loss is more pronounced when onions are kept in water for long time or boiled in water as the pigments are water soluble. Frying makes the onions tender and glossy due to the absorption of oil.

T. Bhavani, PhD, Bengaluru

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Printable version | Oct 17, 2021 1:55:11 PM |

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