Flavor Investigator: Onions

Coming in all shapes and sizes, the onion (Allium cepa L.) is a vegetable from the genus Allium.  This humble vegetable has limitless uses in cooking because when used in small amounts it can add another dimension of flavour.  Typically in raw applications there is a preference for bulbs which are milder as pungent varieties can easily over whelm the dish. In contrast, manufactured products such as sauces and dips tend to contain more pungent variations varieties. In food processing it is not too surprising if you see onion powder or salts used.


As you may be aware there are many different onions- all of which possess their own unique flavour notes.  Scallions also known as green onions are characteristically known for their long, hollow, green leaves with a white bottom. The flavour is sweet and mild with a slight bite. They are able to be used cooked or raw and are seen as a topping for many onion soups. Similar in appearance and flavour as scallions, spring onions have a mild flavour but can be considered slightly spicy when eaten raw and sweet when cooked.

Shallots are a small, slender and lighter in colour than red onions. When compared to red onions, they have a milder flavour but are more assertive than yellow, with a hint of garlic flavour. Vidalia onions are considered to be one of the mildest onions as they are very sweet.  This sweetness makes them ideal for raw applications and when caramelized have an even richer sweetness.

Red onions are known for their inherent spiciness and assertive flavour. Although cooking does help to tone these notes down, it is still present despite becoming sweeter. Yellow onions are not as spicy when compared to red onions but still pack a punch when raw. Cooking yellow onions produces an intense sweet flavour. Finally, white onions have a milder flavour then yellow onions and are somewhat sweeter in flavour.

How the Flavour Intensity of Onions is Determined

There are two factors which determine the flavour intensity of onions: the variety of the onion and the sulphur content of the soil.  As described above there are multiple varieties of onions however, they are able to be broadly grouped into two categories: mild spring onions, and pungent storage onions.

Mild spring onions are planted in the fall and harvested in the early spring while storage onions are planted in the spring and harvested in the late summer or early fall. The flavour of spring onions are sweeter and less pungent when compared to storage onions.

Interestingly, the flavour intensity of an onion is partly dependent on the sulphur content of the soil in which it was grown. In soil, sulphur exists as sulphate salts and is absorbed by the roots of the onion. In the roots the sulphate is converted to sulphur-containing amino acids such as cysteine. Eventually in the onion bulb these sulphur-containing amino acids are converted to thiosulfinates and thiosulfonates. Therefore, more sulphur in the soil means a more intense pungent flavour.

Possible Unique Pairings for Onions

  • Onions and Beets- Beets are naturally sweet and due to this they are able to offset raw onions. However, if raw onions do not appeal to you then try cooking the onions and pairing it with pickled beets.
  • Onions and Cucumber- Although these two may seem like polar opposites the two are able to work together. Cucumbers are cooling, refreshing and have a light flavour this is a lovely contrast to sharp, pungent onions.
  • Onion and Nutmeg- Typically nutmeg is associated with sweeter applications but this should not always be the case. Nutmeg can have a slightly bitter taste which can counter a cooked onions sweetness. To taste this combination try adding a small amount of nutmeg next time you caramelize some onions!

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