How Spicy Are Onions?

Onions are a unique vegetable and spice, possessing a remarkable flavor and taste. As a result, they are an essential ingredient in almost every meal. Depending on how much of their flavor you prefer, you can consume them raw or cooked.

Onions are mildly spicy compared to other spices. However, the spiciness of onions depends on the type of onions and whether the onions are raw or cooked. The reason is that onions contain an enzyme, isoalliin, that quickly catalyzes the production of thio-compounds (thiosulfinates and thiosolfonates) within the onion to yield the spicy flavor.

Since onions are mildly spicy, it is crucial to understand how to preserve their subtle flavor by learning about the different factors and conditions affecting onion spiciness. So read on to know more! 

Are All Onions Equally Spicy?

Red onions

Not all onions are equally spicy. Essentially, the various types of onions differ in their level of spiciness. For example, red onions are the spiciest among all kinds of onions, noted for their intense, aromatic, and slightly peppery flavor in foods and when in contact with any parts of your body. 

Red onions are most likely to make your eyes burn and tear when you cut them open because of their enzyme content and high concentration of sulphonamides. As a result, onions planted in high-sulfur-containing soils possess a more intense flavor.

Additionally, the onion’s time of the year or onion planting season affects their degree of spiciness. For example, onions planted in late Spring are less spicy than the others. In other words, certain factors influence the spiciness of onions. Here they are:

Factors Influencing the Degree of Onions’ Spiciness

  • High soil Sulphur content
  • Onion Planting season
  • Onion physical state (raw or cooked)
  • Onion color, etc. 

Do Onion Planting Seasons Affect Their Spiciness?

Seasonal onions

Planting seasons lend different tastes, flavors, and colors to onions year-round. Due to seasonal differences, some onions can be sweet, spicy, mild, aromatic, red, white, or yellow. 

Fun Fact: Seasonal differences among onion types are more noticeable in March because of the Fall/Winter transition to Spring/Summer of onions.

Below is a table highlighting basic facts about seasonal onions.

Onion seasonAvailabilityOnion characteristicsFlavorBest utility
Spring/SummerMarch-AugustThin skin
Transparent layers
Lighter color
High water content
Shorter shelf-life(30-60days)
Easily bruised
Mild-SweetRaw
Pickled
Light cooking
Grilling
Fall/winterAugust-MayThicker skin
Paper-like layers
Darker color
Lower water content
Longer shelf-life.
Mild-SpicyRoasted
Fried
Long cooking
Caramelizi-ng

How Spicy Are the Spiciest Types of Onions?

Red onions are the spiciest types of onions, often described as having an aromatic or intense flavor due to their high content of thio-compounds. Their dark-purple coloration adds to their taste, causing them to permeate every food recipe. However, their spiciness is equally toned down when exposed to heat during cooking.   

Understanding the science behind onion spiciness helps you decide what to use onions for, whether as part of your recipe or as a medicinal substance.

What Is the Science Behind Onion Spiciness?

You probably didn’t see this coming but believe it or not; there is a science and chemistry behind onion spiciness. The science of onion spiciness allows you to creatively manipulate onions for whatever you want without dealing with their intense flavor. It all starts from the raw onion state and their thiosulfinates.

Understanding the Chemistry of Onion Thiosulfinates

All members of the Allium species have sulfur, and onion is one of them. However, thiosulphinates are the critical compounds behind the spiciness— pungent, aromatic essence of onions. The thiosulphinate compounds are why your eyes burn and tear up when you cut an onion. They also give onions their taste/flavor, odor, and sometimes color. 

Fun fact: Interestingly, the raw onions do not inherently contain thiosulphinate compounds. These compounds only form when you do damage to the onions—cutting!

The enzymes in the onions react with thiosulphinate precursor molecules to create the thio-compounds when you slice onions.

Are Onions Spicy Enough To Be Considered a Spice?

Onions are spicy enough to be considered a spice because they possess the characteristic spicy, somewhat peppery, or hot flavor/feeling as other known spices like peppers, gingers, and garlic. 

Your taste buds are not the only determinant of what should be considered a spice. Essentially, the tactile effect of the substance on your skin and other body tissues determines if it is fit to be a spice. It is the same reason onions make your eyes tear with that slightly painful and peppery feeling, just like any other spice would.

Moreover, there is also the scientific or taxonomic classification and the general spice definition. To this effect, onions belong to the genius, Allium, precisely Allium Sativum, which means cultivated onion when translated from Latin. Allium is a genius covering most flowering plants with one seed leaf and includes tons of species. 

Examples of spices belonging to the genius Allium include but are not limited to

  • Onions
  • Garlic 
  • Chives
  • Leek
  • Scallion, etc.

So scientifically, onions are spice enough to be called a spice. 

In addition, the general definition of spices by Merriam-Webster’s online dictionary is an aromatic vegetable or product used to season or flavor foods. So, from the dictionary standpoint, onions are spicy enough to be considered a spice.

While there are varying degrees of spiciness, it is essential to know if all onion types have the same degree of spiciness and their related conditions.

How To Regulate the Spice Intensity of Onions

Suppose you do not appreciate the spiciness of onions; you can regulate the intense flavor to suit your preference by controlling the thiosulfinate compounds. Similarly, if you prefer to have higher concentrations of the onion flavor, it’s all within your control. Here is how:

To Get More Spicy Onions

Cut raw onions

If you are up for it, you can get more spicy onions by cutting and using them raw. Since the odor and flavor only form from fresh cutting, you’ll get a more intense spicy onion if you make more delicate, thinner slices. Moreover, the longer you leave the cut or sliced onions, the stronger the flavor.

To Get Less Spicy Onions

Getting less spicy and even sweeter onions is all about heat treatment. Cooking onions immediately after cutting prevents the enzymatic reaction and formation of the thiosulphinate compounds. Essentially, the longer you cook, the less spicy the onions. 

The reason is that heat breaks down the enzyme before it can catalyze the reaction and release the thiosulphinate compounds responsible for onion pepperiness or spiciness. 

You can regulate the degree of onion spiciness by subjecting it to varying degrees of temperature. For instance, you will get less spicy or pungent onions under low to medium heat and no onion flavor under high or prolonged heating. 

Frequently Asked Questions

Are Onions Spicy or Sour?

Onions are spicy instead of sour due to their pungent rather than tart flavor. They also belong to the same spice class as ginger and garlic. However, onions’ spiciness is mild and is easily lost with exposure to heat by cooking, frying, roasting, or grilling.

What Kind of Onion Is the Spiciest?

Red onions are the spiciest of all onion types, with a somewhat peppery feel added to their deep-purple color. They are readily available and are the most used types of onions for preparing most meals or as part of most recipes. Red onions are the sweetest between March and September. 

Is Onion Powder Considered a Herb or Spice?

Onion powder is considered a spice because it is a product of fresh onions that have been dried and ground to yield the onion powder. As a result, the onion powder retains the mild, somewhat peppery, and spicy flavor of the previously fresh onions.

Final Thoughts

The degree of onion spiciness rests on a delicate balance between the onion type and the processed state of the onions. However, since everyone’s spice palate is different, onion spiciness is also distinct for everyone. So even though we can generally agree that onions are spices and possess a mild/delicate pepperiness, it is up to every individual to decide how spicy onions are for them.   

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Bill Kalkumnerd
Bill Kalkumnerd

I am Bill, I am the Owner of HappySpicyHour, a website devoted to spicy food lovers like me. Ramen and Som-tum (Papaya Salad) are two of my favorite spicy dishes. Spicy food is more than a passion for me - it's my life! For more information about this site Click

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