Everything You Should Know About Onions

7 Mins
September 11, 2017

Hello! I am Bezalel Adainoo, the author of Stay Well Now. My friends call me Bez. As a professional food scientist, I have been asked many questions by people from different walks of life seeking the right information on what food to eat and how that will affect their health.

Onions are one of the commonest spices used all over the world. Though onions are used as a spice, they are often used as a vegetable too. This is one of the cases where one food commodity can be considered both a vegetable and a spice. Onions belong in the same family as shallot, garlic, chive, Chinese onion and leek. Onions are rich in essential components that promote good health. There’s almost no soup, sauce or gravy that is made without onions. Onions are part of our daily diet, that’s why it is important for you to know everything about them.

Types of onions

Basically, there are three types of onions; the red, the yellow and the white. These types of onions have different physical appearances, flavours, and tastes. Also, they have different degrees of astringency and pungency.

Yellow onion can be used for any recipe at all. Sometimes, it referred to as brown onion. It has a yellow, papery skin colour and white flesh. Generally, if a recipe says onion without specifying which one, it is assumed that the recipe is referring to the yellow onion. It is the most pungent of the types of onions and the one that will make your eye water the most when you cut it. Ideally, yellow onion should be cooked slightly longer than the red and white onions. When cooked, yellow onion has a light golden-brown colour with a sweet taste.

Yellow onions are used for all kinds of recipes

Red onion has a mild flavour and taste; hence, it is not as pungent as the yellow onion. Red onion has a purple/reddish skin which is due to the antioxidant content of the onion. Anthocyanins are the antioxidants responsible for the deep purple colour of this type of onion. The purple skin makes it a colourful option for garnishing foods and salads. Because they are less pungent, red onions can be eaten raw or used in salads.

Red onions are very popular for garnishing foods because of its color

White onion when eaten raw, might overpower the flavour of your dish. So, it is best used by grinding it together with other ingredients to lessen its sharpness. White onion has similar properties as the yellow type; it has a sweet taste when cooked. Yellow onion can be replaced with white onions in recipes that do not specify which type of onion to use.

White onion has such a good taste

Composition of onions

Fresh onions are about 89% water, 9% carbohydrates, 1.7% fibre, with very small amounts of proteins and fats and other components which contribute to the healthful benefits of onions. Onions contain antioxidants and sulphur-compounds as well. The antioxidants and these sulphur-compounds protect against cancers and heart diseases.

In many countries, onion is the most common dietary source of quercetin, a chemical that has the potential to lower blood pressure and boost heart health . Raw red onion contains a little more quercetin than raw white onion which is also higher in quercetin than raw yellow onion. Cooking reduces the quercetin content in onions by about 27%.

(Related: 20 foods proven to support heart health)

Red onions in particular, contain anthocyanins (potent antioxidants) which give them their distinct purple/reddish colour. These healthful chemicals (anthocyanins and quercetin) are more concentrated on the outer flesh of the onions. Over-peeling onions could lead to losses of about 20% of quercetin and 75% of anthocyanins. As much as possible, remove only the papery skin on the onion to help retain its quercetin and anthocyanin contents in the outer flesh of the onion.

In fact, red onions have the highest amount of antioxidants, followed by yellow onions then white onions.

The sulphur compounds are responsible for the smell of onions, the flavour of onions and the reason why your eyes water when you cut onions. Also, these sulphur compounds protect against cancer, inhibit the growth of harmful microorganisms and prevent the formation blood clots. The prevention of the formation of blood clots is particularly important because when your blood clots while flowing through your blood vessels in your body, the blood clots could clog the blood vessel preventing the free flow of blood. This could lead to stroke if the blood clot blocks the flow of blood in your brain or cause serious complications if the clot blocks the flow of blood in other vital organs of your body.

Additionally, onions contain vitamin C, folic acid (vitamin B9), vitamin B6 and potassium. The vitamins are mostly lost during cooking, however, if you’re eating the onions raw, you will certainly get those nutrients.

Why do onions make you ‘cry’ when you cut them?

Earlier, I mentioned that yellow onions make you ‘cry’ the most when you cut them. But it is also important to note that the ability of onions to cause your eyes to water is also dependent on the characteristics of the soil in which it was grown.

Basically, onions make your eyes water because of some of the sulphur compounds they contain. When you cut onions, they release a gas (containing sulphur called lachrymatory factor) which stimulates the nerves in your eyes. This causes a stinging sensation leading to the production of tears to flush out the gas from the cut onion.

The root end of the onion bulb contains the higher amounts of the gas released from cut onions than the flesh. Leaving the root end intact would reduce the intensity of the stinging sensation and irritation.

Also, you may cut onions under running water. This will prevent the gas from the cut onion from being released into the air to sting your eyes. However, the downside of this is that you might lose considerable amounts of the vitamins and antioxidants in the process.

Is it safe to leave cut onions overnight? How safe are onions?

There have been some misconceptions regarding the safety of cut onions left out in the open overnight. Some of these are of the thought that when you leave cut onions out in the open overnight, they ‘absorb’ bacteria, they become ‘poisonous’ and you could die from eating cut onions left overnight, even in the fridge. This is not the correct presentation of what actually happens.

Really, the truth is all foods contain water, even the foods that appear to be extremely dry. The amount of water in the food determines how susceptible it will be to the activity of microorganisms (bacteria, yeasts, moulds). Foods that contain lower amounts of water would be less susceptible to the activity of microorganisms while those that contain higher amounts of water would be more exposed to the activity of microorganisms. Onion is one of the foods that has a high water content (89% water). This exposes it enormously to microbial activity. And any other food with such high water content would be exposed to just as much microbial activity or even more. The microbial activity would lead to contamination and subsequent spoilage.

Irregardless, cutting onions and leaving it overnight before use does not pose any significant health risk if appropriate safe food handling techniques are employed during cutting and storage. Washing your hands and surfaces that would come into contact with the onion before handling the onion (or vegetable or any food at all) will significantly reduce the chances of contamination of the onion. Also, store them in a tightly sealed container in a refrigerator… it is safer there than openly on the kitchen counter . In the fridge, cut onions can stay for about 7 days. Even so, it is better to use stored cut onions within a day or two.

(Related: Safe food vs healthy food which one you should include in your diet)

There have been a few cases where some people have had allergic reactions to onion. In some extreme cases, some people have allergic reactions just by touching onions. Nonetheless, there are fairly common cases of intolerance to raw onions in some people. Symptoms of onion intolerance have included stomach upset, gas and heartburn. Further, people with irritable bowel syndrome are advised to stay away from onions as consumption may lead to unpleasant digestive symptoms like diarrhoea, gas and bloating.

Undoubtedly, not all animals can handle all the components of onions. Some (like dogs, cats, horses, monkeys) might even die as a result of onion consumption so if you have a pet, be advised to keep it away from onions.

Shelf Life of Onions

The conditions under which onions are stored affect how long the onion will remain wholesome. Also, the storage conditions affect the amount of healthful chemicals onions contain. Onions should be stored in a well-ventilated place away from bright light. It should be away from bright light because the light will cause the antioxidants to lose their potency. Storing whole raw onions in plastic bags will make them go bad quickly.

When the shelf life of an onion is up, it begins to sprout. When stored well, whole raw onions should last about 60 to 90 days before they begin to sprout. It is important to note that the longer you store the onion, the more its antioxidant content reduces so don’t store them for too long to enjoy the healthful benefits of the antioxidants.

4 Important Tips on Onions

  1. Leave the root end of the onion bulb whole to reduce the eye stinging that makes your eyes water when you cut onions.
  2. Onions might taste bitter after sautéing because of the high heat so it is better to use low or medium heat while sautéing onions.
  3. To keep the odour of onion out of your mouth, rinse your mouth with a 50% to 50% mixture of lemon juice and water.
  4. To keep the odour of onion off your hands, rub an aluminium (or cast iron or carbon-steel) pot with salt with your hands

To summarize, onions are healthy vegetables/spices that are not foreign to you. They are rich in antioxidants that protect against heart diseases and allow free flow of blood. When you store them well, they last but don’t store them for too long so that you can have the beneficial chemicals they contain.