Why Did My Onion Turn Blue in Slow Cooker? Easy Explanation

While cooking onions in your slow cooker, you may notice that your onions suddenly have turned blue. As blue is not a color we used to see while cooking fresh food, it can be kind of shocking to see it.

Now, do you know why your onions might turn blue while cooking in a slow cooker? It is not a regular case and a slow cooker has nothing to do with it. Your onions can turn blue anytime while cooking them in a normal frying pan also.

In this article, we will explain the reason why & when onions turn blue, how can you turn them back red, and what you can do to prevent them from turning blue. So, without further talk, let’s get started!

Why Did My Onion Turn Blue in Slow Cooker

Why Do Onions Turn Blue in a Slow Cooker?

To understand why onions may turn blue while cooking, you may need to know some basics of chemistry. Red onions contain a natural pigment named “Anthocyanin” which is responsible for their natural reddish-purple color.

While the onion is sliced or cooked, some substances can get access to the cells of the onion and react with the anthocyanin. As a result, the color of the onion might change. However, there can be three possible reasons why your onions might turn blue.

1. Acidic Elements React with Anthocyanin

When onions are cooked in a slow cooker with other foods, the acid present in the food reacts with the anthocyanin and makes them lose their hydrogen ions. As a result, the chemical structure of the anthocyanin pigment changes, and the color of the onion turns pink, purple, or bluish-red.

2. Alkaline Elements React with Anthocyanin

Some foods like legumes, nuts, tofu, leafy greens, dairy, and most proteins contain alkaline elements like magnesium and calcium. While cooking onions with alkaline foods, the anthocyanin reacts with these alkaline or basic elements and makes the color of onions blue or violet.

3. Metal Reacts with Sulphur Compound

Another possible explanation is that onions contain sulfur as a compound which is known as ” Sulfides or Thiosulfate “. With boiling water, it can react with the metal of the slow cooker’s stoneware and make another compound containing Sulphur, hydrogen, and iron ions or copper ions. As a result, the onions turn blue, bluish-green, or cyan.

Except for these reasons, onions themselves have a variety of pigments. For example, white onions contain “Flavanol”, orange onions contain “Lycopene”, etc. So, while cooking different types of onions at the same time, the pigments can react with each other and produce a blue or purple hue.

What To Do If Your Onions Turn Blue?

Now you know all the possible reasons that may turn your onions blue while cooking. So, you need to take steps accordingly to make them red again. Here is what you can do:

  1. For Alkaline Foods: If your onions are turned blue while cooking with alkaline foods like fruits, nuts, legumes, and vegetables, you can turn them back to red by adding a dash of vinegar or lemon juice.
  2. For Acidic Foods: If your onions are turned blue while cooking with acidic foods like meat, poultry, fish, dairy, eggs, grains, and alcohol, you can turn them back to red by adding a little bit of baking soda.

You can also make your red onions blue or blue-green color simply by adding vinegar or baking soda while cooking. As now you have full control over the color of onions, you can make foods with different hues to make your food look special and unique.

Frequently Asked Questions and Answers – FAQs 

Is It Safe to Eat Onions That Have Turned Blue?

Yes, onions that are turned blue are completely safe to eat. However, if the onions have turned blue due to fungus or mold attack, then it isn’t safe to eat. But if a fresh red onion turns blue or green while cooking, it may not look appealing but it is 100% edible.

What Is the Blue Stuff on Onions?

If you store your onions in a damp place, and you notice some blue stuff on onions, then most probably they are affected by a species of penicillium which is also known as blue mold. Onions may get affected by blue molds through wounds, uncured neck tissue, or bulbs of the onion. Blue mold is poisonous and you shouldn’t eat onions that are affected by it. 

Why Did My Shallots Turn Blue?

Shallots and onions are almost the same thing. However, shallots contain more sugar than onions. As a result, they can turn blue easily while cooking with alkaline or acidic foods.

Why Is My Egg Turning Blue When Added with Onion?

Eggs basically contain acidic elements like amino acids which may react with the pigment of the onion that releases a blue pigment into the cooking pot. By observing the blue pigment released by onions, the egg can turn blue.


To conclude, the main reason why your onion might turn blue while cooking in a slow cooker is Anthocyanins contained in the onions. We hope the mystery of blue onions has been solved after reading this article. If you have found this article helpful or have other queries regarding this topic, feel free to leave a comment in our comment box below. Thanks for reading and Happy Cooking!

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