Why Does Cabbage Turn Blue? How Can I Fix It?

on November 2, 2022

Red cabbage is a common ingredient in many dishes such as stir fries and coleslaws. It produces fantastic color and interest in these dishes, however, have you ever had the experience where red cabbage literally turns blue which often happens overnight after the dish has been cooled and refrigerated. Is this dish safe to eat and what has caused the cabbage to turn blue?

The reason that red cabbage sometimes turns blue is that it contains a naturally occurring dye called Anthocyanin which is the compound that causes the red color in the cabbage. This compound has an unusual property where its color can change depending upon the pH or how acidic or alkaline the dish has become.

Typically, red cabbage has a pH that is slightly acidic which produces the red color that we are all familiar with, however when the pH rises to a neutral or even slightly alkaline range the color of the cabbage can change to blue. This most frequently happens when the cabbage is cooked rather than used in a salad.

The cabbage itself is still safe to eat even if the color does look a little bit strange. So what can you do to prevent this from happening?

How To Stop Red Cabbage Turning Blue

If you are concerned about cabbage turning blue there is a simple thing that you can do which will ensure that it remains red in color which is to add an acidifying agent to your dish. This will ensure that the pH remains acidic even after the food is cooked. Examples of an acidifying agent include any citrus juice or vinegar which contain citric and acetic acid respectively.

However, when selecting your acidifying agent you do need to consider how the addition of it is going to affect the flavor of the dish. Generally, you do not need to add much of this acidifying agent; usually a teaspoon in a dish will often be enough, to prevent this from happening.

The other obvious alternative to red cabbage is to use traditional white or green cabbage which does not naturally contain the dye that causes the color change.

Is Red Cabbage The Only Type Of Food That Contains A Red Dye?

The red dye present in red cabbage Anthocyanin is present in a range of common foods which includes things like blueberries, raspberries, beetroot, purple cauliflower and purple carrots. Basically, it is present in most fruits and vegetables that have a purple color and it can be extracted from the food when it is cooked, which is one of the reasons why adding a purple vegetable will often result in other ingredients also being dyed purple in the dish.

This happens because the compound is water soluble which means that it readily leaches into water base liquids which produce the characteristic purple color.

Why Does The Cabbage Turning Blue Only Happen When It Is Cooked?

The main reason that you tend to see this happening after a cooking process Is because the cooking process creates chemical changes within the food that you are eating which means new compounds are formed which can result in the food becoming more alkaline.

Whereas when it is used in a salad such as coleslaw it is extremely rare for the cabbage to turn blue because there is limited chemical change occurring. Additionally, most salads have a dressing which often contains vinegar of some sort which naturally acts as an acidifying agent for the cabbage preventing it from turning blue.

How Can You Make Your Cabbage Turn Blue On Purpose?

If you are cooking for a special occasion where you want to intentionally make the cabbage turn blue you may be wondering whether this is possible. And the answer to this is yes it is in most cases provided that you are not cooking a dish where there is an acidifying agent in the sauce that you are using to create the dish.

There are several ways that you can increase the alkalinity of a dish naturally without having to do anything particularly special. The first is to add a range of vegetables with the cabbage such as other brassicas, carrots and asparagus. Additionally, there is also a range of herbs that can increase pH including basil, cilantro, garlic, ginger, mint, chives, thyme, parsley, and cumin. You will just have to experiment a little bit to achieve the desired effect for your dish.

Is This Dye Useful For Other Applications?

The dye contained within the cabbage is an extremely useful chemical compound because of its ability to change color with pH. This means that red cabbage can be used to create a homemade pH kit which can be useful for testing swimming pools and soil.

A photograph of the different colors that red cabbage producers is shown below. This can be used in the same way as a pH chart in a commercially produced testing kit to determine the pH of a sample.

To create a testing solution it is relatively quick and simple to do and only takes a matter of minutes. The first step required is to cut up the cabbage into fine pieces and then place it into a jar or a bowl. The next step required is to pour boiling water over the top of the cabbage to extract the dye. This will only take a matter of a few minutes and you will typically end up with a purple solution which can be used for testing after the cabbage has been strained out of the solution.

To test the sample, mix it with the indicator solution and then compare the color against the chart above. Amount that you need to mix doesn’t matter all that match, however, I would generally advise putting one part of the indicator solution to three parts of the test solution which should be enough to give you a pretty good idea about what the pH is of the sample.

In cases where you want to use it to test dirt from your garden it is best to make up a slurry solution of the dirt and water which can be allowed to sit for a few minutes before testing the sample.