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How to keep well-coloured fruit and vegetables

By Christina Blais Published March 5th, 2013 (0)
How to keep well-coloured fruit and vegetables
Have you ever noticed that fruit and vegetables change colour when cooked? Blueberries turn green in muffins, broccoli becomes yellowish in pasta salad, braised red cabbage turns blue…
 

These changes can be explained by the presence of chameleon molecules that have the ability to change colour when they come into contact with certain metals or ingredients. Here is a compilation of the most common colour changes and ways to deal with them.

Green blueberries or cherries in muffins, cake, pancakes, clafouti or pudding

Blueberries and many other fruits (grapes, cherries, raspberries…) contain anthocyanin, a molecule that is a reddish colour in its naturally acidic environment: fruit. But, when it comes into contact with alkaline ingredients (the baking soda in a cake batter, for ex.), the anthocyanin turns blue.

What to do?
The colour change occurs mainly when a recipe contains too much baking soda or if it lacks acidity. You can acidify the batter by adding a bit of lemon juice or lower the alkalinity by reducing the amount of baking soda or powder.

Red cabbage turns blue

Red cabbage also contains anthocyanin molecules that turn from red to blue-green in an alkaline environment. The colour change is reversible. Check for yourself. Try our red cabbage experiment below.

What to do?
To keep the beautiful colour of red cabbage, make sure there is also an acidic ingredient in the recipe. Braised red cabbage, for example, is made with vinegar, apple juice, white wine or even apple pieces.

Green vegetables in salad turn yellow-brown

This is the same problem that happens when green vegetables are cooked. However, in this case acidic ingredients added to the vegetables are responsible for the colour change.

What to do?
Wait to pour dressing on the vegetables or add them to the salad at the last minute. You can also try to reduce the acid level in the recipe (vinegar, lemon juice). Blanching vegetables does not slow down the reaction.

 

Potatoes darken when cooked

This colour change is caused by a reaction between iron and a phenolic compound (chlorogenic acid), both naturally present in the potato. The reaction leads to the creation of a greyish-black coloured molecule. Certain types of potato are more susceptible than others, depending on the growing conditions.

What to do?
Add a dash of lemon juice or a pinch of cream of tartar when the potatoes are half cooked (potatoes will take longer to soften if acid is added at the start).

Broccoli, green beans or asparagus turn olive green after cooking

Chlorophyll gives these vegetables a nice green colour but they turn yellowish after coming into contact with acidic substances. Plant cells are protected by a wall, even if they contain a lot of acidic substances. The wall breaks down during cooking. This allows acids to react with chlorophyll and so the vegetable changes colour.

What to do?

An old cooking proverb tells us to simply cook green vegetables uncovered if you want to preserve their beautiful green colour. However, it’s not the lid that is important; cooking time makes all the difference. Whatever the cooking method (steam, water, sautéed), it is important to keep the cooking time to between 5 to 7 minutes. Any longer and cell walls break down too much, making a colour change inevitable. Rinse green vegetables with cold water afterward to quickly end the cooking process and limit any further damage.

 

What is the pH ?

Food acidity is often indicated by pH level. It is the measure of a food’s hydrogen-ion concentration. The pH scale ranges from 1 (very acidic) to 14 (very alkaline) while a pH of 7 is neutral. Few foods have a pH above 9. Here are some examples. 2 lime juice, lemon juice
3 vinegar, rhubarb, grapefruit
3.5 apples, blueberries, oranges, wine
4 pears, peaches
4.5 tomatoes, buttermilk, bananas
5 carrots, cucumbers, squash
5.5 spinach, beef
6 peas, carrots, potatoes
6.5 asparagus, egg yolk, milk, rice
7 water, crab
8 egg white
8.5

baking soda

 

«Heavy metal» effect

Iron, zinc, aluminum and tin present in utensils, pans and sometimes water can also change the colour of fruit and vegetables. For example, compounds called flavonoids (found in veggies like cauliflower and onions) are normally colourless but can turn pink, blue, yellow or grey in the presence of some metals. What to do? Use stainless steel knives and pans.

Try it! Our red cabbage experiment

Cook some finely chopped red cabbage in boiling water for about 5 minutes (this step extracts anthocyanin pigment from the cabbage cells). Drain the cabbage and keep the cooking water. Let the water cool and pour it into two clear glasses. Then have some fun with the following experiment:

• In the first glass, add a bit of vinegar… the cooking water will turn pink.
• In the second glass, add a pinch of baking soda… the cooking water will turn blue-green.

Now, do the opposite:
• In the first glass, add a pinch of baking soda: the cooking water will bubble (the baking soda reacts with vinegar to produce carbon dioxide) and turn blue again.
• In the second glass, add a bit of vinegar… there is more bubbling and the cooking water turns back to pink.

Christina Blais

Christina Blais

For Christina Blais, explaining food chemistry to the masses is as simple as making a good omelet. Holding a Bachelor and Master degree in Nutrition, she has been a part-time lecturer for nearly 20 years in the Department of Nutrition at the Université de Montréal, where she teaches food science courses. She has been sharing the fruits of her experience with Ricardo since 2001, during his daily show broadcast on ICI Radio-Canada Télé. And diehards can also read her column, Chimie alimentaire (Food Chemistry), in each issue of Ricardo magazine.

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