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Why do carrot pieces turn green when cooked in a cake or muffin?

Have you heard of the situation where carrot shreds in carrot cake turn bright green after baking? Some cooks think this is crazy. Do you know what circumstances produce the colour change? Is it the sequence ingredients are added, the baking powder, the type of oven (gas vs. electric) used? People seem to either be familiar with the colour change or deny that it happens. Any ideas?

Answers so far - it could be a reaction to the baking soda. You may try this experiment next time: Bring a pot of water to a boil. Put your shredded carrots in a fine mesh colander, and dip it in the boiling water for 5-10 seconds, and then straight in to ice water (to stop the cooking process). Now they are blanched, and are less sensitive to chemical reactions.

OR Sometimes cakes will turn greenish if left to cool in a metal pan. This is due to the oxidisation of the metal, and can be prevented by turning the cake out onto a cooling rack after about 10 minutes. We have heard of carrots turning green when bought in packages pre-grated. This could also be due to oxidization. You might try peeling your carrots first, before grating them.

OR The colour change in your carrots is probably because they have been mixed or baked in a metal pan. Some aluminium and stainless steel dishes can cause the carrots to oxidise and turn green. Use a glass bowl to mix, and line metal baking pans with parchment.

OR It is simply oxidisation. Sometimes it may also cause a reaction when mixed in an aluminium or stainless steel bowl or even when baked in an aluminium pan. If that's the case, you could just mix it in a glass bowl and line your pan with parchment. It may be even a reaction from the baking soda/powder. Carrots naturally turn green if even left out, especially uncooked shreds.

OR Make sure to peel all of your carrots before shredding them and you won't have bits of green in your cake!

Baking soda can have an effect on the colour of fruits. Baking soda is chemically classified as a base - above 7 pH- in order to work, baking soda must be combined with an acid-below 7 pH- (high school chemistry). Too much baking soda made blueberries turn greyish-green. If you replace the baking soda with cream of tartar (an acid), the cream of tartar makes the blueberries turn more of a magenta colour. So maybe the same with carrots?

Carrots turn green when the batter contains too much baking soda, or when the soda isn't evenly mixed in the batter. Carrots contain pigments that are sensitive to changes in pH balance. When the shreds of carrot come into contact with the alkaline baking soda, a chemical reaction takes place that causes the pigments to change colour. Ipso facto, green carrots!

If your carrots turn inexplicably green with a recipe you've used many times before, the likely culprit is that the baking soda didn't get fully dispersed throughout the batter. If your carrots are turning green every time you make the recipe, the ratio of baking soda in the recipe is probably off. Try reducing the amount by a quarter teaspoon next time you make the recipe. Unappetizing though they may be, cakes and loaves with green carrots are perfectly safe to eat and taste no different than normal cakes! Blueberries, sunflower seeds, and walnuts are also reactive to alkaline environments and experience similar colour changes when the amount or distribution of baking soda is off.


A more in depth scientific discussion on the problem: An analysis of why carrot pieces might turn green in a cake.

As discussed by the Singapore Science Centre www.science.edu.sg

Question:
Any idea why the carrot shreds in a carrot cake would turn (bright) green? I was trying 3 different recipes, and only one of them turned green. It had more baking soda and no baking powder, can orange carotenoids turn green when basic?

I did cook it in an older metal pan, (the other two were in non-stick & glass pans). But the shreds were green all the way through the cake, not only at the outside. Also, that cake was made with oil, not butter as the other two were, and was much darker in colour (brown).

Finally, I didn't actually notice the green colour until the second day, but I don't know if that was because I was distracted at first, or if the colour didn't develop until later.

Answer:

Carotenoids would not turn green in the presence of baking soda or baking powder. A slice of carrot would not change its colour even in a solution of 0.1 M sodium hydroxide. It would be difficult to explain the phenomenon without knowing the recipe of your carrot cake that changes colour. Metal ion and pH could account for the colour change if the ingredient contains some anthocyanins, which are water-soluble cell sap pigments found in many vegetable. For example, red cabbage is one anthocyanin-containing vegetable. The pigment in red cabbage is actually purple in the raw tissue. It is confined to the outer layers of cells of the cabbage leaf. Unless the cooking water is acidified, the pigment will change to a dull and unappetising blue. It is to prevent this that slices of sour apple are sometimes included with red cabbage when it is cooked. Certain minerals react with anthocyanin pigments to give a blue-coloured complex. Red cabbage shredded with a stainless-steel blade, such as a floating-blade peeler, turns blue very rapidly from the reaction between the iron of the peeler and the pigment of the cabbage.

Carotenoids are polyene hydrocarbons biosynthesised from eight isoprene units. They include the yellow, orange, and red-orange fat-soluble pigments. The colour of carotenoids is due to the large number of conjugated double bonds (double bonds alternating with single bonds) in the molecules. Carotenoids are found in the chloroplasts of green leaves, where they are masked by the high concentration of chlorophyll and in such yellow vegetables as sweet potatoes, winter squash, and carrots. The red pigment in tomatoes is a carotenoid, lycopene. Carotenoid pigments are of two types, carotenes and xanthophylls. Carotenes, which include - and -carotene and lycopene, are hydrocarbons with 40 carbon atoms in the molecule. Xanthophylls contain, in addition to carbon and hydrogen, one or more atoms of oxygen. The red-orange -carotene is the most common carotenoid. The concentration of carotenoids in carrot is 54 ppm (on dry weight basis). For information on structure and nomenclature of carotenoids, please refer to the URLs below:

http://dcb-carot.unibe.ch/nomen.htm

http://www.chem.qmul.ac.uk/iupac/carot/

Normally the central portion of the carotenoid molecule is in the all-trans form, which makes this part of the molecule linear. Isomerisation of the molecule occurs when these unsaturated carotenoids are heated in the presence of acid. The all-trans form of the pigment changes over to a cis configuration. The bend occurs at a double bond, in which case the molecule is no longer linear. This change in shape reduces resonance in the molecule and so the intensity of the colour. -Carotene in the cis form, for example, changes from the typical red-orange to a paler yellow-orange.

Carotenoids are generally quite stable in their natural environments, but when food is heated or when they are extracted into solutions on oils or organic solvents they are much more labile. Carotenoids in food are stable even at high temperatures. Fat-soluble carotenoids are not lost to the cooking water, but appreciable amounts may dissolve in the table fat used to season a vegetable such as carrots. When slices of carrot have been in contact with boiling water for 2 or 3 minutes, the hue shifts slightly towards yellow. On heating in the absence of air there is a tendency for some of the trans double bonds of carotenes to isomerise to cis. Longer heating, and especially overcooking, may result in a trans-cis shift and some loss in intensity of colour. The longer the vegetable is cooked and the higher the temperature, the greater the change in hue. Because of the quantity of carotene present, cooked carrots are still bright and attractive.

Baking soda can have an effect on the colour of fruits. Baking soda is chemically classified as a base - above 7 pH- in order to work, baking soda must be combined with an acid-below 7 pH- (high school chemistry). Too much baking soda made blueberries turn greyish-green. If you replace the baking soda with cream of tartar (an acid),  the cream of tartar makes the blueberries turn more of a magenta colour.  So maybe the same with carrots?

See John's visit to Potts the Bakers, leading supplier of carrot cake in the UK. They had a similar issue and overcome it with a change in formula of baking powder and a good clean out of equipment. Click here.


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