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