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I've noticed that when I bake banana bread with certain brands of self-raising flour, the finished product looks more or less like any other cake.

Banana bread without black spots However, when I use flour with sodium bicarbonate as the leavening agent, black spots appear peppered throughout the cooked cake.

Banana bread without black spots

What is the chemistry behind this phenomenon?

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  • $\begingroup$ Can those be banana seeds? :) $\endgroup$
    – andselisk ♦
    Nov 6, 2017 at 0:57
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    $\begingroup$ Totally. It's just that they're only visible when I used sodium bicarbonate as the raising agent. $\endgroup$
    – thelastquagga
    Nov 6, 2017 at 1:00
  • $\begingroup$ Bananas are seedless (at least the ones we eat today). $\endgroup$
    – Raul Luciano
    Nov 6, 2017 at 11:47
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    $\begingroup$ @RaulLuciano Sometimes there are rudimentary/unformed seeds which look similar to poppy seed, at least in those imported in Europe from Africa, and they look pretty much like those stains on the OP's photo. But yes, you are right, it's more like an exception these days. $\endgroup$
    – andselisk ♦
    Nov 6, 2017 at 11:53
  • $\begingroup$ Yes, but judging from the photo above (I am not sure whether this is the picture of the actual banana bread or an image from google) those are definitely not only those "rudimentary seeds" we see in bananas, there's just too much of them. That's why I thought of the two reactions mentioned in my answer. $\endgroup$
    – Raul Luciano
    Nov 6, 2017 at 12:13

3 Answers 3

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There are, at least, two reactions taking place and causing the brown colour of your bread. One is the oxidation of monophenols by an enzyme called PPO and the other is Maillard reaction, which is the same reaction that makes your steak looks deliciously brown.

Reaction 1. Oxidation of monophenols.

Bananas have a lot of polyphenol oxidase (also known as phenolase or PPO), an enzyme that catalyzes the oxidation of polyphenols naturally present in the banana fruit. The product is a colored ortho-quinone, which is brown or dark reddish. When you smash bananas to make a banana bread you end up releasing a lot of PPO which will catalyze the polymerization of the phenols and, therefore, make your bred looks darker then others. Below is a representation of the oxidation of monophenols into the colored ortho-quinone.

enter image description here

Reaction 2. Maillard reaction

The Maillard reaction is one of the most important reactions in food chemistry. Its products can add sensory features to many preparations, from steaks to coffee. Basically, sugars from the ingredients react with an aminoacid's amine group to form a glycosylamine. This compound is then isomerized to a ketosamine, which breaks into smaller substances. These substances are the Maillard products, and one class is called Melanoidins, responsible for the dark spots in your bread. The reason why it is formed when you add sodium bicarbonate to the mixture is because alkaline media speed up the fragmentation of the ketosamine, yielding more Maillard products. 2

I have included an infographic from CompoundChem with other information and the Maillard reaction in chemical notation. enter image description here

2Owusu-Apenten R., Introduction to Food Chemistry, CRC Press, 2004

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  • $\begingroup$ I'm having hard times trying to understand how this answers the question. There is only a general explanation of why bred is brown(ish), but I don't see anything explaining the nature of black spots inside, presumably caused by sodium bicarbonate. $\endgroup$
    – andselisk ♦
    Nov 6, 2017 at 2:07
  • $\begingroup$ @andselisk I am also not 100% sure about the black spots, I have searched everywhere and could find no mention about this effect. However, I believe that small parts of the banana fruit which were not 'dissolved' in the pastry, thus remaining as small banana chunks, would be the source of brown spots... I think this might be the answer because when I bake a banana bread with some banana chunks in it the bananas turn to an intense brown after cooking. I am not sure though if this is best explanation possible although I believe these two reactions are responsible for it. $\endgroup$
    – Raul Luciano
    Nov 6, 2017 at 11:44
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In banana cells they carry amyloplasts. Amyloplasts contain starches and when they are exposed to certain chemicals the starches turn black.

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It’s because bananas are acidic and when it reacts with baking soda it’s creates the black specks in the banana bread

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    $\begingroup$ If you’re saying that there’s an acid-base reaction, most salts (and certainly nearly all sodium salts) are colourless/white, so we’re gonna need a bit more explanation on why that makes black spots. $\endgroup$
    – orthocresol ♦
    Sep 20, 2018 at 8:30

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