Potatoes turning black upon peeling

[Clusterbuck]

I would like to know what the exact chemical reaction involved is please. Of course it’s an oxidation, but of what compound? Starch, the main component of potato, is a non-reducing carbohydrate, so we can safely rule it out. Any ideas?

[RingsOfPylon]

Hi,

It could be one or both of the following:

1. Potatoes were stored in temps that are too cold (below 40F). This causes the starch to turn to sugar and darkens the potatoes. It will also make the potatoes taste sweeter.

2. The potatoes were cooked in an pan/pot made of aluminum or some other reactive metal. The metals interact with the iron in potatoes and causes them to darken.

I found the info here: Whatscooking

Also from the site:

According to the American Journal of Potato Research:

After-cooking darkening (ACD) is one of the most widespread, undesirable characteristics of cultivated potato. With the current expansion of the potato-processing industry around the world, there is a renewed interest in the development of new ways to prevent ACD. After-cooking, darkening is caused by the oxidation of the ferri-chlorogenic acid in the boiled or fried potatoes.

For a more detailed explanation of either the reaction, you’ll need a resident scientist to get into the step by step description.

I opened the thread because I thought it might be a question about growing potatoes; I could probably help with that.

An interesting question, though. Hope you get more thorough answers.

[MrDibble]

Are you referring toenzymatic browning? I believe it’s not the starch (unless iodine was involved) but the other compounds like phenols and esters which are oxidizing - e.g.chlorogenic acid. Note that the same process happens in apples, which are not starchy but do have chlorogenic acid.

It’s not pure oxidation, as the enzymes do the actual work, but oxygen is usually needed for the process to kick off.

[MrDibble]

There’s a bit more info here as to what’s occuring, but note that that’s only one of several simultaneous reactions occuring in the whole browning process in potatoes or apples.

Also note that after-cooking darkening issomething elseentirely (same compound involved, though)

[bibliophage]

I read a book about melanin years ago (I don’t recall the title) and was surprised to learn that it is responsible for a lot of browning in fruits and vegetables, including bananas and apparently potatoes.

This browning occurs due to the oxidation and dehydrogenation of colourless polyphenols present in the plants. The initial reaction catalysed by polyphenol oxidase and produces reddish-brown o-quinones. These are highly reactive, and so they subsequently undergo a series of non-enzymic reactions to yield insoluble black-brown melanin pigments.

source: http://onlinelibrary.wiley.com/doi/10.1016/S0307-4412(99)00033-3/pdf

See also this Google books result.

[Leo_Bloom]

American Journal of Potato Research
January 2004, Volume 81, Issue 1, pp 7-16

Potato after-cooking darkening
Gefu Wang-Pruski , Jerzy Nowak
Abstract
After-cooking darkening (ACD) is one of the most widespread, undesirable characteristics of cultivated potato. With the current expansion of the potato-processing industry around the world, there is a renewed interest in the development of new ways to prevent ACD. After-cooking darkening is caused by the oxidation of the ferri-chlorogenic acid in the boiled or fried potatoes. The severity of the darkening is dependent on the ratio of chlorogenic acid to citric acid concentrations in the potato tubers. Higher ratio normally results in darker tubers. The concentration of the chlorogenic and citric acids is genetically controlled and influenced by environmental conditions. This paper outlines the history of ACD and current status of knowledge of the chemistry of the dark pigment formation and its genetic and environmental determinants. Also discussed are the methods of chemical prevention of ACD presently used by the potato-processing industry and potential strategies for reducing tuber after cooking darkening using molecular approaches.

Article is behind wall, but 89 citations are public on-line.

[Leo_Bloom]

Apples apparently are a motherlode of the enzyme polyphenol oxidase, my new named nemesis. And, thanks to this thread, found this Wiki hopping:Arctic Apples are a suite of trademarked[1] apples that contain a nonbrowning trait (when the apples are subjected to mechanical damage, such as slicing or bruising, the apple flesh remains its original color)[2] introduced through biotechnology.[3][4] They were developed through a process of genetic engineering and precision breeding by Okanagan Specialty Fruits Inc. (OSF).[5] Specifically, gene silencing reduces the expression of polyphenol oxidase (PPO), thus preventing the fruit from browning. It is the first approved food product to use that technique.[6] This is unlike many other genetically modified foods, which insert genes from other species for the purposes of pesticide tolerance or insect resistance.

[Lsura]

I’ve never seen them turn black, but I have seen then turn brown after peeling.

The enzyme behind that is the same one as in Apples, polyphenol oxidase.Here’s some additional explanation.

ETA: this slow response is what happens when I open a tab, type, and then get interrupted before I hit submit.

[Johnny_L.A]

Without having followed any of the links, I’d just like to note that if you rinse or soak uncooked potatoes that have turned grey, they will return to white.

I generally make hashbrowns once a week, and I rinse and soak the potatoes after grating. Unlike freshly-grated potatoes, they do not turn grey immediately after draining and drying.