[paul.fr]

We all know that if you cut a potato in half and leave it out on the side, it will turn brown. Yet mashed potato stays white over the same time period, why is that?

[Bored chemist]

The cooking process destroys the enzymes in the potato that make it go brown.

[eric l]

I have to check on this - and it is not readily available by googling.  What I remember - and of this I am quite sure - is that what turns brown is not the starch in the potato, but the proteins.  What I do not remember and have to check is whether these proteins are only denaturated or mainly leached out by the cooking process.

[Karen W.]

It seems like it is boiled out like .. I always thought it was the starch,, so the protein Idea is interesting. I never think of protein with a potato!
I always soak my potatoes about ten minutes in lemon juice and water. I believe the lemons, lower the ph on the potato outer surface anyway and the water will just prevent the air from getting to it. so no oxygen exposure that way!
I think cooking potatoes might destroy certain enzymes that cause the thing to turn brown when exposed to air itc. especially when you cut the popato.

[Bored chemist]

All living things contain proteins, spuds included.
The enzyme is polyphenol oxidase.
It is inhibited at low pH. Also it needs oxygen, the vitamin C in lemmon tends to reduce the available oxygen.
Another effect is that the citrate complexes with the copper in the enzyme and this inhibits browning. Cyanide would do a better job but most people don't keep it in the kitchen.
http://en.wikipedia.org/wiki/Polyphenol_oxidase

[Karen W.]

LOL! Thanks eric I thought that was it but I did not know what the enzymes were! thanks..

[eric l]

Two more questions for BC :

• cyanides inhibit the reaction, so you will not have it (or only to a lesser degree) with manioc (cassava) ?
• is it the same enzyme that makes apples turn brown ?  To me it appears like a different shade of brown, and in different shapes (more uniform with apples, more or less star shaped with potatoes

I was not too far off after all, enzymes being proteins, but I should have known :  the famous Maillard reaction (which turns things brown when baking or roasting) is classified as "non enzymatic brown colouration".

[lyner]

DO boiled potatoes really go brown faster that mashed?
It may not be a fair test - cos you put salt, pepper, butter and CREAM (yum, yum) in the best mash. There is rarely any left for experimenting.

[eric l]

DO boiled potatoes really go brown faster that mashed?
...

In my experience it is the RAW potatoes that brown faster, when cut and exposed to air.

[Karen W.]

I agree with Eric the raw ones!

[Bored chemist]

cyanides inhibit the reaction, so you will not have it (or only to a lesser degree) with manioc (cassava) ?
Good question. I don't know if cassava makes the browning enzyme at all.
If it does it may have a relatively cyanide-proof version. It might also be that the cyanide producing enzyme doesn't get activated at the same time and place as the browning one.

is it the same enzyme that makes apples turn brown ? 
Again, the simple answer is that I don't know, I know it's a similar one..
To me it appears like a different shade of brown, and in different shapes (more uniform with apples, more or less star shaped with potatoes.
The different colour may be due to differences in the enzyme, the acidity or the substrate. Apples are more acid and have higher tannin levels.
The shape is a matter of where the plant produces the enzymes.