apple - ? (Nov/10/2005 )
hi>>
i have a question in my mind : why apples turn yellow-brown after cutting???
are proteins involved here?
common, anyone? 
hi strawberry!
Oxidative browning is caused by the enzyme polyphenol oxidase (PPO) which, in the presence of O2, converts phenolic compounds in fruits and vegetables into dark colored pigments.
In fresh fruits processing there are some strategies to avoid browning:
- reduction O2: PPO requires O2 to induce cut surface discoloration. Industrial preparation covers vegetables with special packages to prevent oxygen from entering the cut cells.
- acidification: PPO most effectively catalyzes cut surface discoloration at a neutral pH of approximately 7. Ascorbic acid or erythorbate (an isomer of ascorbic acid) are compounds used to prevent PPO-mediated browning converting quinones (formed by PPO from phenolics) back to phenolic compounds. Unfortunately, once all the ascorbic acid or erythorbate is exhausted, PPO browning will proceed uninhibited. If you rinse your cut apple in lemon juice+water, it won't become brown because of ascorbic acid, that is an antioxydant.
Another way to avoid browning is to heat cut vegetables to destroy the enzymes responsible of oxydation, but, obviously it's not always possible.
Cheers!!!
ILA
thanx dear..........but what is the substrate in this case?
polyphenols???
what about polyphenolases?
polyphenols???
what about polyphenolases?
Polyphenolase and polyphenol oxidase (a.k.a. tyrosinase) are the same.
Substrates = polyphenols or phenolic compounds
Ahhhhhhhhha
more last question:
what is polyphenols? i mean are they proteins, carohydrates or what?
Maru--> what do you mean by a.k.a.
My dear,
polyphenols are just chemical compuonds characterized by two or more phenol groups.
A.k.a = also known as
Other questions? Don't hesitate to ask, I'll be please to answer!!! 
hahhahaha
thanx everybody for your replies
i am new in this world and want to know everything
Enthusiast--> sure, i will not....
Not that I know anything about this topic (seems like ila and Maru have it well covered), but I just couldn't resist a thread titled "apple" by a member called "strawberry"...
oh, ye
i notice this just right now
mmmm, yummy
Right!
So do I, HomeBrew!!!
Fortunately this time I could answer too!!!