I always curious about why sliced apple will become brown when left on the open air?

Answer 1

Apples undergo a process known as oxidative browning due to oxygen reacting with polyphenols released from the cut cells. Polyphenol oxidation takes place after physical injuries inflicted to fruits and vegetables during harvesting, storage or due to food preparation activities. For fresh fruits and vegetables, this is often harmful because the yellow and brown pigments impart an undesirable appearance. In some cases this color development can be desired,as during the harvest of dates or for some beverages (tea or cider). The first step of the browning is the oxidation of o-diphenols to highly reactive o-quinones. This reaction uses oxygen and is catalyzed by the enzyme polyphenol oxidase. The o-quinones go on to react with other compounds released at the cut surface by several different pathways to produce secondary products with the characteristic brown color. A technical review of this can be found in Nicolas JJ, Richard-Forget FC, Goupy PM, Amiot MJ, Aubert SY. 1994 Enzymatic browning reactions in apple and apple products. Crit Rev Food Sci Nutr. 34:109-57

Althoughseveral oxidation/reduction enzymes have iron as a co-factor for their function, the actual 'rusting' of iron has nothing to do with apple browning.

A number of compound have been proposed to inhibit browning, mostly working as anti-oxidants to prevent the oxidation catalyzed by polyphenol oxidase. For a technical discussion of this you might read: Son S.M.; Moon K.D.; Lee C.Y 2001 Inhibitory effects of various anti-browning agents on apple slices. Food Chem 73:23-30

I hope this helps...

Answer 2

Apple has some iron in it which oxidizes in contact with air, therefore the apple turns brown when sliced.

Answer 3

You can keep them from turning brown by coating them with lemon juice.

Just add some lemon juice to a bowl and then swirl the cut apples in the bowl to cover the exposed apple. No more brown apples.

Answer 4

the answer is very simple apple contains iron which gets oxidised when cut open thus the color changes to brown the same way as the iron rod gets rusted when left in open .

Answer 5

it has iron in Fe3+ form on oxidation it turns to Fe2+ form
yellow to brown

Answer 6

oxidation...doesn't happen if you add a bit of lemon juice to it.

Answer 7

oxidation