That鈥檚 an interesting question.聽How lettuce turns brown is well known.聽But why this happens is a different story.聽Lets deal with the how first.聽The chemistry that takes place when lettuce leaves become brown is the same chemistry we see when an apple is cut and turns brown, when green guacamole turns brown or when a green olive ripens and turns black. All these reactions occur when chemicals in the fruit or vegetable called polyphenols react with enzymes called polyphenoloxidases.聽Enzymes are specialized protein molecules made within cells and serve as catalysts.聽In other words, they speed up chemical reactions.聽
Normally within a cell the enzymes and the polyphenols are separated, but when cells are damaged as in cutting an apple or tearing lettuce, their contents leak out an the polyphenols and the polyphenoloxidase mix.聽The result is a reaction that oxidizes the polyphenols and allows them to link up with each other to produce a brown pigment.聽When lettuce leaves torn, some cells are damaged and the browning reaction begins.聽But even if there is no tearing, even if we have a whole head of lettuce, there will eventually be browning.聽That鈥檚 because aging of the cells also leads to damage and mixing of the polyphenols with the enzyme.
The age old question about lettuce is what causes more damage, tearing or cutting?聽 Actually it seems not to make much difference.聽Some cooks argue that cutting is more destructive and leads to quicker browning but experiments do not bear this out.聽Some cooks say that they can taste metal in a salad if a knife has been used.聽I think this is culinary snobbery.聽In any case, if the lettuce is eaten soon after it is prepared, as far as potential browning goes, it doesn鈥檛 matter if it was cut or ripped.聽One more item about preparing lettuce.聽The dressing should always go on at the last moment because oil soaks into the leaves readily and makes them soggy.聽And noting that lettuce is soggy is not culinary snobbery.
So that鈥檚 the how.聽What about the why?聽Actually we don鈥檛 know.聽The main theory that crops up is that the brown pigment has anti-fungal and insecticidal properties.聽In other words, the damaged fruit or vegetable is trying to protect itself.聽 he argument is that historically damage was caused by insects and fungi and that at the first sign of such damage the enzymes would swing into action to ward off further destruction.聽The fly in this ointment is that nobody has clearly demonstrated that the brown pigments really do have anti-fungal or insecticidal properties.聽But there certainly is some interesting chemistry there.
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