Chemistry

Why does Fruit Turn Brown



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We have all been there.  Preparing a special salad, or an apple pie, wanting everything to be perfect for our family and guests.  Then, before the blink of an eye, the pristine juicy cut surfaces of the fruit begin to turn brown and ugly.  We all know that this can be avoided by coating the newly cut fruit with lemon juice, but what is the actual cause of this color and texture change? 

All animal or plant cells contain thousands of enzymes which they use to carry out the work of metabolism.  Most fruits contain a specific enzyme called tyrosinase, that reacts with oxygen and ferrous phenols which are also found in fruit.  Their reaction with atmospheric oxygen forms a sort of rust on the surface of the fruit. 

This reaction can be minimized by inactivating the tyrosinase.  This can be accomplished by cooking the fruit, by making the surface of the fruit more acidic (this is the lemon juice approach), reducing the access of the fruit surface to oxygen by submerging it in water or vacuum packing the fruit, or by adding preservative chemicals like sulfur dioxide.  The amount of browning can also be reduced by using quality knifes which will not corrode and thereby provide more iron salts for the browning reaction.

But what is the brown stuff?  The tyrosinase reacts with the ferrous phenols to form o-quinones, which are the precursors to the brown coloration on the cut fruit surfaces.  The o-quinones then form the familiar brown coloration by reacting with amino acids or proteins, or simply by forming brown polymers.  The brown coloration is in no way harmful, but does distract from the enjoyment of the dish. 

How is a cook best to evade this problem.  Probably the best way is to avoid cutting fruit until just before the dish is served (or just before cooking).  This gives the reaction with oxygen in the air very little change to ruin the fruit.  If this is not possible, the lemon juice trick can be used, but at the cost of introducing an additional flavor element into the dish.  Perhaps a better approach is to submerge the cut fruit in water so that the atmospheric oxygen cannot reach the cut surface.  And, as mentioned previously, using stainless steel knifes will promote browning less than using carbon steel knifes. 

Most fruit is served cut into bite-sized pieces rather than as whole fruit.  These simple tips allow you to retain the wholesome appearance of the fruit and greatly enhance the overall enjoyment of your meal!




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