Enzyme Browning

This typically occurs in fruits and vegetables that have been cut or bruised, breaking open their cells and allowing the natural enzymes in them to decompose other substances into compounds with a dark colour and, often, an ‘off’ flavour. This is seldom desirable; an exception is the preparation of ‘black’ tea, where the leaves are deliberately bruised to allow enzymes to work.

We normally try to frustrate the enzymes responsible for this kind of browning. The simplest way is to cook the food, which destroys the enzymes. Freezing, on the other hand, does not, so vegetables to be frozen must be thoroughly blanched beforehand to prevent them from slowly browning after they are frozen. Salt blocks the action of the enzymes, but such large quantities are needed that it is not generally used for the purpose, although sliced apples may be left in brine for a short time before they are used. Acids have the same effect and are more practical. Lemon juice, vinegar, and ascorbic acid (vitamin C) are all used. Sulphur dioxide (which forms sulphurous acid in water) is used for commercial dried fruit. Finally, enzyme browning can be prevented by the exclusion of air, since, unlike the other kinds of browning, it needs oxygen to work. Putting foods in water is not effective by itself, because of the air dissolved in the water. Syrups afford better protection. Vacuum packing, although the vacuum is never total, is quite effective. The use of an inert gas, as when apples are stored in nitrogen, is best of all.