When an apple is cut in half, the exposed surface quickly turns brown. Do the same thing to an orange, and nothing happens. The noted Hungarian biochemist Albert Szent-Gyorgyi was intrigued by this observation because the brown colour seemed to him to be very similar to the skin pigmentation often noted in patients suffering from Addison's disease. He was studying the disease which had been described by Thomas Addison in 1855 and knew that it was characterized by an underactive adrenal gland. Szent-Gyorgyi wondered whether there was some special substance in oranges that prevented the fruit from browning and could perhaps be used to treat Addison's disease.
Dr. Szent-Gyorgyi soon demonstrated that the juice from fruits that do not turn brown can delay browning in others. He went on to isolate the chemical he believed was responsible for this effect and was elated when it turned out to be the same substance he had previously extracted from adrenal glands. Surely there must be some connection!
He quickly wrote up his research and sent it in for publication to a scientific journal. Unfortunately, he had not been able to determine the exact molecular structure of the newly discovered substance and suggested it be called "Godnose." The editor of the journal did not share Szent-Gyorgyi's sense of humour and they eventually agreed on "hexuronic acid" as the new name. Later, when it was determined that hexuronic acid was the compound in citrus juice that prevented the "scorbutic condition," or scurvy, the name was changed to “ascorbic acid.” Today, we also know the substance as Vitamin C.
The initial excitement over finding ascorbic acid both in adrenal glands and in citrus juice was short-lived. The juice, which readily prevented browning reactions in fruit, had no effect on the skin discoloration of Addison's victims. No connection between Vitamin C and the disease was ever found but Szent-Gyorgyi did go on to unravel the way the body uses Vitamin C and received the 1937 Nobel Prize in medicine and physiology for his efforts.
So lemon juice may not cure Addison's disease, but there is no question that it can prevent browning reactions in fruits. But why do some fruits turn brown anyway? A set of naturally occurring compounds called “polyphenols” are responsible. When these react with enzymes in the plant, known as “phenolases,” they are converted into quinones. These quinones then react with each other to form giant molecules, or polymers, that are brown in colour. The polyphenols and the enzyme are stored separately in the fruit and are brought together only when tissue damage such as slicing, peeling, bruising or fungal attack occurs.
The quinones formed when the tissues are damaged have antifungal properties. The appearance of the brown colour actually is a signal that the fruit is trying to protect itself from fungal attack on the newly exposed surfaces. The greater the phenolase activity in a fruit, the greater the resistance to fungal disease. An apple, which turns brown readily, is more resistant to attack than a cantaloupe that does not discolour.
The brown pigments do not affect either the flavour or the nutritional value of the fruit in question. It may be desirable, however, for esthetic reasons to prevent discoloration. This can be achieved in a number of ways. Heating rapidly, as when fruits are blanched, inactivates the phenolase enzymes as does the addition of sulphur dioxide. Since the enzymes require oxygen to carry out their work, immersing the fruit underwater is also effective. The phenolase enzymes have copper incorporated into their structure and any substance that can remove the copper can destroy the enzyme. Citric acid and ascorbic acid can both do this. Furthermore, acidity inhibits the enzyme, so that the aforementioned compounds actually have a dual effect. Ascorbic acid, or Vitamin C, prevents discoloration in yet another fashion. It can actually convert quinones back into colourless phenols. When you check the label on a box of dehydrated potato flakes and note that Vitamin C has been added, you can rest assured that it is not there to increase the nutritional value of the product.
There is one instance in which the formation of the brown colour is actually promoted. This is in tea manufacture. Green tea leaves contain a very high percentage of polyphenolics that are brought into contact with the phenolase enzymes when the leaves are macerated resulting in the leaves turning brown. A variety of chemical reactions take place during this period, but basically, the colourless phenols bind together into molecules called tannins. Depending on how extensively the phenols bind together, the resulting tannins can exhibit a variety of colours, which are subsequently extracted when tea is brewed.
The pigments are acid sensitive and become much lighter when lemon juice is added to the tea. On the other hand, a base, like baking soda, will turn the tea almost blood red. One last point. Tannins combine with iron to make ink. Since tap water always contains some iron, tea made from tap water tends to be dark brown. Anyone who objects to drinking ink can use filtered or distilled water, from which iron has been removed. So as far as chemistry is concerned, it seems that brown isn't such a boring colour after all.