Hazel Harrison, of Nasbey, asks :-
While eating two different apples I noticed that an apple from a very old tree turned brown after about six bites but that a new variety Pacific Rose did not. Why does the flesh of the older variety become brown and what has been modified in new varieties to delay this unsightly process?
John Walker, a biochemist in the Plant and Microbial Sciences department at the University of Canterbury, responded.
The flesh of most fruits, and some vegetables, turns brown after it has been damaged in any way. Common examples are apples, bananas, pears, mushrooms and potatoes. This phenomenon is called enzymic browning and is brought about by the action of an enzyme (called 'catecholase' or 'diphenol oxidase') upon a special group of chemicals (diphenols) naturally present in the plant's cells.
This reaction requires oxygen so does not occur until the plant cells are damaged in some way, such as by cutting or bruising, and the final products of this reaction are usually dark brown to black pigments. It is these that you see as `browning'. This enzymic browning reaction is thought to play an important role as one of the plant's defence mechanisms against infections by disease-causing microorganisms since it can 'seal-off' the infected area in an attempt to isolate the organism causing the disease. A similar set of enzyme-catalysed reactions occurs in animals during the formation of melanin, the brown/black pigment in hair and skin. I should add also that this enzyme-catalysed browning is quite different from the non-enzymic browning reactions that occur during the baking of bread, biscuits and similar products.
Now, turning to the question of the observed differences in rates of enzymic browning between apple varieties. This observation is nothing new and may be seen if you compare low-browning apple varieties like Golden Delicious or Gravenstein against high browning cultivars like Red Delicious, Sturmer or Braeburn. Nothing has been modified; the differences are quite natural and are due to differences between cultivars in both the amount of the browning enzyme (catecholase) and the chemical substrate(s) (diphenols) upon which this enzyme acts. The higher the levels of enzyme and substrate; the greater the degree of browning.
Obviously the control of enzymic browning is of great concern to the food processing industry. Thus ascorbic acid (Vitamin C) may be added during the processing of fruit juices to prevent enzymic browning until the juice is pasteurised. Sulphur dioxide has been, and still is, used to control browning in some dried fruits (apples, apricots) but its use is under strict control and there is a real need for alternatives. However its not all bad news; enzymic browning plays a vital role in the manufacture of tea, cocoa and some wines. Researchers at the University of Canterbury have been actively seeking new compounds from micro-organisms to control enzymic browning and to help their understanding of the mechanisms of plant disease resistance.