A rotten apple spoils the whole barrel. That’s not just an old adage, it’s a scientific fact. And it all has to do with ethylene, a gas produced internally by a fruit to stimulate ripening. Basically, ethylene is a plant hormone. Our word “hormone” derives from the Greek “hormon” meaning “to set in motion” and that is just what ethylene does. It sets in motion a large number of enzymatic processes that, in general, are responsible for ripening. An increase in ethylene concentration enhances tissue respiration, which is the process of producing energy to drive biochemical processes through the reaction of stored sugar with oxygen. These reactions lead to a breakdown of the green pigment chlorophyll and the synthesis of other pigments. Starch is converted to simple sugars. At the same time pectin, a type of fibre that cements cell walls together, begins to disintegrate, softening the tissue. Rotting is the end stage of ripening, with more ethylene being released into the surrounding air. This stimulates the ripening of nearby fruit, setting off a chain reaction that spoils the whole barrel.
Apple producers are commonly plagued by this problem because fruit has to be stored for months to meet year-round demand. This has necessitated the development of various technologies to counter the effects of ethylene in a storage facility. Since the 1960s, growers have kept stocked apples firm by reducing oxygen and raising carbon dioxide levels to slow respiration. This has allowed some varieties of apples to be sold all year, although they don’t keep their full flavour and can go soft.
Another idea is to prevent ethylene from stimulating respiration in the first place. One obvious method is the use of substances that can remove ethylene either by absorbing it or by eliminating it through a chemical reaction. Activated carbon or minerals called zeolites can absorb ethylene effectively. Zeolites derive their name from the Greek words for “boil” and “stone” because, back in the 18th century, Swedish mineralogist Axel Fredrik Cronstedt found that heating a type of naturally occurring mineral called stilbite released copious amounts of steam. The mineral had absorbed water from its surroundings and stored it until it was released by heat. It turned out that zeolites were capable of absorbing a variety of other chemicals as well. This provided an explanation for the tradition of storing apples in caves around the Japanese town of Oya. The town is located on the site of an ancient volcano that once spewed lava that hardened into Oya stone. This rock is a complex mixture of various minerals and is rich in zeolites, which are efficient at absorbing ethylene gas.
Today, pellets of zeolite are used by various companies to remove ethylene from the atmosphere when produce is being stored or transported. These minerals are also incorporated into plastic bags sold to consumers for produce storage at home. The plastic is impregnated with potassium permanganate, a chemical that removes ethylene by reacting with it to yield harmless potassium oxide and carbon dioxide.
There is yet another way to tackle the ethylene problem. After being generated within the fruit, the gas travels through the tissues until it fits into cell receptors, much like a key fits into a lock. When there is a proper fit, the ripening reactions are triggered. If the receptor can be blocked by another molecule, ripening can be prevented. The problem is to find a substance that is similar to ethylene, enabling it to bind to the receptor but different enough to prevent stimulating the receptor. An analogy would be a key that fits a lock but cannot unlock it.
Such a molecule has been found. Since 2002, 1-methylcyclopropene has been successfully used to block the action of ethylene. The fruit is placed in a chamber where it is exposed to the gas before being stored in a facility for up to a year. The amount of 1-methylcyclopropene absorbed by the fruit is very little and presents no health problem. However, while the texture and colour are well maintained, there is a slight loss of flavour. If you want the taste of a freshly picked apple, you have to go pick one. We cannot do that in the middle of winter, so thank goodness for the chemical ingenuity that allows us to eat apples 12 months
Joe Schwarcz is the director of McGill University’s Office for Science and Society. Read his blog at www.mcgill.ca/oss.