Why Apples Turn Brown
Apples have a tendency to turn brown when they are peeled, cut, or bruised in transport. You probably know this is a result of oxidation. When plant tissues come into contact with oxygen in the air a chemical reaction changes them, much the same way oxygen exposure causes rust in some metals.
What's happening is that polyphenolic compounds – chemicals responsible for the colour and taste of the apple – react with oxygen. This reaction starts because of an enzyme called polyphenol oxidase, or PPO, which was first discovered in mushrooms. It is also present in other fruits and vegetables like bananas, grapes, pears, and potatoes.
Enzymatic browning starts when the flesh of the apple is damaged in any way. Normally the PPO and the polyphenols are kept separate, but peeling, biting into, slicing, or bruising an apple brings the two types of chemicals together and starts the browning reaction. The result of the reaction is melanins, the very same compounds that are responsible for tanning when our unprotected skin is exposed to the sun.
Melanins are present in a number of plants and animals, and are associated with a number of protective responses. Melanin can form a barrier to prevent the spread of damage or infection from one part of the organism to another. It also helps to protect both plants and animals from adverse climatic conditions, and viral and microbial infections.
Gene Silencing: The Benefits of Switching Off PPO
A recent article on genetically modified foods called my attention to the development of non-browning apples in my home province of British Columbia. Canadian biotechnology company called Okanagan Specialty Fruits (OSF) has licensed a technology that switches PPO off to eliminate enzymatic browning. First developed in Australia for application in potatoes, OSF has learned to use the process to eliminate oxidative browning in its genetically modified (GM) apples. Suppression of the PPO leaves important polyphenols intact, but eliminates browning of the apple when its flesh is exposed to the air.
OSF founder Neal Carter says he was inspired by the baby-cut carrot market. Despite the overall increase in consumption of fruits and vegetables, fewer apples are currently being sold. Carter recognized the benefits of a non-browning apple for the fresh-cut market, and hopes this will revive consumer interest in eating apples. The absence of browning will also reduce the loss of fruits damaged during picking and transport, which he sees as a significant benefit for fruit growers and packers.
No Chemicals Required – Well, Kind Of
One of the biggest concerns with other genetically modified foods is the heavy reliance on pesticides and other chemicals. Monsanto's Roundup Ready crops are designed to resist the herbicide glyphosate, and assume the use of this chemical to control weeds. By comparison with GM crops already on the market, OSF's non-browning apples – called Arctic apples for their white flesh – do not involve resistance to any sort of pesticide or herbicide. The apples act just like any other fruit trees in the orchard, and do not require application of any chemicals in order to grow.
Another concern with GM crops is the creation of transgenic organisms by extracting genes from one organism – like the Bacillus thuringiensis (Bt) bacterium – and injecting them into the DNA of another organism like corn or cotton. Transgenesis also involves introducing a tracer, often a protein that makes the GM plants resistant to a specific antibiotic. Because only a fraction of the attempts at transgenesis are successful, scientists attach this marker to the gene that produces the desired change in the plant. They can then test all their samples to determine which ones were successfully transformed.
While Arctic apples don't incorporate any DNA from other species to silence POP, they are still transgenic rather than cisgenic (the difference being whether the genetic material introduced is from a completely different organism, or from within the species.) OSF uses the NPTII protein to create seedlings that are resistant to an antibiotic called kanamycin. Tests show that no novel proteins are expressed in the ripe apples, but this could still affect how consumers view the new apples.
Credit: Gunnar Magnusson/Flickr CC BY-ND 2.0
Home Grown Business
Unlike the multinational biotech firms responsible for genetically modified corn, soy, cotton or sugar beets, OSF is a small, grower-owned company. Carter and his wife have lived in B.C.'s Okanagan Valley for decades, and are fruit growers themselves. Their vision stems from a concern with the drop in apple sales, and the fact that apple growers in the region are abandoning the fruit for more profitable crops.
BC's apple industry is in crisis. Increased competition from Washington State and lower profits are discouraging enough that the once 20,000 acres of apple orchards in the Okanagan have dwindled down to less than half that. And no wonder, with some varieties actually dragging growers into the red. A group of six or seven varieties that account for more than 40% of apples grown in B.C., actually cost money to grow rather than bringing in a profit.
OSF has targeted four of these varieties for its non-browning apple breeding program. Arctic Granny Smiths and Arctic Golden Delicious apples are currently undergoing the approval process in both the United Sates and Canada. Non-browning versions of the Gala and Fuji are still in the works.
Credit: Einstein02/Wikipedia CC BY-SA 3.0
Respecting the Consumer's Choice
Non-browning Arctic apples will likely take a few years to reach your local grocery store, as OSF is not planning to grow enough of the fruit themselves for commercial distribution. By the time there are enough growers to introduce the fruit commercially, OSF figures they will be the most studied GM food on the market. And they will be labelled!
Out of respect for the consumer's right to choose, Arctic apples will bear the company logo – the snowflake inside a red apple – and will be labelled regardless of any legal requirement to identify them as genetically modified food. Processed foods that contain more than 5% of the fruit will also bear a label. However in foods that have undergone heating that denatures the DNA – like pasteurized apple juice or applesauce – there will be no specific label.