Many the flesh of several common fruit including apples, pears, avocados and melons, turns brown when cut and exposed to air. An enzyme is responsible for this browning. In the Arctic Apple, the production of this enzyme has been reduced using gene silencing, resulting in a fruit that undergoes less browning.
Many fruits turn brown or lose color when they are bruised or cut and exposed to oxygen in the air. The browning of a sliced apple is a good example of this phenomenon. Browning of apples occurs primarily through an enzymatic process. Polyphenol oxidase is an enzyme that, as its name suggests, catalyzes the oxidation of phenols to reactive quinones. These quinones are reactive and can oligomerize to form pigments, resulting in the observed brown color. Polyphenol oxidase may also be involved in the degradation of anthocyanins, a major class of plant pigments that can have vibrant red, blue, or purple color. Typically, polyphenol oxidase is present in the plastid while the phenol substrates are present in the vacuole. Upon wounding of the fruit (cutting or bruising) cell compartments are broken down (possibly indirectly) and the enzyme comes into contact with the substrate. The browning reaction is likely closely linked to the bruising response, in which fruit tissue becomes soft following a mechanical impact or abrasion. Bruising and browning are not indicative of bacterial or fungal growth.
The browning reaction is observed in many plant species and is found in leaves as addition to fruits. It is believed to play a role in pathogen defense. Polyphenol oxidase, the primary enzyme involve d in browning, is increased in response to treatment with pathogen response regulators such as systemin or jasmonate. It is believed that the reactive quinones produced by polyphenol oxidase may have a direct cytotoxic effect or may react with other cellular components to create molecules with antinutritive properties. The overexpression of polyphenol oxidase in tomato leads to a reduction of bacterial growth in leaves. Brown and bruised fruits are less appealing to consumers and therefore less valuable. Consumers may also discard browned fruits prior to spoilage because they are less appetizing.
Tradition prevention of fruit browning
Many strategies can be used to reduce browning of fruits. One is to treat the exposed flesh with an antioxidant, such as ascorbic acid (vitamin C). This does not affect the activity of polyphenol oxidase but rather quenches the reactive quinone products before they can react to form brown pigments. Fruit can be treated with polyphenol oxidase inhibitors, such as 4-hexylresorcinol. Acid, heat treatment, and salt treatments, and oxygen deprivation have also been shown to reduce browning. A common home remedy is to coat apply slice with lemon juice (high in ascorbic acid, low pH) to prevent browning. NatureSeal is a company that makes ascorbic acid and calcium based formulations to slow browning of fruits. Products such as this are commonly used in the commercial preparation of sliced apples.
Rather than inhibiting the browning reaction post harvest, as described above, it is possible to slow down or prevent browning by reducing or eliminating the production of the polyphenol oxidase enzyme. The Canadian company Okanagan Specialty Fruits developed an apple, termed the Arctic Apple, that has eliminated or reduced browning. Because eight polyphenol oxidase genes are believed to be present in the apple genome, a gene silencing approach was used to reduce the expression of the native polyphenol oxidases. An antisense copy of a polyphenol oxidase gene was transformed several apple varieties. The antisense version of the gene leads to silencing of the native polyphenol oxidase genes. More detailed information on how this apples was created will be posted as available. Currently the company has produced non-browning versions of Golden Delicious and Granny Smith and is working on Galas and Fujis. Okanagan has applied to the USDA for approval of the Arctic Apple in the US and is planning to apply for approval in Canada.
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