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Prevention of Browning in Fruits and Vegetables
Jiangsu Center of International Technology Transfer
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Bruising or other mechanical wounding of fruits and vegetables cause many post-harvest problems; including the browning of injured and adjacent tissue. Currently, the only way to control browning induced by mechanical wounding is with refrigeration or chemical treatment. While these treatments slow down browning, such procedures can add significantly to costs and may have a negative effect on product quality; e.g. they may leave chemical residues and alter texture. Scientists have discovered that activation of another biosynthetic pathway inhibits the activity of the browning reactions. An inexpensive, easy to apply treatment lasting only a few minutes at most is sufficient to prevent the synthesis of wound-associated compounds in freshly-harvested or lightly-processed fruits and vegetables. The UC treatment is non-chemical and can be administered by a number of well-known techniques, so it offers a preferred and easily-implemented means for anti-browning treatment of produce.
Browning of injured plant tissue occurs through both enzymatic and non-enzymatic reactions. In both cases, browning results when colorless phenolic compounds in the tissue, when exposed to oxygen, are oxidized into colored compounds. Some fruits and vegetables have naturally high levels of phenolic compounds and brown rapidly after being injured; e.g. artichokes, and some apple and potato cultivars. Low temperatures and strong anti-oxidants are necessary to control browning in these tissues. Other tissues (e.g. celery, lettuce, some potato cultivars) have naturally low levels of phenolic compounds and only become prone to browning after an injury induces the synthesis and accumulation of the phenolic browning precursors. In these tissues, preventing wound-induced phenolic synthesis is an effective way of preventing browning.
In the case of iceberg lettuce, a 90 second treatment (administered up to 36 hours before a wounding event) is sufficient to provide an anti-browning effect that lasts for at least 15 days when the lettuce is stored at 5 degrees Celsius. This length of storage is similar to that achieved with modified atmosphere packaging, but the UC method does not require low oxygen-modified atmospheres or expensive packaging. The UC treatment is effective if applied up to 24 hours before or up to 2 hours after the injury. In addition, this technology permits the use of vacuum cooling to cool and dewater the processed lettuce, thereby avoiding mechanical damage caused by the current method of centrifugation. The UC treatment does not alter the texture or taste of the lettuce.