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Citric secret |
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Question |
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Why does lemon juice stop cut apples and pears from browning? |
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Brian Dobson , Alton, Hampshire, UK |
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Answers (http://www.newscientist.com/lastword/article.jsp?id=lw1028) |
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To answer this question first we need to understand why some plant tissues go brown when cut. Plant cells have various compartments, including vacuoles and plastids, which are separated from each other by membranes. The vacuoles contain phenolic compounds which are sometimes coloured but usually colourless, while other compartments of the cell house enzymes called phenol oxidases. In a healthy plant cell, membranes separate the phenolics and the oxidases. However, when the cell is damaged by cutting into an apple, for example phenolics can leak from the vacuoles through the punctured membrane and come into contact with the oxidases. In the presence of oxygen from the surrounding air these enzymes oxidise the phenolics to give products which may help protect the plant, favouring wound healing, but also turning the plant material brown. The browning reaction can be blocked by one of two agents, both of which are present in lemon juice. The first is vitamin C, a biological antioxidant that is oxidised to colourless products instead of the apple's phenolics. The second agents are organic acids, especially citric acid, which make the pH lower than the oxidases' optimum level and thus slow the browning. Lemon juice has more than 50 times the vitamin C content of apples and pears. And lemon juice, with a pH of less than 2, is much more acidic than apple juice as a quick taste will tell you. So lemon juice will immediately prevent browning. You could also prevent cut apples browning, even without lemon juice, by putting them in an atmosphere of nitrogen or carbon dioxide, thus excluding the oxygen required by the oxidases. An excellent vegetable for observing browning is celeriac. It is possible to cut a large, relatively uniform slice of this root tissue and then lay several small filter paper discs on the cut surface, each soaked in a different solution such as lemon juice, apple juice, vitamin C, other antioxidants, citric acid, other acids and suchlike. A disc soaked with an agent that blocks the action of oxidases will leave a white circle on an otherwise brown surface. Stephen C. Fry , University of Edinburgh, UK ----- Lemon juice can also prevent the transmission of the AIDS virus: |
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