Citric secret
Why does lemon juice stop cut apples and pears from browning?
• 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.
Advertisement
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
Institute of Cell and Molecular Biology
University of Edinburgh, UK
• Polyphenol oxidase (PPO) was discovered in mushrooms in 1856 by Christian Schoenbein. It is widespread in nature and found in humans, most animals and many plants. In plants its function is to protect against insects and micro-organisms when the skin of the fruit is damaged. The dark brown surface formed by the skin is not attractive to insects or other animals, and the compounds formed during the browning process have an antibacterial effect.
In some foodstuffs made from plants this browning effect is desirable. For example, in tea, coffee or chocolate it produces their characteristic flavour. However, in other plants or fruits such as avocado, apples and pears, browning is an economic problem for farmers, because brown fruit is not acceptable to consumers and it doesn’t taste good.
Angeles Hernández Y Hernández
Laboratory of Crystallography
Andalusian Institute of Earth Sciences
Granada, Spain
Sweet heat
While looking through old recipe books for new things to try on a glut of summer fruit that I am trying to preserve, I was intrigued by descriptions of the different properties of sugar at different temperatures. Between 105 and 107 °C the sugar syrup will form a fine, thin thread suitable for decorative use when cooled. Between 115 and 121 °C the syrup will form a hard ball useful for making grained products such as fudge. At 138 °C the syrup becomes brittle when cold, which is good for toffee-making. What chemical and physical properties change in the sugar and water molecules to make these different products?
• A sugar solution can only be heated as far as its boiling point. As the sugar solution boils, however, water is lost and the resulting increase in the sugar concentration means the solution boils at a higher temperature. When sufficient water has been driven off, the solution will harden to a solid after cooling because sugar is more soluble in hot water than cold. In general, the less water that is present, the higher the boiling temperature and the harder the resulting solid after cooling.
The solid confection produced on cooling consists of crystallised sugar suspended in a supersaturated sugar solution. The texture after cooling also depends on the size of the sugar crystals. Allowing confections to cool in moulds in an undisturbed fashion produces small sugar crystals, which are imperceptible to the palate and therefore seem to have a smooth texture. By contrast, stirring the mix during initial cooling produces the large sugar crystals characteristic of traditional fudge. If this same recipe is allowed to set without stirring, a smooth toffee-like texture is produced. And if stirring the fudge recipe is delayed until it is nearly cold, the higher viscosity of the cooler mix partially inhibits crystal formation and a texture intermediate between the original course-textured fudge and toffee is produced.
Both the milk protein and fat in toffee and fudge tend to reduce the size of the sugar crystals formed during mixing and cooling. By controlling the amount of such ingredients and the process conditions, the confectioner controls the size of the sugar crystals and hence the texture of confectionery. The only significant chemical change that occurs in sugar after prolonged heating at neutral pH is that the resulting solution becomes darker as a result of partial burning, known as caramelisation.
M. V. Wareing
Braintree, Essex
This week’s question
Pearly whites
My dentist tells me that the stains on my teeth are caused by black coffee and red wine. Is this correct and, if so, what is in these drinks that is not in their white coffee and white wine counterparts. I also read on toothpaste tubes that whitening toothpastes can help to remove these stains. How do these work?
Miles Weston
Haslemere, Surrey