I recently made nettle wine using an old recipe. Nettles (genus Urtica) are green, yet when they were boiled, as the recipe suggested, the resulting liquid was red. Why? Was this a property of the plants or of the aluminium pan, or some residue in the apparently clean pan? Nothing else, such as wine-making yeast, had been added at that stage.
鈥� Nettle wine is normally slightly greenish-yellow, so the red coloration is probably to do with the aluminium pan. Nettles contain a substance called , which will yield a red colour when it reacts with aluminium ions released in the presence of acidic compounds.
Paul Hudgins, Jacksonville, Florida, US
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鈥� To teach my Year 9 classes (children aged 13 and 14) the principles of scientific investigation, I set them the task of answering this question.
The question did not say what recipe to follow, so we simply made an extract of the nettle leaves using boiling water. When we added this extract to solutions at a range of pHs, we found that the most acidic ones gave a pink to orange colour. It is therefore likely that the recipe involved adding an acid, and indeed most recipes I have found include lemon juice.
The colour change occurs because boiling allows substances in the nettle cells, including anthocyanin pigments from the stem and flowers, to diffuse out into the water. are natural pH indicators, like litmus. In acidic solutions they turn pink or orange, and in alkaline conditions yellow or green.
鈥淎nthocyanins found in nettles are natural pH indicators and in acidic solutions they turn pink鈥�
One of my pupils noticed that nettles growing in full sunlight turned reddish while those in the shade were green. This is significant because anthocyanins accumulate in plants that are adapted for shaded conditions and so need a 鈥渟unscreen鈥� in bright conditions. It is the anthocyanins that act as the sun block.
The reason for the colour difference depending on where nettles grow is that anthocyanins build up in vacuoles 鈥� chambers inside cells that are also the most acidic part of a cell. If the nettles that your correspondent used were from an exposed site, then they could have been somewhat reddish to start with.
Peter Scott, Classes 9.2 and 9.4, Worth School, Sussex, UK