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THE LAST WORD Colour that stings 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. n Nettle wine is normally slightly greenish-yellow, so the red coloration is probably to do with the aluminium pan. Nettles contain a substance called fisetin, which will yield a red colour when it reacts with aluminium ions released in the presence of acidic compounds. Paul Hudgins Jacksonville, Florida, US n 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. Anthocyanins are natural pH indicators, like litmus. In acidic solutions they turn pink or orange, and in alkaline conditions yellow or green. 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 “sunscreen” 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 – 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 Hot in the hay I have always assumed that the belief that haystacks can burst into flames spontaneously was a convenient myth to cover for careless farm workers having a crafty cigarette break while forgetting their surroundings, but a friend insists that it can happen. Surely, the only way hay can warm up significantly is if it is wet and bacteria begin to heat the stack as part of the process of biodegradation. But I’d be amazed if this could generate temperatures hotter than about 40 °C. So how else could ignition take place? (Continued) n It was interesting to read earlier letters discussing how damp hay leads to internal heating in haystacks. As children growing up on a smallholding in west Wales, we had to build hay ricks using either small bales or loose hay. Hay was always carted and baled after the dew had evaporated from the fields, but occasionally rain, or the threat of rain, meant we had to build the ricks sooner than desirable. So occasionally there were some damp bales, and occasionally one got into a rick. If there were any worries about damp hay, the rick was tested every few days for any heating in its centre. This was done by pushing a pole between the bales or through the loose hay, and then someone would stick their arm in to take the temperature of the interior of the rick. Only once did we have cause for concern, when the interior was very hot. All hands immediately pulled the rick apart and removed what appeared to be a smoking bale. It was also damp and had inadvertently been tightly packed in. We discarded the offending bale, allowed the others to cool, and rebuilt the rick more loosely. I recall that another mishap involving fire and hay occurred in an outlying field by a railway line. The hay had been formed into a continuous row to make baling easier, when a spark from a steam train set it alight. Fortunately it was spotted, the burning hay was isolated and neighbouring householders brought water to damp it down, so only a small amount was lost. Mary Sinclair Narberth, Pembrokeshire, UK n An earlier reply stated that insurance companies in the 18th century were willing to cover farmers’ haystacks, implying that the insurers clearly believed hay could catch fire spontaneously. Actually, it doesn’t imply spontaneous combustion of hay is possible or that insurers believed it to be. Instead, the companies may have offered this insurance in the belief that it was not at all possible for hay to catch fire like this. They would thereby attain insurance- company nirvana: collecting a premium to cover a non-existent risk. Ian Cargill Leatherhead, Surrey, UK This week’s question ALARMING NOISES Why do heavy thunderstorms and fireworks set off car alarms? Mike Preston London, UK “Anthocyanins found in nettles are natural pH indicators and in acidic solutions they turn pink” Last words past and present at newscientist.com/topic/lastword Win £100 by answering our monthly question about energy issues Why do wind turbines have such thin blades? Would it not be more efficient for them to have wider blades in order to capture and convert more of the wind’s energy? Answers should reach us by 23.59 BST on 15 September 2013 to [email protected] or visit www.newscientist.com/topic/ energy. Terms and conditions are also at this url. The writers of answers published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. We are pleased to acknowledge financial support from Statoil in producing The Last Word. New Scientist retains total editorial control over the content. Reed Business Information Ltd reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to lastword@newscientist. com or visit www.newscientist.com/topic/ lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this url. THE LAST WORD ON ENERGY sponsored by

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Colour that stingsI 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.

n Nettle wine is normally slightly greenish-yellow, so the red coloration is probably to do with the aluminium pan. Nettles contain a substance called fisetin, which will yield a red colour when it reacts with aluminium ions released in the presence of acidic compounds.Paul HudginsJacksonville, Florida, US

n 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. Anthocyanins are natural pH indicators, like litmus. In acidic solutions they turn pink or orange, and in alkaline conditions yellow or green.

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 “sunscreen” 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 – 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 ScottClasses 9.2 and 9.4Worth School, Sussex, UK

Hot in the hay I have always assumed that the belief that haystacks can burst into flames spontaneously was a convenient myth to cover for careless farm workers having a

crafty cigarette break while forgetting their surroundings, but a friend insists that it can happen. Surely, the only way hay can warm up significantly is if it is wet and bacteria begin to heat the stack as part of the process of biodegradation. But I’d be amazed if this could generate temperatures hotter than about 40 °C. So how else could ignition take place? (Continued)

n It was interesting to read earlier letters discussing how damp hay leads to internal heating in haystacks. As children growing up on a smallholding in west Wales, we had to build hay ricks using either small bales or loose hay. Hay was always carted and baled after the dew had evaporated from the fields, but occasionally rain, or the threat of rain, meant we had to build the ricks sooner than desirable. So occasionally there were some damp bales, and occasionally one got into a rick.

If there were any worries about damp hay, the rick was tested every few days for any heating in its centre. This was done by pushing a pole between the bales or through the loose hay, and then someone would stick their arm in to take the temperature of the interior of the rick.

Only once did we have cause for concern, when the interior was very hot. All hands immediately pulled the rick apart and removed what appeared to be a smoking bale. It was also damp and had inadvertently been tightly packed in. We discarded the offending

bale, allowed the others to cool, and rebuilt the rick more loosely.

I recall that another mishap involving fire and hay occurred in an outlying field by a railway line. The hay had been formed into a continuous row to make baling easier, when a spark from a steam train set it alight. Fortunately it was spotted, the burning hay was isolated and neighbouring householders brought water to damp it down, so only a small amount was lost.Mary SinclairNarberth, Pembrokeshire, UK

n An earlier reply stated that insurance companies in the 18th century were willing to cover farmers’ haystacks, implying that the insurers clearly believed hay could catch fire spontaneously. Actually, it doesn’t imply spontaneous combustion of hay is possible or that insurers believed it to be.

Instead, the companies may have offered this insurance in the belief that it was not at all possible for hay to catch fire like this. They would thereby attain insurance-company nirvana: collecting a premium to cover a non-existent risk.Ian Cargill Leatherhead, Surrey, UK

This week’s questionAlArming noisesWhy do heavy thunderstorms and fireworks set off car alarms?Mike PrestonLondon, UK

“Anthocyanins found in nettles are natural pH indicators and in acidic solutions they turn pink”

last words past and present at newscientist.com/topic/lastword

Win £100 by answering our monthly question about energy issues

Why do wind turbines have such thin blades? Would it not be more efficient for them to have wider blades in order to capture and convert more of the wind’s energy?

Answers should reach us by 23.59 BST on 15 September 2013 to [email protected] or visit www.newscientist.com/topic/energy. Terms and conditions are also at this url.

The writers of answers published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. We are pleased to acknowledge financial support from Statoil in producing The Last Word. New Scientist retains total editorial control over the content.

Reed Business Information Ltd reserves

all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.

Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.newscientist.com/topic/lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this url.

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