A Cocktail of Biology

DNA is not just found at crime scenes. It’s in every living thing – you, your cat, the bacteria on your hands and the grass under your feet. It’s in every meal you’ve ever eaten and, under the right circumstances, it’s even in your cocktail.

Supermarket strawberry. Hell yes I took this hazy photo with a $30 phone and a droplet of water.

The DNAquiri is a cocktail recipe that won Best in Show at the 2011 Science Hack Day San Francisco because it is also a protocol for extracting DNA from strawberries. The recipe calls for only three ingredients – frozen strawberries, pineapple juice and high-proof rum – but the final product is a complex cocktail of curiosities.

This post is another of my recycled school assignments, in which I was given the slightly daunting task of writing a story in the form of a list. I decided the best kind of list is a recipe, and the best kind of recipe is one that involves both science and cocktails. A deep bow and a tip of the hat to Margot at Hypatian Axis for drawing my attention to the quirky back story of the modern strawberry.

1) Strawberries

Strawberries are a convenient source of DNA because they are delicious and they are octoploid. Octoploid means that each cell in a strawberry contains eight copies of every gene. In contrast, human cells are diploid, with only two copies of each gene, one from mom and one from dad. In the same way, strawberry cells have four copies from mom and four copies from dad, and yes, strawberry plants do have parents. In fact, according to G.M. Darrow in The Strawberry: History, Breeding and Physiology,¹ the mother and father of the cultivated strawberry were the stars of an unlikely transcontinental plant romance.

It all started in 1712, when some unusually plump strawberries caught the eye of a French spy in Chile. The spy was Amédée François Frézier, an engineer in the French Army Intelligence Corps who was spying on Spanish defenses in South America. He noticed that the strawberries cultivated by the locals were much larger than European strawberries and selected several specimens with excellent fruit to take back to France.

Although these plants survived an arduous 6-month voyage to Europe, they did not immediately live up to Frézier’s enthusiastic description. The king’s gardener could not get them to reliably produce oversized fruit “as big as a Walnut, and sometimes as a Hen’s Egg”. By choosing only fruiting plants, Frézier had inadvertently selected only females. At that time, nobody realized that strawberry species often have separate sexes, which is relatively unusual for plants. Male plants carry pollen, while female plants bear fruit, so those female strawberry immigrants were effectively infertile. This is where we meet dad.

Dad was the Virginia strawberry, a wild species still common in the Eastern US that was introduced into Europe in the 1600s. While botanical gardens initially struggled to produce Chilean strawberries, enterprising farmers succeeded by alternating rows with other strawberry varieties. These included some male plants with pollen that could fertilize the Chilean females. The Virginia strawberry gave the best results and the descendents of this pairing became the modern strawberry, which is strongly flavored like its Virginian ancestors, but large like the Chileans. Part of the reason the match worked genetically is that while the European strawberry varieties were mostly diploid (with two copies of each gene per cell), both the Virginia and Chilean strawberries were octoploid².

This octoploid romance means that every cell of a supermarket strawberry is jam-packed with DNA, making it easier for us to extract enough DNA to see with the naked eye. But first, you need to get the DNA out of the cell.

When strawberries freeze, the water in the cells forms ice crystals that puncture the cell membranes. So if you put frozen strawberries in a Ziploc bag and then squash them, warm them to 50°C then chill them again, some of the DNA will spill out of the leaky cells. Force the pinkish mush through a strainer to get rid of the lumps and you’ll have a strawberry cell extract, otherwise known as juice.

2) Pineapples

The strawberry juice contains DNA, but it also contains all kinds of other stuff from inside the cells, including proteins. Proteins translate the genetic information in DNA into action; DNA sequences carry the instructions for making lots of different kinds of proteins, which in turn do many different jobs. But when all those strawberry proteins are released from the cell into the juice, some of those jobs interfere with the process of DNA extraction.

For example, some of the proteins wrap around the DNA strands, winding them into bundles that don’t extract well. Other proteins chop up the DNA. To counter the effects of all these proteins, you can add “protein-chopping” proteins called proteases. Proteases chop other proteins into bits and although they can be found in all cells, they are found in conveniently large quantities in pineapples. In fact, pineapples have such high concentrations they are used in industrial applications. They are even available in the supermarket, sold as a meat tenderizer that works by digesting the collagen protein that gives meat its structure.

Pineapple close-up. By Uri_Breitman (Flickr) [CC BY-NC 2.0]

This is also why pineapple has a bit of a bite to it — that burning sensation when you eat too much pineapple is from proteases damaging proteins on the surface of your tongue.

To digest away the protein in your strawberry mush, simply add some cold, unpasteurized pineapple juice and let the proteases do their work. Now your Ziploc bag contains strawberry DNA and strawberry proteins being chopped up by pineapple proteins. But the DNA is still invisible.

3) Rum

Your next step is to force the DNA to stick together into a clump that you can see. To do this you pour a layer of ice-cold, high-proof rum over the strawberry cell extract. “High proof” just means a high concentration of alcohol, in this case 70%, which is about twice as high as normal rum. The reason you need such a high concentration is that you want to change the chemical environment that surrounds the DNA.

The interior of a cell is watery and DNA is a ”water-loving” or hydrophilic substance that dissolves readily. But just as oil and water don’t mix, hydrophilic substances don’t mix well with “water-hating” or hydrophobic substances, like ethanol. (@EricGumpricht called me out on my decision to call ethanol “water hating,” which was convenient for my word limit, but of course, not remotely true — cocktails wouldn’t be much fun if spirits weren’t miscible with water! I was trying to describe ethanol precipitation without turning this into a chemistry lesson and talking about dielectric constants and whatnot, but I will work on coming up with a better compromise).

When immersed in ethanol, DNA molecules stick together into much larger, visible blobs.

Understandably, ethanol at a high enough concentration to cause DNA to stick together is highly toxic to cells (you could use high-proof rum as effective but expensive disinfectant). But the ethanol in your bottle of rum was originally made by cells — yeast cells.

Yeast are fungi, like mushrooms, but they are microscopic, consisting of only a single cell. They excrete ethanol during the process of extracting energy from fruit sugars. Grapes and other fruit are the natural habitat of yeast, at least during the summer. According to scientists at the University of Florence, yeast’s winter home is in the guts of wasps that feed on grapes. These wasps provide a warm and moist refuge from the elements while fruit is out of season and once the fruit ripens they provide transportation to the site of the annual sugar feast.

Grapes. By Scharks [CC-BY-SA-2.0], via Wikimedia Commons

Preventing other microbes from joining in on the feast is part of the reason why yeast cells excrete ethanol. Yeast get their energy from sugar by converting it to ethanol and carbon dioxide, a process called fermentation. Fermentation generates much less energy than respiration, the method that humans use to metabolize sugar, but fermentation is much faster and has the added benefit of producing a toxic by-product, ethanol. When a wasp inoculates a grape with yeast cells, the yeast rapidly convert their surroundings into an acidic, alcoholic soup that is inhospitable to most other microbes.

Humans have taken advantage of yeast’s curious lifestyle to produce alcoholic drinks, but we have also used it to advance scientific knowledge. For example, studies of yeast fermentation overturned Louis Pasteur’s hypothesis that cells were powered by a mysterious vital force that fundamentally distinguished living from non-living things.

The basic unit of life is the cell, which can grow and divide to make more cells. Pasteur had found that some chemical reactions — like fermentation of sugar to alcohol — could only be catalyzed by live yeast. Vitalists like Pasteur argued that live cells possessed a “vital spark” that allowed them to become more than the sum of the biochemical reactions occurring inside their cell membranes.

As recounted by Christian Reinhardt in Nobel Laureates in Chemistry, 1901-1992, Pasteur’s strain of vitalism was dealt a death blow by Eduard Buchner, a German chemist who was trying to extract proteins from yeast cells without damaging the proteins in the process. He managed this in 1896 by grinding yeast with sand and then filtering out the slurry of broken cells to produce a non-living “yeast juice.” There were no yeast cells remaining in the juice, but it did contain many of the biochemicals, like proteins, that had once been inside the cells. To stop this mix from spoiling before it could be used in other experiments, Eduard, who had once worked in a cannery, tried the method that preserves fruit in jams — ­adding a high concentration of sugar. To his surprise, within 15 minutes of adding sugar to the yeast juice, it started to froth like a fermenting beer.

Bubbling yeast fermentation. By Jim Champion (Flickr: Rising bubbles) [CC-BY-SA-2.0], via Wikimedia Commons

Buchner was able to show that the proteins in the yeast juice could produce carbon dioxide and ethanol from sugar in a way that was identical to fermentation by live cells. Pasteur was wrong, and no vital force was necessary to explain the metabolic activities of the cell. Cells were really just the sum of their parts.

Buchner’s experiments were a key moment in the dawn of biochemistry, the field that uses non-living extracts of living things (like your strawberry juice) to understand the chemical reactions that allow cells to function. One of the most important discoveries of biochemistry has been that many of these chemical reactions are similar in all organisms, and that all living things share the same genetic code. So, once you see a white film form in a layer between the pink strawberry juice and the amber rum, take a cocktail stick, twirl it around in the film and remove a blob of DNA. This unassuming clump of slime is the hidden genetic material that helped make those delicious strawberries. Be sure to give your drink a good stir before you toast the shared heritage between yeast, pineapples, strawberries and yourself.

Blob of unknown identity. By me.

Sad taster’s note and party poopery: I’ve tried the DNAquiri recipe twice and each time have achieved a very respectable yield of white slime and an almost drinkable cocktail (it’s a bit strong for me). However, I’m going to guess that the slime is not actually DNA, but instead is pectin, a cell wall carbohydrate that gives fruit jams and jellies their gel-like consistency. I base this guess on a suggestion from the UK’s National Centre for Biotechnology Education and the fact that the slime isn’t as stringy as I’m used to for DNA. Please weigh in, all you fruit DNA experts! I want to know!

¹More party poopery: Darrow’s story about the origin of the three strawberry flowers on the Fraser coat of arms is probably false. Darrow recounts an old myth that Frézier and the Scottish Fraser clan were descended from one Julius de Berry, who was knighted in 916 by the Emperor of France, King Charles the V. According to the legend, de Berry was knighted in reward for his miraculous ability to provide unseasonably ripe strawberries for a feast. He was given a new coat of arms and a new name, Fraise, which is French for strawberry.

ScienceSeeker Awards: The best of the best

The results of the  inaugural Science Seeker awards have been announced! Nominated by your excellent selves and judged by superstars  Fraser Cain, Maggie Koerth-Baker, and Maryn McKenna, the list of finalists and winners is a wonderful sampling of fine science blogging from the past year. Congratulations to all!

I got strangely nostalgic going through the list because it reminds me how fun it has been making my ScienceSeeker picks over the past year or so, even if I was a bit unreliable about blogging them!

 

HORSEPOWER

Step 2 of my strategic plan: Post my favourite class assignments from the past year, which will start this week with manure and will eventually end with sewage. Enjoy.

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We tend to think of nineteenth century cities like Pittsburgh as industrializing under the power of steam. But Joel Tarr argues that an older technology also drove the development of the great cities of the steam age.

In 1775 James Watt patented the steam engine, a machine that would become a symbol of the industrial revolution. Forty years later, Benjamin Latrobe opened a steamboat engine workshop on the banks of the Monongahela River in Pittsburgh. The power source that Latrobe used to build his engines? Two blind horses.

Horses like Latrobe’s were a central cog in the nineteenth century urban economy. They were hooked up to engines through circular sweeps, rotating platforms and treadmills, and harnessed to vehicles on wheels and tracks. City horses hauled steel, powered ferries, pressed bricks. They were the source of valuable manure and even more valuable carcasses. They were the catalysts for the paving of streets and the suburbanization of cities.

But, like the combustion engine, their great success as a technology also contributed to their eventual decline. Thanks to the work of Joel Tarr, a professor of history and policy at Carnegie Mellon University, and his colleague Clay McShane, a professor of history at Northeastern University, we are rediscovering how horses hauled cities like Pittsburgh into the modern age.

Horses at Cincinnati subway construction site, probably the 1920s. Image credit: University of Cincinnati Library.

Rediscovering the horse as an urban technology

Joel Tarr thought he was done with horse manure back in 1971. The Jersey City native had joined the faculty at Carnegie Mellon in 1967 with a background in urban political history and an interest in how the modern city had been shaped by transport technologies. During his research, he kept coming across historical complaints that he thought might be interesting to a general audience.

The article he submitted to the magazine American Heritage was a vivid account of the problems faced by a horse-driven city, including the staggering scale of manure logistics:

…as health officials in Rochester, New York, calculated in 1900, the fifteen thousand horses in that city produced enough manure in a year to make a pile 175 feet high covering an acre of ground and breeding sixteen billion flies, each one a potential spreader of germs.

“Urban Pollution— Many Long Years Ago”, American Heritage, 1971

Tarr noted that the eventual solution to these problems was the adoption of a new technology, one that harnessed machines rather than animals. “Apparently the editorial board got into a big fight about it because some of them thought that it was an apology for the automobile,” he recalls.

Controversial from the start, the article prompted several newspaper editorials and is still cited today in debates about pollution. But after creating a stir, Tarr moved on, reasoning that “someone else could worry about the horse manure.” And so he pursued his interests in the environmental and technological history of cities and left the subject of horses alone for more than twenty years.

But while the details of the manure problem lived on in the public imagination, Tarr knew that there was much more to say about the importance of horses in the history of our cities. In 1995, he refused to allow the manure article to be reprinted in an anthology and instead asked if he could write a new article on the topic with his friend McShane, who had recently published a history of cars in cities. From that first article, the project ballooned out into a decade of scholarship and co-authorship of their 2007 book on horses as an urban technology, The Horse in the City: Living Machines in the Nineteenth Century. The book is a detailed examination of the centrality of horses to cities, focusing on New York City, Boston, and Tarr’s adoptive home, Pittsburgh.

From steam power to horse power

Tarr and McShane emphasize that the horse was viewed as a “living machine,” valued primarily for its ability to provide power. As machines, horses were an integral part of the economy, even after the advent of the steam engine.

After refining the steam engine, James Watt invented a standard measure of mechanical work – 33,000 foot-pounds of work per minute, or 1 horsepower. This unit allowed customers to estimate how many horses an engine could replace and to gauge whether replacing their horses would be economic. In many cases, it wasn’t. For much of the nineteenth century, horses were the engine of choice for applications that required flexibility or mobility and for businesses that could not afford a large capital outlay.

But the one application in which horses were irreplaceable was ground transport within the city. Goods from the expanded railway and steamboat lines could only be distributed to their final destinations under the power of horses, which meant that horse-drawn transport grew more efficient in parallel with steam technology. Innovations in breeding produced larger and larger horses in the pursuit of (as one agricultural reformer put it) “the best machine for turning food into money.” These industrial-strength horses could pull even larger loads after the development of lighter vehicles made with modern materials.

One resident of Pittsburgh remembered the “pandemonium of noises” produced by horse transport in the 1860s:

Numerous wagons, hauling heavy pigs of iron and iron products, timber wheels with anywhere from six to fourteen horses from which huge and unwieldy vehicles hung castings of many tons’ weight, the clattering omnibus, the rattle of the mail wagons, drays [...] and other conveyances common to traffic.

George Thornton Fleming,1904

This was the cacophony of Pittsburgh’s developing steel industry, the sound of a modern city propelled by coal and hooves.

Horse-drawn wagons and carriages, an electric trolley car, and pedestrians congest a cobblestone Philadelphia street in 1897. Image credit: National Archives and Records Administration, 30-N-36713.

Shaping the city

The structure of Pittsburgh’s neighborhoods today still reflects the age of horse-drawn vehicles. Public transport began with road vehicles called omnibuses, but gathered momentum with one of the most influential urban innovations of the nineteenth century, railed “horsecar” lines. These tracks, the precursors of the cable car and electric streetcar systems, provided a smooth ride that omnibuses could never achieve on cobbled pavements that were optimized for horseshoe traction. The benefits of the tracks were not just to the spines of riders, but to the speed the horses could travel, the number of riders they could haul, and the amount of profit their owners could make.

The first lines were laid in 1863 and by 1890 the average Pittsburgh resident took 192 horsecar trips per year. The tracks had grown along the lines of least resistance, following valleys and avoiding the worst of Pittsburgh’s steep hills. As the lines expanded, ridership increased at a rate much faster than population growth, reflecting Pittsburgh’s shift to the suburbs; Residents could now live further away from downtown and make a daily commute to work. Wards within an hour’s smooth ride of downtown were suddenly more desirable than when they were a longer, more expensive and more bone-jarring omnibus ride. The relatively flat Eastside saw the biggest growth – between 1870 and 1890 it grew from 5,350 dwellings to 17,604. Construction boomed in areas within a ten-minute walk of a horsecar line. Tarr and McShane write that “the greater speeds allowed Americans to fulfill the new dream of the middle class, a detached home with a yard on the outskirts of a city.” Meanwhile, downtown was losing residents to the new suburbs and slowly transforming into a true central business district.

Tarr and McShane point out that the horsecar alone did not cause these changes in Pittsburgh and other growing cities. Factors like economic expansion, population growth and a new appreciation for suburban life played an important role, but the horsecar was the technology that allowed these trends to play out, and it set the patterns that were extended in the twentieth century by the streetcar and the gas-fueled automobile.

Problems with the living machine

In 1872, American horses came down with a terrible case of the flu. Several Northeastern cities ground to a dramatic halt. The horse flu epidemic cut off city supplies, grounded fire departments, and isolated suburbs from their vital horsecar lines. When one commentator later warned that another epidemic would reduce New York City to “straits of distress,” he concluded that although “cities have been made by building around the horse there is no necessity for keeping him permanently as their centre.” As the century progressed, more and more objections were made to the city’s dependence on horses.

Like all technologies, horses had their downsides. They were living creatures, susceptible to disease, unreliability, and even personality. They required an enormous infrastructure of foul-smelling stables, with stockpiles of hay that posed a significant fire hazard. But above all, horses were prolific polluters. The average city horse unleashed 25-35 pounds of manure and two to three gallons of urine per day.

Horse manure started out as just one of the many hazards of urban life, but as the century progressed, the exploding city horse population became a source of public angst and newspaper editorials. To make matters worse, by the 1880s the bottom had fallen out of the manure market.

Fresh manure had long been a valued commodity, sold by stable owners and street sweepers to farmers on the urban periphery. But thanks partly to competition from new guano and rock phosphate fertilizers, the price of manure had fallen to less than a quarter of its worth. A New York Times editorial from 1881 conveys the confusion caused by a city decision to declare summer dumping grounds off-limits amidst the glut of manure: “Public health nuisance: No place for stable manure—What is to become of it?” By 1908, one journalist claimed that 20,000 New Yorkers died each year from “maladies that fly in the dust, created mainly by horse manure.” The biggest problem was that the accumulating piles were a favorite breeding ground for flies, a vector for life-threatening diseases like typhoid.

Manure on New York City streets in 1893 from sanitary engineering hero George E. Waring‘s book “Street-cleaning and the Disposal of a City’s Wastes: Methods and Results and the Effect Upon Public Health, Public Morals, and Municipal Property” Doubleday and McClure 1897.

Part of the solution to the manure problem was technological. By 1902 most horsecar lines had transitioned to electric trolleys only a decade after they had been first introduced. But the manure problem itself was not necessarily responsible for the speed of this change. Tarr and McShane argue that in many cases, the new technology was rapidly embraced by horsecar companies because these companies did a tidy side business in land speculation. Horsecar lines had the reliable effect of pushing up property prices wherever they were laid, but by the late 1880s, horsecar lines had mostly expanded as far as they could within a one-hour commute of downdown. With the increased speed of electrified trolleys however, horsecar companies could expect to double that radius and reap the rewards in real estate. As a result, these companies became intimately involved in urban politics and in many cases bought themselves influence on city councils to ensure they received the necessary franchises. Within a decade, most of the lines had switched over to electric.

For a few more decades, horses were still favored for tasks like fighting fire, hauling waste, and making neighborhood deliveries. But by the end of World War II, even these jobs fell to the automobile. The horse manure problem was solved and the age of the car had begun.

The technological solution

Despite the initial optimism that cars were a clean and efficient alternative to the horse, the new technology has also become a victim of its own success. The burning of fossil fuels generates air pollution that can be as hazardous to human health as the diseases spread by flies, and it releases carbon dioxide that contributes to climate change. A century after the decline of the horse, we are again facing a chronic pollution problem.

Embedded among the engineers and policy faculty of Carnegie Mellon, Tarr has consistently pursued historical questions that provide perspective for contemporary policy debates, particularly the problems of urban waste. But ever since Tarr published that first article on the horse manure problem, commentators have repeatedly used the story as a parable about the wonders of technological fixes to environmental problems. For instance, Steven Levitt and Stephen Dubner used the story of the horse in their 2009 book SuperFreakonomics to justify the use of radical geoengineering solutions to climate change.

Tarr himself doesn’t believe technological change is always a panacea. “Why do we automatically assume that every new device will be better?” he asks. He has made urban technological change one of his specialties because he believes it is important that we understand the drivers of change. “History circles,” he explains.

This particular circle has come around quickly. In Tarr’s office there is a reproduction of a magazine photo hanging prominently amongst the accumulated books and papers; in it stands his father in a worker’s cap, cigarette between his lips, at work under the harsh light of the night shift at a shipyard. He had been one of those workers who built the urban landscape with the help of a living machine. “He had a horse,” Tarr says, “back when he was in the scrap business in New York. He had a horse called Shivers, and that’s just about all I know about it.”

Where to Find Out More

The Horse in the City: Living Machines in the 19th Century by Clay McShane and Joel Tarr Johns Hopkins University Press, 2007.

“The Centrality of the Horse in the Nineteenth-Century American City,” Clay McShane and Joel Tarr, In Raymond A. Mohl (ed.), The Making of Urban America Scholarly Resources, Inc., 1997.

“The Horse Era in Pittsburgh,” Joel Tarr, Western Pennsylvania History, Summer 2009, 28-41

Blobologist-approved reads: oops, is it May?

Apologies, my dear, neglected readers, for posting so infrequently, but things have been a little hectic in the House of Blob this semester. Sadly, this meant there was no time for blogging. On the positive side, I did at least spend that time writing about science, InDesigning up a storm, creating a hella professional social media strategy for a clinical research group and building a pretty respectable looking website for a local non-profit.

But now I’m left with a blobology backlog that I will address with the following strategic plan:

1) Present my ScienceSeeker picks from the past few months, even though they are now horrifically out-of-date

2) Post a couple of my favourite science writing class assignments

3) Mix myself a Dark ‘n’ Stormy

I might not follow this plan in the prescribed order.

My ScienceSeeker editor’s picks: The semester shell-shock edition

The Unresolved Mysteries of the Mold in Your House
Contains the answer to the question, “What do your dishwasher and fruit bat’s colon have in common?”
Rachel Adams at Your Wild Life (guest)

When #chemophobia isn’t irrational: listening to the public’s real worries.
Part of the on-going conversation about chemophobia, which is the blanket distrust that many people feel towards anything they consider “chemical.” But shouting at people that they don’t understand what that word means doesn’t help anyone, least of all chemists.
Janet D. Stemwedel at Doing Good Science

How genetic plunder transformed a microbe into a pink, salt-loving scavenger
A tale of genetic thievery on an epic scale.
Lucas Brouwers at Thoughtomics

How to protect lions?
Can we really protect lions by fencing them in or by hunting them?
Colin Beale at Safari Ecology

Ducks Meet the Culture Wars
A beautifully written defense of basic science and the point of studying duck penises.
Carl Zimmer at The Loom

The Narcissism of De-Extinction
If you follow me on Twitter you might have noticed that the TedX DeExtinction conference got me uncharacteristically irritated and even made me break out some ALL CAPS OUTRAGE. Thankfully, by the time I extricated myself from TweetDeck Hannah Waters was already ON IT, explaining much more thoughtfully and lucidly than I could why it was just ALL SO ANNOYING.
Hannah Waters at Culturing Science

Roller Derby Teammates Give Each Other Bacterial Hugs
Roller Derby teams are close and so are their skin microbes.
Kate Clancy at Context and Variation

The two ideas to fix the gender balance that do not make me cringe
Two recent (at least, they were recent when I made this pick) initiatives for addressing the fact there are not enough women in the most powerful positions in science.
Eva Amsen at Occam’s Typewriter (guest)

There Should Be Grandeur: Basic Science in the Shadow of the Sequester
On the risks posed to basic research by the sequester. Featuring the line: “paying for basic research is a bet a society makes on its future.”
Tom Levenson at Scientific American Guest Blog

Buzzsaw Jaw Helicoprion Was a Freaky Ratfish
So paleontologists finally solved the mystery of where to put the the spectacular buzzsaw jaws on their Helicoprion reconstructions.
Brian Switek at Laelaps

Carey Morewedge serves up an imaginary feast

Dear Blobologist,

I know I’ve been neglecting you recently. As a peace offering, here’s a class assignment I did a few weeks ago, based on an interview with a researcher from my university. I hope it will tide you over.

love,

Cristy xx

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Imagine your favorite food.

Chocolate, cheddar cheese, chicken tikka masala, whatever your weakness, picture it just out of reach, glistening enticingly.

Although this food isn’t real, your body might be responding as if it were. Perhaps your mouth is watering or maybe you’re feeling the pang of cravings. But Carey Morewedge, a psychologist at Carnegie Mellon University, says food fantasies can have an even stranger effect; he has shown that we can be satisfied by imaginary food.

When Morewedge and his collaborators, Young Eun Huh and Joachim Vosgerau, began their research into imaginary feasts, most psychologists believed that the more you thought about food, the more you craved it. The problem with this idea was that when you eat food in real life, you crave it less rather than more. Our first mouthful of a favorite dish makes us desire it more, but as we eat bite after bite, we start to lose interest. This loss of interest is called “habituation” and is part of every pleasurable experience, from food to sex to watching Gangnam Style.

So why doesn’t imagining food also make us lose interest? Morewedge and his group asked this question because when we imagine an experience, our bodies and minds often respond as if that imagined experience were real. They guessed that the reason previous studies had not observed habituation was because study participants didn’t take their imaginary experiences far enough.

“If I’m thinking about a Chipotle burrito, I’m thinking about the shape, what’s inside it, what it will taste like on the first bite, what it might smell like, or how warm it might be,” says Morewedge. “But I do not think about biting, chewing and swallowing the whole burrito.”

So the researchers asked people to think about biting, chewing and swallowing M&Ms. Each person in their study imagined performing 33 repetitive actions: either inserting 33 quarters into a laundry machine, inserting 30 quarters into the machine and then eating 3 M&Ms, or inserting 3 quarters into the machine and eating 30 M&Ms. Inserting quarters was chosen as a control for imagining an action similar to picking up candy.

After their mental exertions, the participants were allowed to eat as many M&Ms as they wanted during preparation for a fictitious “taste test.” Psychologists often include these kinds of deceptive scenarios to prevent people guessing what behavior is being measured, which can influence their response.

After each experiment, Morewedge’s team weighed the M&M bowl to see how much the participant had eaten. The results showed that people who had imagined eating 30 M&Ms ate almost half as many real M&Ms as those who imagined eating only three. In effect, they had satisfied their desire for M&Ms without actually eating any.

This only worked when people pictured eating the M&Ms. When they instead imagined moving the candy into a bowl, the people who moved more imaginary M&Ms ended up eating more real M&Ms, rather than fewer. That kind of imagery only whetted their appetites.

But was this really habituation? To test this, the researchers looked for one of the hallmarks of habituation, called sensory specificity or the “dessert stomach” effect.

“We’ve all heard of this phenomenon,” says Morewedge. “When you go to a restaurant, you finish your entree and you can’t even imagine eating another bite. And then someone rolls out a cart of cheese or cakes, and all of a sudden you have a renewed interest in food.”

Just like real habituation, the imaginary M&Ms did not affect participants’ desire for other types of food, in this case, cubes of cheddar cheese. It worked the other way as well: eating more imaginary cheddar cheese meant people tended to eat fewer real cheese cubes, but it had no effect on how many M&Ms they ate.

So does this mean you can think yourself thin? Probably not. The dessert stomach effect is one reason why Morewedge doesn’t think we’ll see their results become the next diet craze. Imaginary eating habituates you to the food you have imagined, but makes other foods seem even more appealing.

Instead of using the research to come up with a diet miracle, the group is trying to apply their results to other contexts, like cigarette smoking, to see if mental habituation might be a useful tool for modifying addictive behavior.

But even if we are never able to harness the power of that imaginary chicken tikka masala for practical use, Morewedge and his colleagues have made an important theoretical advance. The line between imagination and physical experience is blurrier that we used to believe — a pleasant idea to contemplate the next time you get a hankering for something just out of your reach.

Photo credit: Flickr user Zen. Shared under this Creative Commons license

Shameless self-promotion: me at the SA incubator

Thanks to the magnificent Bora Zivkovic, I did a Q&A at the  SA Incubator at Scientific American Blogs yesterday. If you want to see several photos of me grinning like an idiot, plus one photo of a Vaccean cremation tomb, then that is where you will find them.

Blobologist-approved reads: Privilege, pigeons, polyester, paleontology & pythons

My ScienceSeeker editor’s picks: The between conferences edition

A field guide to privilege in marine science: some reasons why we lack diversity by Miriam Goldstein at Deep Sea News

A much-needed list of the barriers that can prevent people from making it in science. If you’re a scientist, I insist that you read this.

A Dream Deferred: How access to STEM is denied to many students before they get in the door good by Danielle Lee at The Urban Scientist

Miriam Goldstein’s field guide prompted an important post from Danielle Lee, in which she shares her stories from the trenches of high school science fairs. Even if you’re not a scientist, I insist that you read this.

Pigeons: Darwin’s Unappreciated Avian Assistant by Michael Wheelock at The Incubator

Why pigeons and not finches should be remembered as Darwin’s best feathered friends.

My Sweaty Valentine by Rebecca Guenard at Scientific American Guest Blog

Why is it that polyester always make you smell so bad? Rebecca turned her sweaty synthetic experience at ScienceOnline2013 into a wise-cracking journey into the science of B.O.

Why Paleontology Is Relevant by Sarah Werning at The Integrative Paleontologists

Turns out it’s not just because paleontologists often teach anatomy to med students.

Calories aren’t right on labels and maybe that’s OK by David Despain at Evolving Health

Does a doughnut = an apple? Does a calorie = a calorie? Does a whole rat = a blended rat? The answers to these questions and more in David’s fascinating write up of a AAAS meeting session on counting calories.