Archive for the ‘academic articles’ Category

Drink your tea without milk!

Wednesday, January 10th, 2007

Experimental and clinical studies indicate that tea exerts protection against cardiovascular diseases. However, a group of German researchers (abstract, European Heart Journal 2007, ASAP contents) have found that the beneficial effects of drinking tea may be reduced if milk is added to the tea.

milk drop hits coffee
(Picture by IreneM entiteld “coffee with a “drop” of milk” from DPchallenge – OK, it’s not tea, but I just love this picture!)

By measuring the blood vessel’s ability to expand (and thereby reduce the blood pressure) the researchers found that this ability was improved by tea, but the effect was completely blunted if milk was added to the tea. It was found that the caseins were responsible for the observed inhibition, probably by formation of complexes with catechins. It is believed that catechins (polyphenolic compounds, belong to the group of flavonoids, structure of epicatechin shown below) trigger the release of other active substances that are responsible for the expansion of blood vessels (also known as vasodilation).

epicatechin

The results of this study are not limited to tea, because catechins are found in many other foods, including citrus fruits, wine and chocolate.

Happy New Year with the Science of Champagne!

Sunday, December 31st, 2006

Have you ever though about how far you can shoot a champagne cork? The swedish physicist Hans-Uno Bengtsson has actually done the necessary calculations in the wonderful Swedish book “Kring flaskor och fysik” (which translates to something like “Among bottles and physics”, it was written together with sommelier Mischa Billing). Assuming a bottle pressure of 6 atmospheres, a cork length of 25 mm (the part in contact with the bottle), a radius of 9 mm and a mass of 7.5 g, this gives an initial cork velocity of approximately 20 meters per second or 70 km/h! This translates into a maximum shot length of around 40 m (if we neglect air resistance). In case you prefer not to shoot the cork, you could of coarse turn to a saber or a heavy kitchen knife instead to open the bottle.

When opening a bottle of champagne, you might have noticed the cloud forming right above the bottle neck (see picture below). This is due to a significant temperature drop, caused by gas expansion when we open the bottle. Assuming an adiabatic expansion (meaning no heat exchange with the surroundings), Hans-Uno Bengtsson has calculated a temperature drop of 112 °C! No wonder the vapor around the bottle neck immediately freezes forming a small cloud.

cloud at neck of champagne bottle
(picture by polarunner at flickr.com)

If this doesn’t satisfy your craving for champagne science, there’s a whole book on the subject: “Uncorked – The Science of Champagne” by Gérard Liger-Belair. He’s an associate professor of physical sciences at the University of Reims Champagne-Ardenne and probably knows more about champagne bubbles than anyone else! In addition to many fascinating pictures of bubbles, the book has many interesting facts. Did you know that:

  • 0.1 liters of champagne (the contets of an average flute) contains approximately 0.7 liters of carbon dioxide which must escape to restore equillibrium – assuming an average bubble size of 500 micrometers in diameter this corresponds to 11 million bubbles!
  • Contrary to popular belief, nucleation sites for bubbles are not found on scratches or irregularities on the glass itself, but on impurites stuck on the glass wall. These impurities are typically fibres from paper or fabrics.
  • From the point when a bubble leaves the nucleation site till it reaches the surface, the volume increases by a factor of 1 million. This is due to diffusion of carbon dioxide from the solution and into the bubble.
  • Surfactant molecules in champagne form a protective shield around the rising bubbles. This stiffens the bubbles and significantly increases the drag on the bubble as it rises (which gives us more time to admire the trail of bubbles!).
  • The surfactant coating of the bubbles helps keeping them in line as they rise. In pure water, the bubbles would jostle around.
  • The bursting bubbles play an imporant role in flavor release as they collect and concentrate surface active molecules which are thrown against your nose once the bubble bursts, creating a cloud of droplets.
  • (these facts should be perfect conversation starters!)

    trail of champagne bubbles
    (photo by Gérard Liger-Belair)

    An interesting article by Gérard Liger-Belair, “Effervescence in a glass of champagne: A bubble story” is available from Europhysics news.

    Happy New Year!

    Odor recognition by shape or vibration?

    Thursday, December 28th, 2006

    Over Christmas I have been reading Luca Turin’s book “The Science of Scent”. This became a real eye-opener for me with regards to my understanding of how the sense of smell works. (BTW, Luca Turin was also featured in Chandler Burr’s book “The Emperor of Scent” which I haven’t read yet).

    secret of scent cover

    The first part of the book includes a lot of basic chemistry (which can be skipped if you’re familiar with chemistry) plus descriptions of many perfumes and perfume ingredients which made we wish the book came with it’s own smelling strips. But then comes the interesting part. I have always thought of smell to be a result of molecular recognition – a typical interaction between a drug and a receptor or and enzyme and a substrate. The reason I guess is that this seems very intuitive – just like a shape sorter toy for children! I had also read a review article on “Structure-Odor Relationships” (Rossiter, K. J. Chem. Rev. 1996, 3201). Now the interesting thing is that this might be wrong (or at least not the whole truth – and scientific controversy is always exciting)!

    shape sorter

    Luca Turin suggests that it is the molecular vibrations of a molecule that we recognize as it’s smell. In an easy accesible article on this (at least for chemists), Turin puts up pro’s and con’s for both theories, including the following:

  • isosteric molecules smell different (ie. similar shape, different smell)
  • most enantiomeric pairs smell the same, or similar (this is contrasted by medical drugs where enantiomeric purity is often crucial)
  • we smell functional groups (for example alcohols (OH) never smell like thiols (SH), regardless of molecular shape and concentration, this supports the idea that we smell vibrations rather then shape)
  • deuterated molecules (probably) smell different from their protio analogues (experiments here are not trivial to perform, as purity is a major issue here)
  • A very recent review entitled “The Nose as a Stereochemist. Enantiomers and Odor” (Bentley, R. Chem. Rev. 2006, 4099.) mentions Turins work, but with a short dismisal:

    … a theory by L. Turin proposes inelastic electron tunneling to account for the biological transduction of molecular vibrations.[35][36] Recent experiments to test predictions of the theory found no evidence to support it [37]

    Reference 35 and 36 can be downloaded from www.flexitral.com – the latter needs to be saved/renamed as a pdf before opening. The contents of 37 is described here. It’s surprising however that Bentley uses ref 37 to disprove the vibration theory, because the authors refer to their own work as “… a paper of solely negative results”. What they did was to perform experiments, partly outlined in Turin’s book, that suggest that molecular vibrations alone cannot explain all aspects of smell. Despite the controversy, Luca Turin and his company Flexitral have been quite succesful in designing new odorants, especially stable odorants which imitate other, less stable molecules. The development of these new odorants is based on designing stable molecules with vibrations similar of the molecule it’s supposed to imitate.

    But the story doesn’t end here: Very recently, physicists at University College London reported that they have discovered a physical mechanism that would allow a receptor to distinguish different molecular vibrations (read press release, preprint and SciAm news report). Put simple, the researchers have shown that when a molecule with the correct vibration binds to a receptor, a switch closes allowing electrons to flow. This means that there is experimental theoretical evidence that supports the vibration theory!

    Now what does all this have to do with molecular gastronomy and food? When we talk about taste, it’s actually 80% aroma and 20% taste (more on this page). And with aroma, we’re talking about the smell of volatile molecules. Luca Turin touches upon this on the very last pages of the book were he writes that “An area which, in my opinion, is ripe for revolution is that of flavours”. Perhaps it will be possible one day to “synthesize” any desired odor (or aroma!) with a set of molecules (or condiments) with different molecular vibrations?

    The Joy of Evidence-Based Cooking

    Sunday, December 3rd, 2006

    In a recent Science article (Science 2006, 314 (5803) 1235 (requires subscription, but text has been posted in a newsgroup), Martin Enserink writes about Hervé This and molecular gastronomy. One of his projects is to rid cook books of the many errors.

    One of This’s obsessions is that chefs, despite knowing so little about science, have developed such elaborate laws. Over the years, he has meticulously collected more than 25,000 instructions, called précisions in French, from cookbooks, many of which are useless, he says. So where do they come from? “Our parents love us. Why are they teaching us all these rules that make no sense?” His hypothesis: Cooks, using trial and error, remembered the circumstances in which they created a successful dish, even if they were irrelevant, and made them part of the recipe.

    The article also touches upon the different views Harold McGee and Hervé This have on what molecular gastronomy is and/or should be. Whereas This wants the help of cooking schools to test his précisions, McGee is more reluctant: “I’m not sure I’d spend so much time studying misunderstandings of the past”.

    Hervé This giving a demonstration
    (picture from Science, Credit: Ppierre Beachemin/ITHQ)

    One more article by Hervé This

    Monday, November 13th, 2006

    Now that I’m at it, I found yet another article by Hervé This entitled Molecular Gastronomy and the Foundation “Food Science and Food Culture”, published in Comprehensive Reviews in Food Science and Food Safety, 2006, 5, 48. About the name “molecular gastronomy”, Hervé This writes:

    Molecular gastronomy, why such a pompous name? And is it some useless activity of the idle rich or wealthy foodies? Of course not! First, a differentiation should be made between cooking and gastronomy. Cooking means preparing dishes, whereas gastronomy, according to the promoter of the word, means “intelligent knowledge of whatever concerns man’s nourishment” (Brillat-Savarin 2006). When this knowledge is history, the activity is “historical gastronomy,” but when it comes to the study of chemical and physical transformations involved in culinary practice, then it is “molecular gastronomy.”

    He goes on to distinguish it from culinology (which BTW is a trademark – “How can science be patented?” Hervé asks). What is even more interesting, is that he includes a modification of the original five points that were published in his PhD thesis (and also included in the 2002 article “Molecular gastronomy” in Angewandte Chemie):

    1. investigate recipes
    2. collect and test culinary proverbs, old wives’ tales, and so on
    3. invent new dishes based on 1 and 2
    4. introduce new tools, ingredients, and methods in the kitchen
    5. use cooking to show that the physical and biological sciences are wonderful

    He writes that this was a major mistake because 3 and 4 are technological, not scientific, and 5 is political. Because of this, he has recently changed the objectives of what he thinks molecular gastronomy should be. He notes that a dish contains a “love” component, an “art” component and a “technical” component. And molecular gastronomy should investigate these three, but only from a scientific point of view. Read more about definitions of molecular gastronomy.

    New article by Hervé This

    Thursday, November 9th, 2006

    In a recent issue of EMBO Reports, Hervé This (who coined the term “molecular gastronomy”) writes about “Food for tomorrow? How the scientific discipline of molecular gastronomy could change the way we eat” (free download: html or pdf). He asks:

    What is molecular gastronomy? Is it only a temporary trend for people who are prepared to spend a small fortune on the latest in fine food, or is it here to stay? Is it a useful technique for both the average chef and anyone preparing dinner for their family? What does it mean for the future of food preparation? What are we going to eat tomorrow?

    Higly recommended reading!

    Gastro physics

    Monday, October 23rd, 2006

    There is certainly some overlap between molecular gastronomy, kitchen chemistry, gastro physics, culinary physics and everyday chemistry… That’s why I thought the January 2004 issue of Physics Education would be of interest. It features a section on food physics, covering topics such as melting of chocolate, popping of popcorn, photographing food with visible and infrared light etc. Most of the material is for subscribers only (your local university library probably has a subscription!), but the free material includes a nice article by Jon Ogborn (entitled “Soft matter: food for thought”) on foams, gels and emulsions. Did you for instance know that mayonnaise is thixotropic?

    This means that it only flows after a certain minimum stress has been applied (figure 6). This is unusual. Liquids usually flow even under the smallest stress.

    Non-drip paint is also thixotropic. It retains its shape, but becomes fluid when enough stress is applied, for example when a paint-roller moves through it. Once the stress is removed, the paint becomes stiff again, as it is then only affected by gravity, and does not flowdown the coated surface. It contains large molecules that form a gel, keeping the paint in place. The gel structure breaks down if enough stress is applied, only to re-form quickly once the stress has been removed. So, paint is liquid on the brush and solid on the wall. Try painting with mayonnaise!

    Thixotropic materials are also referred to as shear thinningpedia. However, according to this page, the terms thixotropic and shear thinning are easily confused, so here’s the IUPAC definitions:

    Shear thinning: If viscosity is a univalued function of the rate of shear, a decrease of the viscosity with increasing rate of shear is called shear thinning, and an increase of the viscosity shear thickening.

    The application of a finite shear to a system after a long rest may result in a decrease of the viscosity or the consistency. If the decrease persists when the shear is discontinued, this behaviour is called work softening (or shear breakdown), whereas if the original viscosity or consistency is recovered this behaviour is called thixotropy.

    Ketchup is shear thinning (or was it thixotropic?), and an amusing website has even been set up to investigate “The great Ketchup mystery”.

    k_hz.jpg

    Their conclusion so far is:

    … the next time you whack the bottom of a ketchup bottle [consider this:] Even supercomputers can’t predict the outcome.

    IgNoble prize for food chemistry!

    Sunday, October 8th, 2006

    Slightly off topic, but quite amusing: Last night the IgNoble prizes were awarded. Their slogan reads “First it makes you LAUGH, then it makes you THINK”. The 2006 IgNoble prize in chemistry was awarded to a research group studying “Ultrasonic Velocity in Cheddar Cheese as Affected by Temperature”. Just in case you were wondering, melting fat is the reason for the varying ultrasonic velocities observed. And yes – this could be useful for determining mean temperatures in heating/cooling processes acording to the abstract. But why not just use a thermometer?

    There was also a nutrition prize awarded to researches who showed that dung beetles are finicky eaters… yuck! My favorite IgNoble this year is the ornithology prize awarded to a team who explored why woodpeckers don’t get headackes!

    Suppression of bitterness

    Sunday, October 1st, 2006

    I received an email last week from a supertaster (read more: BBC, Wikipedia) with an interesting question: Certain foods contain bitter substances that only a fraction of the population can taste. Examples include a group of compounds called cucurbitacins, found in melon and cucumbers, and propylthiouracil in broccoli. The question was whether these compounds could be neutralized by any means.

    A very simple chemical that neutralized/modifies bitter taste is salt – and the best thing is that you don’t have to be a supertaster to test this. For a simple experiment, take tonic water, taste it and then stir in some salt (start with 1/2 teaspoon). Taste it again – if you can still taste the quinine, add a little more salt. At one point the bitter taste has almost disappeared! This principle might work for cucumbers and melons as well, but of cource there could be totally different taste mechanisms responsible for the bittertaste in the two cases.

    tonic water

    It might sound strange to add salt, but in Asia, it is not uncommon to eat different fruits with salt. I am aware of unripe mangoes, guavas and honey dew melon are eaten with salt, a salty spice and soy sauce respectively. Also – some people add a small amount of salt to the water when brewing coffee – this reduces bitterness and rounds of the taste. One last example is how salty food can make a young red wine with plenty of tannins more pleasent to drink. Tannins (polyphenolic compounds) can be both astringent and bitter, depending on their molecular weight (low molecular weight tannins are predominantely bitter whereas larger molecules are more astringent).

    BTW, this has also been treated scientifically. See for instance: Breslin, P. A. S; G.K. Beauchamp, “Suppression of Bitterness by Sodium: Variation Among Bitter Taste Stimuli” Chemical Senses 1995, 20, 609-623 (link).