Tuesday, December 7, 2010

Lemon-honey dressing and handedness

My son "invented" a very simple salad dressing with two ingredients, lemon juice and honey.  I put the invented in quotes since a simple internet search reveals a number of recipes for this already: Jamie Oliver, Great Grub,  and even Martha Stewart.  Many of these add extra ingredients - olive oil, pepper, herbs and spices.  My son's was pure - lemon juice and honey...
I actually like his recipe on one of my favourite salads.  It is a mix of romaine lettuce sliced finely, chopped apple, chopped avocado, green or red peppers, and raisins (and sometimes pomegranate)

The dressing makes the salad very light and refreshing!

Limonene is the chemical that makes oranges and lemons smell so lovely - but it is also the piney smell of turpentine...how can this be?

Well, limonene is actually what is called a chiral chemical - it has two forms with almost the same structure:
The D-limonene (also called R-limonene) is the chemical found in the skins of oranges and lemons and has been produced in about 95% purity from the waste of orange juice production for many years!

The L-limonene ( or S-limonene) has been extracted purely from turpentine and from the plant Eucalyptus Stageriana. Some is present in the skins of oranges and lemons too.  

The R and S refer to the "handedness" of the molecules. Your right and left hands are chiral - they are un-superimposable mirror images.  R comes from the latin Rectus and S from the latin Sinister - right and left.

Chiral compounds often are very similar in properties to each other but sometimes exhibit quite different properties.  R- and S-limonene have this effect - R-limonene is orangey smelling while the S version is more lemony smelling.  The same is true for a related chiral molecule: R- and S-carvone.
R-carvone has the odor of spearmint and the S-carvone is the smell of caraway!


Theodore J. Leitereg, Dante G. Guadagni, Jean. Harris, Thomas R. Mon, Roy. Teranishi (1971) Chemical and sensory data supporting the difference between the odors of the enantiomeric carvones. Journal of Agricultural and Food Chemistry, 19 (4), pp 785–787 DOI: 10.1021/jf60176a035
L. Friedman & J.G. Miller, Odor incongruity and chirality, Science, 172, 1044-6 (1971)

David J. Willock (2008) Molecular symmetry, 

pg 41

A.F. Thomas and Y. Bessiere (1989) Natural Product Reports, 6291.
DOI: 10.1039/NP9890600291

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