Savoury proteins

Even if we take our senses for granted, they can still hide surprises. Eastern chefs discovered first, at the beginning of the century, that sweet, bitter, salty and sour are not the only tastes that our buds can perceive. We can enjoy a fifth side of flavour in our plates, the “savouriness”. The world now refers to it more and more as umami, the name given by its japanese discoverer. It comes especially from glutamate; and thus, from proteins-rich food: meats and their broths, algae, cheese, soy and mushrooms. Umami is the secret of oriental cuisine and its controversial condiment, MSG – monosodium glutamate – that gives our palates a hearty feeling.
A century after its first official description by professor Ikeda, Umami was still keeping secrets from the culinary scientist. An in it these lied one of its most wonderful potential for the adventurous cook : its taste can be multiplied exponentially by a variety of ingredients. Notably inoside and guanoside monophosphate, as well as certain sweeteners greatly enhance its flavour.
The umami taste receptor, mGluR4, presents an extensive similarity with the sweet taste receptor. Both are G-coupled heterodimeric protein complex and share a ligand-binding domain, TIR3, while the umami taste receptor possess a unique glutamate-binding domain, TIR1. The association of glutamate to TIR1 elicit the signal that our brain interprets as a hearty flavour. Interestingly, when conserved residues between TIR1 and TIR2 are mutated, the synergetic effect of IMP and GMP is lost. Using molecular modelling, chemists showed that the mystery resides in the phosphate group of the molecules. They allow IMP and GMP to bind near the entrance of the crevice to positively charged residues, stabilizing a closed conformation around the glutamate and thus stimulating the production of the signal to the brain.
A wonderful examples of molecules working together for our enjoyment!

For further reading :

Zhang F. et al. 2008 Molecular mechanism for the umami taste synergism Proc. Natl. Ac. Sci. 105(52), 20930-4

Flavors from the devil

My final, considered judgment is that the hardy bulb [garlic] blesses and ennobles everything it touches - with the possible exception of ice cream and pie.” Angelo Pellegrini, 'The Unprejudiced Palate' (1948)

Sulphur compounds do not only keep in check the reduction potential of our solutions in the lab: Nature brings them to our noses and palate as well, leaving their stamp on our cuisine. The pungent smell given by crushed garlic, the tears flowing while cutting onions and the aftertaste of Sauvignon Blanc all share their origins in them. In many foods, the potential of sulphur for redox chemistry makes it subject to the enzymatic transformation of its organic derivatives in flavorful components.
In the cells of the Alliaceae family (which include garlic, onion, shallot, and chive) the enzyme alliinase is sequestered from the cytoplasm. When a bulb of garlic (Allium sativum) is crushed or cut, the cells are damaged and alliinase is released in the cytoplasm. Once there, it comes in contact with alliin, a cysteine derivative without particular smell. Alliinase catalyze oxidize it to give allicin, which gives its… pungent... smell to garlic. Apparently, somewhere along its evolution, the Alliaceae family found the trick handy when animals decided to grab a bite at their expense.
This might help the resourceful biochemist cooking with garlic. For garlic with the flavor but without the bad breath, wrap your bulb in the aluminium foil with olive oil and put in the oven at 325 °F for an hour. The heat will easily inactivate the alliinase.
The alliinase released in onions has a different effect. It allows ultimately the formation of syn-propanethiol-S-oxide from sulfenic acid. The molecule is not only volatile: it reacts with water to give sulfuric acid, giving their lachrymal properties to onions. The good news is that, as this sulfinyl is combustible, lighting a candle next to your cutting board should eliminate most of the annoying molecule.
Sulfur compounds can mean a more enjoyable experience too. A characteristic element of the taste of the Sauvignon Blanc wines rests on certain thiols. Interestingly, those thiols are absent from the wine itself. They originate from S-(R/S)-3-(1-hexanol)-L-cysteine, a component of the grape, which is transformed by salivas’ microflora in volatile thiols. Sulfur can have its upside too after all.

Further readings:
Jones MG et al. 2004 Biosynthesis of the flavour precursors of onion and garlic, J. Exp. Bot., 55 ( 404), 1903
Srarkenman C. 2008 Olfactory Perception of Cysteine-S-Conjugates from Fruits and Vegetables J. Agric. Food Chem., 56, 9575-80

Kitchen Blues

If a biochemist can be spotted by its blue-stained fingers, the color blue might be often overlooked in the kitchen. For this first column, we will turn our attention to two blue ingredients in our kitchens: red wine and bananas.
Blue hides in red wine. Anthocyanins give its color to the royal drink: those polyphenols gives in fact their colors to many flowers, fruits, berries and vegetables. Among which we can name pansies, eggplants, cherry, apples, raspberries, blueberries and grape. Acting as a natural sunscreen, anthocyanins protect the plants from the lights radiations that chlorophyll itself does not absorb or in other occurrences from other oxidative damage. Funny thing about them: their absorbance spectrum vary in a pH-dependant manner; red for acidic solutions, blue for alkaline and purple around neutral (anyone ever wondered where the Litmus test came from?).
Here is the funny hands-on part : to make some blue (and unpalatable) wine, just add some alkali to your red wine! 1M NaOH should do the trick (watch out : too much of it just gives a nasty brown). And duh… do I really need to tell you not to drink the wine afterward?
We have seen that red can be blue. Now, let’s see why, when bananas are involved, yellow also can be blue… Yes, they are blue. No? You were not looking at them under the right light: try UV instead (have you never brought a banana in a bar?). Your banana will turn a bright, electric blue. During ripening, as chlorophyll is degraded, bananas lose their green color, with only some carotenoids left to give the usual yellow. Chlorophyll being a porphyrinoid, its breakdown products can present surprising light-absorption characteristics. It appears that the secret of that fluorescent surprise resides in one of those metabolites, only recently discovered and termed FCC-56 (Fluorescent Chlorophyll Catabolite). Now, biochemists just have to figure the exact role of those metabolites for the plant... Piece of cake

For further reading about anthocyanins and wine color:

Jensen J. S. et al. 2008 Prediction of Wine Color Attributes from the Phenolic Profiles of Red Grapes (Vitis vinifera) J. Agric. Food Chem., 56 (3), 1105–15
The discovery of the fluorescence in bananas (how come no one saw that before?): Moser S. et al. 2008 Blue Luminescence of Ripening Bananas Angewandte Chemie. 47(46), 8954-7