Guardian Weekend – Part of a series of 6 articles on taste and cooking with co-author Peter Barham

The combination a sweet and a sour taste provides some of the most piquant dishes in the culinary repertoire. Whether it’s in an oriental sweet and sour dish or a dessert of sweet fruit with yoghurt, sweet and sour together remain popular favourites.

The sourness comes from an acid. In Oriental cooking, it’s vinegar, which is no more than a dilute solution of acetic acid in water, perhaps with some caramel colouring added. In yoghurt (or sour milk) it’s lactic acid, arising from bacterial action. In the Margharita that you have later, it’s lemon or lime juice, a weak solution of citric acid. In the regurgitating stomach that follows all this eating and drinking it’s hydrochloric acid, fresh from the stomach wall and strong enough to strip the paint from a car, as anyone who has ever had a car-sick passenger with their head out the window will know.

Sourness is the least understood of the basic taste qualities. Why, for example, does vinegar add the sour quality to sweet and sour pork, and yet convey a rather different taste sensation when used to pickle gherkins? No one seems to know. One of the reasons why we scientists get together with chefs and food writers for a biennial meeting in a remote Sicilian monastery is to thresh such matters out. It’s surprising how much we still have to learn about tastes and flavours in cooking. As the person who started our meetings, the eminent physicist Nicholas Kurti, once said in a lecture in London ” … while we can and do measure the temperature in the atmosphere of Venus, we do not know what goes on inside our souffles.”

We know even less about how acids work in our taste processes. In the mouth, an acid breaks up into electrically charged ions in the same way that salts do. Salt produces positively charged sodium ions (the ones that have an electrical effect on the tongue, perceived eventually by the brain as “saltiness”) and negatively charged chloride ions. An acid produces positively hydrogen ions (smaller than sodium ions, but otherwise similar) and negatively charged ions that differ with different acids. It is the hydrogen ions that produce “acidity”, which we may perceive as “sourness” taste (in German, an acid is known as “Sauerstoffe”). The tongue, though, doesn’t seem to have specific “acidity” receptors in the same way that it has sweetness receptors or bitterness receptors. The effect appears to be a more general one, as with salt.

One thing that we do know, just from simple chemistry, is that we can neutralise the effect of acids by adding something alkaline. Alkalis are the opposite of acids. If you mix the two, you get a salt. That is the theory behind taking bicarbonate (an alkali) when you have a stomach-ache that you suspect is due to excess acidity. “Bicarbonate” is actually sodium bicarbonate, and when it reacts with the hydrochloric acid in the stomach you get sodium chloride, which is common salt. Your body already contains a substantial amount of this, so the little bit extra doesn’t hurt. As well as salt, though, the reaction produces carbon dioxide, and this can emerge as a satisfying burp. Such a burp is regarded as good manners in some societies, so it might be a good idea to carry a bottle of bicarbonate when you have your next authentic sweet and sour.

You can easily produce a burp in a bottle by adding some bicarb, a little at a time, to vinegar until the fizzing stops. Try tasting it as you go. You will start with an acid (or sour) taste, and end up with salt.

Even though acids act in a similar way to salts in the mouth, their effect on bitterness or sweetness perception seems to be quite different. Tiny amounts of salt can enhance sweetness perception, but sourness and sweetness do not seem to interact in the same way. Try this for yourself by adding a drop of vinegar to (say) a sweet drink. If it was salt that you were adding, the sweet taste would be increased but you wouldn’t taste the salt. With vinegar, though, acidity and sweetness both appear to be present, just as it is in a sweet and sour dish.

That’s what we think, anyway, from our own experience. So far as we know, though, the experiment has never been done properly on a large scale. Try it yourself and, if your own results are different, do let us know. The science of good food, known pompously at our Sicilian meeting as Molecular Gastronomy, is the province of us all!

© This article is copyright Len Fisher. Please email Len Fisher to seek permission to reproduce part or all of the above article.

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