A touch of acid

Kilawin (a vinegar based ceviche) and grilled pork with calamansi. An excellent combination. 

You hear it quite often nowadays on those cooking competition shows. A judge eats a bit of a contestant's food, and pronounces "It could use more acid". Well, what exactly do they mean by that? In general, I think the judge just wants an additional sour note to the food - after all, that's what we taste as sour are acidic substances. But not all acids are equal.

Industrially, mineral acids are the norm - examples of these are sulfuric acid, or muriatic (hydrochloric) acid or nitric acids. These are strong acids, which is to say just a little bit of mineral acids can wreak havoc on naked metal (and yes, just about all metals are susceptible to acid attack - take note about your fancy knives), and scar exposed skin. But these are rarely (if ever) used in culinary applications.

What are found in foods, the endogenous acids that make them sour, are weaker organic acids. Used in a scientific context, organic has nothing to do with being pesticide-free or other scaremongering imagery, rather, organic compounds have carbon backbones, and almost all chemicals emanating or involved with life are organic in nature. Ironically, that means that most pesticides are organic in nature.

Maybe the simplest culinary acid is carbonic acid, formed by combining a molecule of carbon dioxide with a molecule of water. Carbonated water is already acidic by this reaction, although it tends to be unstable, and breaks down easily. Nonetheless, formation of carbonic acid is an important step in buffering of blood chemistry, and given how easily it is formed, is the key compound in making limestone caves.

A bit more stable is acetic acid, the key souring component of vinegar. It's formed by bacteria eating the alcohol formed by yeast in fermented juices like wine. Although more stable than carbonic acid, it is volatile, meaning that it will evaporate. So, how does a salt and vinegar chip taste sour without being soggy? More than likely, the manufacturer used a crystallizable acid: citric acid. As you can tell from the name, citric acids is the key souring compound in citrus fruits, although it can be found in other foods as well. Powdered citric acid crystals are readily available, and are a food safe method of cleaning off the white crust from hard water that accumulates in kettles and faucets. What balances out citric acid is tartaric acid, the souring notes in fruits like grapes and apples. In fact, the ingredient cream of tartar is basically solid tartaric acid. It also has a certain astringency that is characteristic of the fruity flavors.