Acid

Have you ever wondered why acids taste sour? Acids are chemical compounds that are likely to donate positively charged protons, usually hydrogen ions. Since the presence of free hydrogen ions can have a dramatic effect on other molecules around it, our bodies have a specialized taste sensation for estimating the concentration of protons in a solution - sourness. Your taste buds detect the level of hydrogen ions and send a signal to your brain indicating the level of sourness. The acid concentration in the cells in your body is carefully regulated. Since the liquid contents of a cell are a buffer system that resists changes in pH, a common reaction to the taste of acid is to drink water to dilute it.

Depending on concentration, acids can titillate the palate, affect the colour of cooking vegetables, coagulate milk into cheese, aid in digestion, or corrode metal. Almost every food we eat is acidic to some degree, from meat to orange juice, (even milk is slightly acidic) and so the chemistry of acids affects most recipes.

• Chemical leaveners depend on the reaction between acids and alkalis to produce enough carbon dioxide gas to raise baking batters.
• A little lemon juice or vinegar added to the water for a poached egg helps to coagulate albumin protein, insuring that the egg white remains firm and compact.
• Dried beans cooked with tomato, lemon, or other acidic ingredients will take longer to soften because acid stabilizes the cell walls of legumes. This can be problematic when cooking particularly hard beans, like chickpeas and soybeans, but for cooking less fibrous beans, like lentils, or for long-cooking preparations, like baked beans, a little acid helps beans keep their shape as they soften.
• In a ceviche, citrus juice and/or vinegar denatures the delicate proteins of fish and shellfish the same way cooking with heat does. The flesh becomes firm, opaque, and dry enough to be eaten without ever being heated.
• As the active agent in marinades, acid decreases the number of chemical bonds between proteins, encouraging meat fibres to bond with flavourful components in the marinade, and tenderizing tough meat fibres.
• Adding a pinch of acid, usually cream of tartar or lemon juice, to egg whites while beating them brings their pH to a point where the proteins coagulate easily. The resulting albumin foam stays flexible and glossy, and therefore rises higher when baked.
• Bacteria used when making fermented dairy, feed off sugars in milk, producing lactic acid that coagulates the casein milk proteins, causing the milk to thicken into buttermilk or yogurt and eventually separate into cheese curds.
• Acid turns the chlorophyll green vegetables khaki colour, which is why marinated green beans or asparagus lose their bright colour, and why an acid, such as lemon juice or tomato, should never be added to a cooking green vegetable until the last minute. Likewise, green vegetables should be cooked in an open pot. Covering them with a lid concentrates the natural acidity of the vegetable and dulls its colour. Although adding an alkali like bicarb soda to the water will neutralize the acid, don't do it. It will brighten the green colour but it will also destroy the vegetable's vitamins and degrade its texture to mush.
• Acid brightens anthocyanin, the colour of red-purple vegetables, which is why red cabbage is usually cooked with vinegar or some acidic fruit.
• Enzymatic browning, the chemical reaction that causes some cut fruits and vegetables to discolour, proceeds slowly if the food is tossed in a weak acid, such as a solution of lemon juice and water, as soon as it is cut.
• Acids catalyze the breakage of starch molecules into smaller pieces so they have less thickening power, which is why sauces that contain wine or citrus juice will not thicken as much as nonacidic sauces.
• Root and tuber starches, such as potato starch, tapioca, and arrowroot, are more sensitive to acids than are grain starches like wheat flour and cornstarch.
• Acid in a poaching liquid, marinade, or frying batter helps to dampen volatile aromas in fish. It also breaks down muddy-smelling compounds that farm-raised fish accumulate from algae in their feed.

Acidity is measured on a pH scale, a standard measure of hydrogen ion activity that ranges from 1 (extremely acid) to 14 (extremely alkali or basic). Pure water is considered neutral, neither acidic nor alkaline, with a pH of 7. Though H20 molecules are fairly stable, at any given moment a small percentage (about 0.0000001) dissociate, allowing free hydrogen ions to bond with nearby intact water molecules, creating positively charged H30 molecules. The more this happens, the more acidic a solution becomes. Strong acids like hydrochloric (HCI) fully dissociate into hydrogen ions and chloride ions. Weak acids like the organic acids found in foods (acetic, citric, lactic, etc.) partially dissociate so only part of the total amount of hydrogen is donated.