Acids and Bases
Acids and bases
Acids are what make foods taste sour. In fact, the name comes from the Latin word for sour, acidus. Bases are substances that neutralize acids.
An acid is any molecule that can easily lose a hydrogen ion. A base is a molecule that accepts a hydrogen ion. Since a hydrogen ion is just a simple proton, acids are sometimes referred to as proton donors. Protons don’t exist in the free state in water – the protons from the acid combine with the water to make H3O+, called a hydronium ion. (There is a class of acids called Lewis acids where a pair of electrons are transferred instead of a proton, but we will be discussing the types of acids found in the kitchen, which all involve proton donors.)
Water itself is both an acid and a base. Any two molecules in water always have a small chance (one in ten million) of spontaneously converting into a hydronium ion and a hydroxide ion (OH-) when one of them donates a proton to the other.
Because a proton moves from one molecule to another, when an acid reacts with something, the result is generally two ions, one with a positive charge, and the other a negative charge. When the water is removed (say, by evaporation), the two ions combine to form a salt. The familiar table salt NaCl, forms when hydrochloric acid reacts with sodium hydroxide:
HCl + NaOH → H+ + Cl- + Na+ + OH- → H2O + NaCl
The hydrochloric acid (HCl) dissociates into a proton (H+) and a chloride ion (Cl-), and the sodium hydroxide dissociates into a sodium ion (Na+) and a hydroxide ion (OH-), and when the proton and the hydroxide ion combine and leave as water vapor, the resulting sodium and chlorine combine to form table salt.
Strong acids easily lose protons. Weak acids hold onto their protons a little better. Strong acids in water lose all of their protons to the water (making hydronium ions). Weak acids reach an equilibrium where some of the molecules lose protons, but others do not. Acids in food such as vinegar (acetic acid), soda water (carbonic acid) and lemon juice (citric acid) are weak acids.
Some acids can lose more than one proton. For example, carbonic acid can lose two protons, while citric and phosphoric acids can lose three.
Bases, called alkalis if an OH- is involved, accept protons. Alkalis do this by donating hydroxide ions. These can accept protons from acids to make water. Lye (sodium hydroxide) is a familiar strong base, and it is used to make soap, and to clean drains by making soap out of the grease that has clogged the drain. The soap then rinses away down the drain.
Ammonia is another familiar base, used for cleaning oils and grease from windows. Baking soda (sodium bicarbonate) is another base found in the kitchen, used because it reacts with acids to produce carbon dioxide gas.
To measure how acidic or basic a solution is, we look at how many hydronium ions and hydroxide ions there are in the solution. If there are one in ten million (as there is with pure water) we say the pH of the solution (think percent Hydronium as a memory device) is 7, because 107 is ten million.
Smaller numbers for pH indicate a more acid solution, and larger numbers indicate a more basic solution. Vinegar has a pH of about 2.4. Baking soda has a pH of about 9. Other examples are shown in the table below.
Effect of acid and heat on sugar
Sucrose (table sugar) is a disaccharide, meaning it is two simple sugars, glucose and fructose, that have reacted in such a way that they join together, losing a molecule of water in the process. A reaction that produces water is called a condensation reaction. The fructose acts like an acid, and donates a proton. The glucose acts like a base, and donates a hydroxide ion. The proton and the hydroxide ion combine to form water, and the two simple sugars combine to form sucrose.
The reverse reaction, called hydrolysis, is where a water molecule is added to a molecule to break it into two parts. Hydrolysis of sucrose in water happens very slowly all by itself. But if an acid is added, it acts like a catalyst, promoting a faster reaction, but not getting used up in the process. Heating up the solution makes the reaction go even faster.
The result of heating sucrose in water with a little lemon juice or vinegar in it is that much of the sucrose is converted into the two simple monosaccharides. Since fructose is a lot sweeter than sucrose, the result is a sweeter solution, even though glucose is not quite as sweet as sucrose. Since the acid is not used up, the solution is also a little tart, but that can be fixed by adding a weak base like egg whites or baking soda. If there are proteins in the solution, they can also react with the acid to neutralize it.
Effect of acid on proteins
We have seen the effects of acid on proteins when we made yogurt. The casein proteins in milk stay in solution because they have water loving protein strands on the outside of the micelles (clumps of protein), and calcium phosphate inside holding the micelles together. But at a pH of 4.6 (less acid than vinegar, but still quite sour), the calcium phosphate dissolves and the proteins denature, and the micelles clump together and form a gel.
While many cheeses are made with enzymes like rennet, which snip off the ends of the water loving strands in the micelles to get them to clump together, other cheeses are made using acid.
Cheeses like the Indian paneer, or the Italian ricotta, are made using acid (or acid and heat together) to coagulate the proteins in the milk. Since acid coagulates the whey proteins as well as the casein proteins, the resulting cheese does not melt when fried or baked, and the yield from a gallon of milk is higher (the whey proteins hold water better than casein proteins do). The resulting curd is not as firm as a rennet cheese, due to the different way the proteins bind together as they coagulate.
Cooking with acid
Acids are used to denature other proteins in cooking. Fish is ‘cooked’ in lime juice in traditional ceviche recipes, and eggs are ‘hard boiled’ by pickling in vinegar.
Most acid cooking is done with seafood, where the meat does not have as much of the tough collagen connective tissue as beef does. Acid cooking does not soften connective tissue in the same way heat does (by making gelatin out of it). But the acid does denature the proteins in the fish or shellfish, giving them a cooked mouth-feel and a cooked appearance. The acid also does not do as good a job of sterilizing the meat, so cleanliness and freshness are very important.
Acids are also widely used in marinades. Vinegar, tomato juice, citrus juices, and yogurt are often used to denature the outer proteins on the meat, so they open up and absorb the other flavor elements in the marinade. Marinades do not penetrate very far into the meat, so often the meat is pierced many times with a fork to get as much surface area exposed to the liquid as possible.
Cooking with alkali
As with acids, alkalis can be used to “cook” foods. However, if the food contains fats, the alkali turns them into soap, which people seem to object to eating. Something about the taste.
But since soap washes away easily (along with any water soluble vitamins and minerals), washing the result can make it palatable.
A notable use of alkali in cooking is the process known as nixtamalization. This wonderful word refers to the practice of cooking maize (the name for what we Americans call corn) in alkali. The name comes from the Aztec words nixtli (for ashes) and tamalli (for uncooked dough made from corn). The ashes from a cooking fire are alkaline, containing potassium hydroxide. In modern corn processing, calcium hydroxide (lime water) is used instead.
Boiling corn kernels in alkali softens the hard outer hull, and the starches absorb water and swell, then form a gel. Enzymes from the germ of the seed are released and act on the starches and proteins, improving the workability of the resulting dough.
The cell walls of plants are made of cellulose and pectin, which dissolve in hot alkali solutions. But the hot alkali also denatures the corn proteins, making them more available to human digestion.
Nutritionally, the corn is further improved (from the perspective of human digestion) by freeing the niacin that is otherwise bound to proteins in the corn. When the proteins are denatured, the niacin can be digested. Other animals can make their own niacin from the amino acid tryptophan, or they can digest the proteins that hold onto the niacin, and get niacin from corn that way. But humans have lost that ability, and need to cook the corn in alkali to free those nutrients.
Minerals in the lime water or in the ashes are also absorbed by the corn in the process of steeping it in the liquid after boiling. This increases the calcium content of the dough, along with increasing the iron, zinc, potassium, and copper.
There is fat (corn oil) in the kernels, and some of this turns to soap in the process. For this reason, the liquid remaining is usually discarded, and sometimes the dough goes through an additional rinse.
Sodium bicarbonate (baking soda) is probably the most familiar alkali used in cooking. It is actually the salt of a strong alkali and a weak acid, so that it acts as a weak alkali. The strong alkali is sodium hydroxide (lye), and the weak acid is carbonic acid (soda water). Combining the two together first creates a related compound, sodium carbonate (washing soda).
CO2 + 2NaOH → Na2CO3 + H2O
Combining sodium carbonate with more carbonic acid makes sodium bicarbonate.
Na2CO3 + CO2 + H2O → 2NaHCO3
The two molecules are similar. The bicarbonate has a hydrogen where the carbonate has a second sodium. Thus another name for the bicarbonate is sodium hydrogen carbonate.
Sodium bicarbonate is made by bubbling carbon dioxide gas into a solution of sodium carbonate. But the sodium carbonate used is not generally made using lye and soda water. It is either mined in the form of a mineral called trona (a mixture of sodium carbonate and sodium bicarbonate), or it is made by the Solvay process, which is more complicated (salt water, ammonia gas, and calcium carbonate are used in a multi-stage process that turns out to be cheaper than the simpler reaction).
Most of the uses of baking soda in the kitchen involve its alkaline nature, although it is also used as a mild abrasive in cleansers and toothpaste. It reacts with acids to release carbon dioxide for leavening baked goods, it reacts with acids in the stomach to relieve indigestion, and it reacts with acid molecules in the air to freshen refrigerators.
At 158° F (70° C) sodium bicarbonate breaks down into sodium carbonate, water vapor, and carbon dioxide gas, and thus can be used to make foamed candy by adding it to a very hot syrup. For the same reason, it can be used as a fire suppressor in fire extinguishers, or simply by dumping a box of it on a kitchen fire.
pH sensitive colors
Many colored molecules react with acids and bases in ways that change their colors. Chemists use this color change to indicate how acidic or basic a solution is. Litmus paper is one such indicator, but there are a large number of other indicators available.
A very common class of indicators is the group of pigments called anthocyanins (from the Greek roots for flower and blue). These molecules reflect red light in acid solutions, and blue light in basic solutions. Many of the colored fruits, leaves, and flowers we encounter owe their color to anthocyanins, and to the levels of acid in the plant. Examples are the red of apple skins and cherries, the red or purple of red cabbage leaves and eggplant skins, the blue of blueberries, the red of red wines, the purple of purple corn, the blues and reds of pansies and violets, and many more.
A common high school chemistry demonstration is making a broth from red cabbage, and showing how it turns red when vinegar is added, and blue when baking soda is added. The same effect can be seen when washing a glass that has had red wine or grape juice in it. Since most tap water is neutral to slightly alkaline, the red wine will turn blue when the water is added and the acid is diluted or neutralized.
Acidic foods taste sour, unless there is sugar along with the acid. We make lemonade by taking acidic fruit juice and adding sugar. Most lemons are actually no more acidic than orange juice. The orange just has more natural sugar than lemons do.
What this tells us is that the tongue and the brain give us different information about acid, depending on how sweet the food is. Acidic things that are not sweet are often not good things to eat. They make be spoiled food, polluted water, or some other thing we should not be putting in our mouth.
But fruits are often both sweet and sour at the same time. Our sense of taste has adapted to this situation, and we find the combination pleasant, an indicator of something good to eat. Sweet things have much needed energy, and not eating them because of their acidity is not a good survival mechanism.
Recipe: Lemonade with Chameleon Eggs