12 12 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.
Water itself is both an acid and a base. Any two molecules in water always have a small chance—one in 10 million—of spontaneously converting into a hydronium ion (H+) 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 with 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:
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-). 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 in which some of the molecules lose protons but others do not. Acids used 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. 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 dissolves and rinses away.
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, look at how many hydronium ions and hydroxide ions there are in the solution. If there are one in 10 million (as in pure water), we say the pH of the solution (think “percent Hydronium” as a memory device) is 7, because 107 is 10 million.
Smaller numbers for pH indicate a more acidic 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.
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, occurs when 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, such as egg whites or baking soda. If there are proteins in the solution, they can also react with the acid to neutralize it.
You have seen the effects of acid on proteins in making yogurt (page 141). 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, causing the micelles to 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 such as 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.
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 mouthfeel 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.
As with acids, alkalis can be used to “cook” foods. However, if the food contains fats, the alkali turns them into soap, which people often 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. The starches absorb water and swell, and 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 we 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 content.
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).
Combining sodium carbonate with more carbonic acid makes sodium bicarbonate.
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 carbonate
Sodium bicarbonate
Sodium bicarbonate is made by bubbling carbon dioxide gas into a solution of sodium carbonate. But the sodium carbon ate 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 (saltwater, ammonia gas, and calcium carbonate are used in a multistage 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.
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 you 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 then 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. Lemonade is made 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 to eat. They may 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 contain much needed energy, and not eating them because of their acidity is not a good survival mechanism.
RecipeA popular topic in cooking these days is a neat trick for making little balls of gel that look like caviar or salmon eggs. They are made out of a gelling agent called sodium alginate. This compound is extracted from kelp, and when added to water it makes a thick syrup. When calcium ions are added to this syrup, they exchange with the sodium, and a tough gel forms. Using an eyedropper or a pipette to let the syrup fall into a solution of a calcium salt a drop at a time makes wonderful little balls of gel that look like fish eggs or frog eggs.
Now take that trick a step further and make these little eggs slowly change color from blue to red when they are stirred into a glass of lemonade. You can do this by using the anthocyanin pigments from the skins of red grapes as a pH indicator. Start the eggs out at a neutral to slightly alkaline pH, and let the acid in the lemonade bring the acidity up until the anthocyanins turn from blue to red.
You can buy sodium alginate at health food stores for about $4 for a 2-ounce jar, which is six times as much as this recipe needs. Or you can buy it on the Internet for about $40 a pound; it will keep for years. I use a tablespoon of powdered sodium alginate, which is about 9 grams (a third of an ounce).
Ingredients:
If the grape juice is frozen, let it thaw until it is liquid. You want to make sure the juice has not had extra acid added to it. If you can’t find concentrate, you can use regular grape juice. The result will still be good, but it will not burst on the tongue in quite the same way.
Next, mix the sodium alginate and the sugar together. This will make it a lot easier to dissolve the sodium alginate in the liquid. Since the alginate will start to gel as soon as it gets wet, it will form clumps and lumps that will take a lot of stirring and mashing to dissolve. The sugar helps by letting the liquid get into the mass of powder.
Now you want to neutralize the acid in the grape juice, without going so far that you can taste the baking soda. The alginate will not dissolve in an acid liquid, and you want the grape juice to be quite blue in color, as it gets when it has a neutral pH. Add the baking soda to the grape juice 1/8 teaspoon at a time. Stir until all the bubbles have disappeared before adding the next 1/8 teaspoon. At some point you will notice that fewer bubbles came up from that last bit of baking soda. This is the time to stop. You don’t need to go all the way to neutral. Be patient. Waiting for the bubbles to go away can take a while. Don’t be tempted to use the antacid tablets instead of the baking soda—they will cause the alginate to solidify immediately.
Put the alginate powder and sugar in a jar, pour the liquid grape concentrate on top, and begin to stir. Use a jar so that once the main lumps have been broken up with the spoon, you can put a lid on it and shake it. Even with all this trouble, you will have to let the mixture sit for an hour or two to let the gel fully hydrate. Overnight is even better. Plan ahead; do this part the day before you want to impress your friends.
When the powder has all dissolved into a somewhat thick syrup, dissolve the calcium chloride and crushed antacid tablets in a large glass of water (12 to 16 ounces). Stir well, and leave the solution swirling for the next step.
With the eyedropper or pipette, suck up some of the grape syrup and hold it above the antacid solution so that single drops can be squeezed out to fall into the antacid. But don’t let the eyedropper touch the antacid, or the syrup will begin to gel in the eyedropper and clog it up.
The drops might be red or purple as they form spheres in the antacid solution. Leave them in the solution until they turn blue. Then you can remove them with a slotted spoon, or pour the solution through a strainer into another glass. The chameleon eggs will remain in the strainer.
You will want a teaspoon of eggs for each glass of lemonade. This will take a few minutes to make using the eyedropper. If you are planning a big party, you may want to purchase a portable showerhead with a hose at a hardware store and make a lot of drops at a time.
Serve the eggs in a spoon next to the glass of lemonade. Your guests can then stir them into the lemonade and watch them slowly turn red during the meal.
Putting them in a lemon-lime soda also works—not only are they easier to see, but the carbonation makes the eggs float up and down as the bubbles first attach to the egg and make it rise, and then pop when it reaches the top, letting them fall back down, like little submarines.
Chameleon eggs will keep for a long time in a jar in the refrigerator.
If you want a quicker treat, without the color change, try adding alginate to strawberry syrup and dropping that a drop at a time into a calcium chloride solution. You get pink caviar that tastes like strawberry syrup. Serve it on a spoon, or spread it on crackers with cream cheese.