Ahh, salt: responsible for the salvation of many a food (or is that salivation?). The oldest chemical in use, salt was used in prehistoric times and there are records of its use for drycuring hams in the third century BC by the Roman Cato the Elder. The use of sugar as a preservative wasn’t far behind; the Romans used honey to preserve foods as well. And another historical preservative is vinegar, used as an acidity regulator (sounds delicious when I put it that way, no?).
Chemical preservation has the fundamental purpose of preventing microbial growth. While there are plenty of other ways to preserve food, like smoking or drying, using chemicals doesn’t necessarily change flavors as much. Sausages, vinegar pickles, and fruit preserves all rely on chemicals to keep them safe for eating. Chemicals prevent microbial growth by either disrupting cells’ abilities to function, as nitrite does to sausage, or by changing any of the FAT TOM variables to be inhospitable, such as increasing acidity with vinegar or reducing moisture with sugar in fruit preserves.
Salt’s ability to kill pathogens and preserve things isn’t limited to foods. For an adult human, the lethal dose of table salt is about 80 grams—about the amount in the saltshaker on your typical restaurant table. Overdosing on salt is reportedly a really painful way to go, as your brain swells up and ruptures. Plus, it’s unlikely the emergency room physicians will correctly diagnose the cause before it’s too late.
While the chemistry of preservatives may not seem important to everyday cooking, it’s revealing to understand how these ingredients work, and the basics of preservation apply to how most other food additives work. First, a quick refresher on a few definitions that’ll pop up throughout this post:
Basic building block of matter. By definition, atoms have the same number of electrons and protons. Some atoms are stable in this arrangement (e.g., helium), making them less likely to form bonds with other compounds (which is why you don’t see any compounds made of helium). Other atoms (e.g., sodium) are extremely unstable and readily react. A sodium atom (Na) will react violently with water (don’t try licking a sample of pure sodium—it’d ignite due to the water on your tongue), but when an electron is removed it turns into a delicious salty sodium ion (Na+).
Two or more atoms bonded together. H = hydrogen atom; H2 = two hydrogen atoms, making it a molecule. When it’s two or more different atoms, it becomes a compound (e.g., H2O). Sucrose (a.k.a. sugar) is a compound with the composition C12H22O11—12 carbon, 22 hydrogen, and 11 oxygen atoms per molecule. Note that the composition doesn’t tell you what the arrangement of the atoms is, but that arrangement is part of what defines a molecule.
Any atom or molecule that’s charged—that is, where the numbers of electrons and protons aren’t equal. Because of the imbalance, ions can bond with other ions by transferring electrons to (or from) each other.
An atom or molecule that’s positively charged. Pronounced “cat-ion”—meow!—a cation is any atom or molecule that has more protons than electrons; it’s paw-sitively charged. For example, Na+ is a cation—an atom of sodium that has lost an electron, giving it more protons than electrons and thus a net positive charge. Ca2+ is a cation—a cation of calcium—that has lost two electrons.
An atom or molecule that’s negatively charged (i.e., one that has more electrons than protons). Cl– is an atomic anion—in this case an atom of chlorine that has gained an extra electron, giving it a net negative charge.
From these definitions, you’ll hopefully deduce that a lot of chemistry is about ions interacting with each other based on differences in electrical charges. Sodium chloride, common table salt, is a classic example: it’s an ionic compound composed of a cation and an anion. In solid form, though—the stuff in your salt shaker—salt is more complicated than one anion plus one cation. It takes the solid form of a crystal of atoms arranged in an alternating pattern (like a 3D checkerboard) based on charge: cation, anion, cation, anion. In water, the salt crystals dissolve and the individual ions are freed (disassociated). The anions and the cations separate out into individual ions, which can then react and form bonds with other atoms and molecules. That’s why salt is so amazing! Sucrose doesn’t do this. Sodium chloride is one particular type of salt, made up of sodium (a metal, and one that in its pure form happens to react violently when dropped in water) and chloride (chlorine with an extra electron, making it an anion). There are many other types of salts, created with different metals and anions, and they don’t always taste salty. Monosodium glutamate, for example, is a salt that tastes savory and boosts the sensation of other flavors. Epsom salt— magnesium sulfate—tastes bitter.
Multiple types of salts are used to preserve foods. Salmon gravlax is cured with a large amount of sodium chloride, which preserves the fish by increasing osmotic pressure, dehydrating and starving living microbial cells of critical water as well as creating an electrolytic imbalance that poisons them. Many sausages, hams, prosciutto, and corned beef are cured using small quantities of sodium nitrite, which also gives these foods a distinctive flavor and pinkish color. Unlike gravlax, in which the sodium does the preserving, sodium nitrite works because of nitrite; the sodium is merely an escort for the nitrite molecule. Nitrites inhibit bacterial growth by preventing cells from being able to transport an amino acid, meaning they can’t reproduce. (Incidentally, nitrites are also toxic to us at high levels, for presumably the same reason; but without the nitrites, microbial growth would be toxic to us too—dosage matters!)
Sugar can also be used as a preservative. It works like sodium chloride, by changing the osmotic pressure of the environment (see page 386 for more on osmosis in food). With less available water, sugary foods such as candies and jams don’t require refrigeration to prevent bacterial spoilage. Think back to the M in the FAT TOM rule: bacteria need moisture for growth, and adding sugar reduces their ability to drink.
Sugar’s osmotic properties can be used for more than just preserving food. Researchers in the UK have found that sugar can be used as a dressing for wounds, essentially as a cheap bactericidal. The researchers used sugar (sterilized, please), polyethylene glycol, and hydrogen peroxide (0.15% ﬁnal concentration) to make a paste with high osmotic pressure and low water activity, creating something that dries out the wound while preventing bacteria from being able to grow. Whoever thought of rubbing salt in a wound should’ve tried sugar!
Besides salts and sugar starving microbes of vital water, enzymatic inhibitors and acids are used to prevent their growth. Benzoate is one of the most commonly used modern preservatives, often used in breads to prevent mold growth. (Fans of The Simpsons may recall potassium benzoate as part of the curse of frogurt—see cookingforgeeks.com/book/frogurt/.) Like nitrite, benzoate interferes with a cell’s ability to function (in the case of bread, by decreasing fungi’s ability to convert glucose to adenosine triphosphate, thus cutting off the energy supply).
Compounds that lower a food’s pH also preserve the food, and are so critical that acidity regulators get an entire section in the E numbers list. Many of these compounds don’t have uses interesting to the home cook, who already has citric acid (thanks, lemon juice!) and acetic acid (from vinegar) on hand. For industry, the other acidity regulators give a wider range of flavoring options and functional properties, but for home use, there isn’t much re-purposing to be explored beyond a few baking tricks like using a pinch of vitamin C (ascorbic acid) to give yeast a boost during fermentation.
Check back next week for my recipe for Salmon Gravlax.