This NBC Learn video, one in a 6-part "Cheeseburger Chemistry" series, examines the role of salty brine, fermentation, lactic acid and pH in the process of pickling food to preserve it.
The Chemistry of Pickles
AL ROKER, reporting:
It’s one measure of how much most Americans like pickles: they’re standard on burgers sold by all the big fast food chains. A few slices of dill pickle.
A dill pickle starts out like this: as a cucumber a common garden plant and salad ingredient, generally considered to be a vegetable although, like tomatoes and other plant parts with seeds, it’s technically a fruit. Anyway, like most fresh fruits and vegetables, cucumbers don’t have long lives: bacteria that naturally live in and on them cause them to rot in a few days, about 10 days if refrigerated.
JULIE YU (The Exploratorium): Pickling is the process of preserving these vegetables such that bacteria that might spoil them don’t grow on them.
ROKER: Preserving and preserve are words we get from the Latin servo, meaning to save or to keep. The solution to keep our cucumber from spoiling? It actually is a solution: a liquid solution.
KENT KIRSHENBAUM (New York University): When we talk about a pickle here in the United States, what we talk about is a piece of cucumber or a whole cucumber that’s been put in brine. What is brine? Brine is just salty water.
ROKER: Which, by the way, is why the ocean is sometimes referred to as the briny deep, although brine isn’t seawater. What makes brine salty is sodium chloride, or what we know as table salt, extracted and refined from sea salt or rock salt. Soaking foods from cabbage to herring in this salty solution to preserve them, has been common practice around the world for thousands of years, a process known here, at least since New York was New Amsterdam, as pickling. The word pickle is thought to come from the Dutch word pekel, meaning piquant or sharp-tasting. How does pickling work? Think of it as the Battle of the Bacteria, or microbes, same thing, over the cucumber.
KIRSHENBAUM: The process is kind of like a race between two armies of microbes. Some of those microbes we consider good -- they give rise to attractive flavor profiles. Other types of bacteria we would consider bad because they can either create poor flavors or in some cases they can even cause disease such as botulism or salmonella. So what we want to do is enhance the growth of the good bacteria and allow it to out-compete that bad bacteria.
ROKER: More specifically, we want a class of good bacteria, called Lactic Acid Bacteria, to colonize the cucumber and start to preserve it, before the bad bacteria invades and starts to rot it. Both sides use a form of natural chemical warfare, starting with fermentation. In fermentation, bacteria of all kinds, good and bad, eat and digest starches like the natural ones in cucumbers and other plants and convert them into sugars they use as food and fuel to grow and multiply. As soon as a cucumber is put into salty brine, the good microbe army has the edge.
YU: Each bacteria strain has its favorite environment to grow in. As a cucumber sits in a salty, briny, sour solution, lactic acid bacteria are able to grow.
ROKER: And as they grow, they consume even more plant sugars feeding themselves, while leaving less sugars for the bad army to eat. As Napoleon once said, an army marches on its stomach. The better-fed microbe army advances quickly; the food-deprived one weakens, and falls back.
But this is a two-front war. Back to fermentation. Just as we do after digesting food and turning it into fuel, bacteria produce by-products: gas, carbon dioxide or CO2, alcohols, or organic acids like lactic acid, which, not surprisingly, is what the good lactic acid bacteria produce and produce and produce, the more good bacteria grow.
And that, also not surprisingly, changes the levels of acid, the acidity, of the brine and what’s soaking in it, what’s called the pH.
KIRSHENBAUM: pH is just a way that we refer to the acidity of a solution: the measure of the concentration of hydrogen ions that are present in a solution.
ROKER: Just so you know, those ions of hydrogen are positively-charged, abbreviated as H+, and called hydrogen cations.
KIRSHENBAUM: As we acidify a solution we will increase the amount of hydrogen ions that are present, increase the acidity and create a sour flavor.
ROKER: Ok in case that went by too fast, the higher the concentrations of H+ ions in a substance, the higher the acidity level but the lower the pH. A bit of seeming reverse logic that has confused generations of science students, but not you, of course, after watching this video. Actually, the pH scale is a pretty good battle map, showing how our ‘good’ army prevails. Raw cucumbers have a pH of around 8 – a pH level where ‘bad’ bacteria thrive. Generally, the less acidic the pH, the easier it is for bad bacteria to grow. By growing rapidly and producing all that lactic acid, the good bacteria increases the acidity of the brine and what was soaking in it, which decreases the pH to below 4.6, a level acidic enough to pickle our pickle, and keep it preserved for months, even years. So, to recap: Cucumbers. Brine. Good vs. bad bacteria. Fermentation. Lactic acid. Higher acidity. Lower pH. Pickles!
Humans don’t rule the planet. Humans don’t even rule their own bodies. During the past 20 years or so, it’s become apparent that the guys in charge of everything are a nanometer across and run in packs, or perhaps more accurately, hang out in mobs.
Pickle, Dill, Cucumber, Preserve, Preservative, Bacteria, Microbe, Spoil, Spoilage, Brine, Salt Water, Sodium Chloride, Salt, Botulism, Salmonella, Lactic Acid, Fermentation, Plant Sugar, Starch, Carbon Dioxide, Acid, Acidity, pH, pH Level, Hydrogen, Ions, Cations, Kent Kirshenbaum, New York University, NYU, Julie Yu, The Exploratorium, San Francisco, National Science Foundation, Cheeseburger, Bun, Chemistry of Food, Food, Chemistry Now