The Science of Lactic Acid Fermentation: Pickles, Kraut, Kimchi, and More

When fermentation experts Mara Jane King and Sandor Katz traveled to Sichuan to research industrial and home-style fermentation in the region, they had no idea where to find a local who was well-versed in at-home fermentations. They had no connections there, no guide, no personal acquaintance. Fortunately, the pair didn’t have to look very far. As they wandered the streets on that first day, Katz found some sausages curing on a front porch. While he took photographs, a woman, Mrs. Ding, emerged from her house. After a quick chat, she graciously invited them into her home. “And there she was, in her kitchen,” says King. “She’s got one pot fermenting with chiles; she’s got one pot fermenting with jujube and ginger; she’s got another pot fermenting with her own homemade doubanjiang. And it’s no big deal for her. It was just a part of her life.”

What do foods like kimchi, sauerkraut, umeboshi, yogurt, half-sour dill pickles, Mrs. Ding’s pickled chiles and jujubes, or even the sour blueberries at Noma have in common? They’re all sour, sure, but not oppressively so; they’re funky and pungent, but plainly delicious; and their existence hinges on a dynamic relationship between bacteria and the surrounding environment. These foods are fermented by lactic acid bacteria (LAB), or lacto-fermented. But what is LAB fermentation? How does it work? And why is it important to us?

The History of Lacto-Fermentation

Lactic acid fermentation—and fermentation in general—is not a fad. Despite what social media would have you believe, fermentation is not some new-school, fleeting chef technique reserved for restaurants with white tablecloths (or white chefs), $200 dinner tabs, or Michelin stars. Fermentation is as old as civilization, as expansive as the air we breathe. Broadly defined, fermentation is anaerobic metabolism: the conversion of nutrients to energy in the absence of oxygen. Central to this process are fungi, enzymes, and fermenting bacteria—all found in the ground, in the air, on plants, on us and in us—which are believed to have emerged from the primordial soup of Earth’s fledgling years, long before there was an atmosphere to support aerobic life.

It stands to reason that LAB fermentation is one of the oldest preservation methods out there. Archeologists trace the earliest instances of pickled foods as far back as 2400 BC in ancient Mesopotamia. Today, lactic acid fermentation is woven into the fabric of virtually every culinary tradition and culture. The list includes well-known pickled products like kimchi, sauerkraut, and dill pickles; several styles of hot sauces and chile pastes; cultured dairy products like yogurt, crème fraîche, and cheese; kombucha, salami, and, yes, even sourdough bread. As Sandor Katz writes in The Art of Fermentation, “I have searched—without success—for examples of cultures that do not incorporate any form of [lactic acid] fermentation. Indeed, ferments are central features of many, perhaps even most, cuisines.”

How and Why Does Lactic Acid Fermentation Work?

“Lactobacillus eats carbohydrates and poops out lactic acid,” writes Christina Ward. In truth, she’s not far off. More precisely and generally, lactic acid bacteria are acid- and salt-tolerant (or halo-tolerant) bacteria that digest simple carbohydrates to produce lactic acid, in addition to carbon dioxide, ethanol, and sometimes acetic acid. LAB can be found on decomposing plants, dairy products, on the skins of vegetables and fruits, and even on your own hands. All of this microbial fermentation is anaerobic, occurring in the absence of oxygen.

In a nutshell, LAB fermentation transforms sweet (simple carbohydrates) into sour (acid). Take some produce, chop it up, add salt or a brine, put it in a container, cover it, and wait a week or two. Over that time, acid levels increase and lower the food's pH, giving pickled foods their characteristic pucker; a lower pH also inhibits undesirable microbes that can cause spoilage, preserving the food and improving its shelf life. Let’s meet the players involved.

Meet the LAB Family

Lactic acid bacteria are classified under the taxonomic order Lactobacillales, which includes dozens of species of bacteria. That kind of diversity makes generalizations about the process difficult. For instance, two families within that order are relevant to pickling and preserving: Leuconostocaceae and Lactobacillaceae. Meanwhile, dairy and cheese production involves microbes in the Streptococcaceae family, of which bacteria in the Lactococcus genus represent the primary fermenting microbes. For this article, we'll just focus on pickling and preserving—LAB-fermented dairy like yogurt and cheese would require several articles of their own. Within the Leuconostocaceae and Lactobacillaceae families, Leuconostoc, Pediococcus, and Lactobacillus are the genera most often associated with LAB fermentation.

Before going in depth, it helps to understand that these bacteria fall under two groups: homofermentative and heterofermentative strains.

• Homofermentative (or homolactic) LAB only produce lactic acid. They prefer temperatures between 86 to 95°F (30–35°C), though they grow at lower temperatures as well. They produce flavors characterized by dairy, cream, or yogurt notes.
• Heterofermentative (or heterolactic) LAB produce lactic acid, but also acetic acid, ethanol, and even carbon dioxide, depending on conditions. These bacteria thrive at temperatures between 59 and 72°F (15–22°C), but can grow over a much wider range as well. They impart a sharper, more vinegar-like tang to foods, likely due to the extra production of acetic acid.

Leuconostoc

Bacteria in the Leuconostoc genus are usually spherically shaped. They are heterofermentative bacteria, able to produce ethanol, lactic acid, acetic acid, and carbon dioxide, among other metabolites. When it comes to LAB fermentation, L. mesenteroides is a common species involved in initiating fermentation. It thrives over a wider range of salt (and sugar) concentrations than any other lactic acid bacteria; it can tolerate temperatures from 39°F (3.9°C) to 86°F (30°C); and it can grow within a wide pH range (4.5-7.0, though it thrives at 5.5-6.5). Finally, L. mesenteroides is a facultative anaerobe: It can survive in the presence of oxygen, but it will only ferment in the absence of oxygen.

Pediococcus

Bacteria of this genus are spherical and homofermentative. They primarily produce lactic acid. They tolerate salt concentrations up to 8 percent (and higher in some cases), can grow over a similarly wide pH range (4.5-8.2, though they fare better than Leuconostoc as pH decreases), and survive temperatures from 60°F to 95°F. The most pertinent species to lactic acid fermentation is P. cerevisiae.

Lactobacillus

By far the most well-known genus involved in fermentation, Lactobacillus are generally rod-shaped, and include both heterofermentative and homofermentative species. The two most relevant species are L. brevis and L. plantarum—though L. fermentum, L. delbrueckii, and L. pentosus are common as well, depending on the fermented product. Generally, Lactobacillus is the most acid-tolerant genus among the Lactobacillales order. For example, L. plantarum has been shown to grow between pH 3.3 and 8.8, within a temperature range of 53.6°F (12°C) to 104°F (40°C). It tolerates salt concentrations as high as 18 percent in some cases.

The Life of a Dill Pickle

To best understand the intricate dance between these microbes, let’s look at the humble dill pickle. Fermentation occurs as soon as you harvest and cut open a cucumber. Leuconostoc, Pediococcus, and Lactobacillus species reside on the cucumber’s skin, averaging less than one percent of the plant’s total microbial population. After submerging cut cucumbers in a salt brine, you have effectively created an anaerobic, oxygen-free environment ideal for salt-tolerant, anaerobic microbes to grow. Salt inhibits growth of many microorganisms through osmotic shock, which draws water out of microbial cells via osmosis, effectively killing those microbes.

In the beginning stages, L. mesenteroides gets to work, initiating fermentation.* These bacteria metabolize sugars and nutrients in the cucumber to produce lactic acid, carbon dioxide, ethanol, and acetic acid. The pH lowers from 7.0 (neutral, like water) to 4.5 (sour, like soda water or coffee), the ideal range for this particular species. Within a few days, the mixture begins bubbling, indicating carbon dioxide formation. L. mesenteroides also produces a protein called bacteriocin, which further inhibits growth of unwanted microbes.

*In many cases, such as sauerkraut production, other microbes like Klebsiella and Enterobacter lower the pH initially, providing better conditions for L. mesenteroides to take hold.

As acids accumulate and pH lowers further (dipping below 4.5), more acid-tolerant bacteria take over. P. cerevisiae, L. brevis, and L. plantarum begin to proliferate, outcompeting L. mesenteroides. These three species lower the pH further still, until the system reaches as low as 3.3 (roughly the pH of orange juice). Over time, the population of P. cerevisiae may diminish, depending on pH and salt concentration. For example, in an 8% salt brine, P. cerevisiae activity ceases as the pH falls below 3.7. L. brevis and L. plantarum are left to complete the final stage of fermentation. In the end, the cucumbers have transformed into pickles, boasting a sour punch and attractively crisp texture. Most importantly, these pickles are shelf stable, with minimal risk of spoilage.

It should be noted that these microbial interactions are specific to cucumber fermentation. Other ferments, such as olives, are powered by a different pairing of microbes (L. plantarum and L. pentosus). For cheese, L. lactis and L. cremoris are the primary strains in a starter culture (though, as noted above, dairy fermentation is a vastly different lactic-acid fermentation process that deserves its own discussion).

Fermentation in Practice: Key Concepts

The techniques required for specific ferments vary widely between cultures and traditions. For instance, the traditional process for making kimchi is a far cry from that of Indian lime pickles. But when it comes to pickling, certain core concepts are constants.

Remove Air

For any successful LAB fermentation, it is essential to create an anaerobic environment. Little to no oxygen means microbes like yeast can't proliferate and potentially ruin the flavor of your ferment. Of course, there are several ways to remove air, depending on your comfort level and the equipment available.

The most straightforward method involves submerging a product in a salt brine. Anything below the surface of the brine is deprived of oxygen. A second method is to create a mash by blending or crushing a product (such as a hot pepper mash). In addition to greatly expanding the potential surface area exposed to microbes, making a mash will typically release liquid. When the mash is packed tightly in a fermentation vessel and covered with a weight, the liquid fills any gaps or pockets of air, effectively creating an oxygen-free environment.

Finally, there are higher-tech methods that involve physically removing oxygen, like an airlock. An airlock is any device that allows carbon dioxide to escape the vessel; at the same time, the device prevents any oxygen from entering. As fermentation proceeds, carbon dioxide accumulates—replacing any oxygen in the vessel—and escapes, ensuring an oxygen-free environment.

If you’re really keen on kitchen gadgetry (looking at you, sous-vide enthusiasts), or work in a restaurant, a vacuum sealer offers an ideal solution for removing oxygen. (Though it also creates an ideal solution for trapping the carbon dioxide that's created during fermentation—keep your eyes on it, because longer ferments can eventually lead to bags popping like overinflated balloons. To avoid this, open any overly puffed up vacuum bags and transfer their contents to new ones.)

Select Appropriate Ingredients

The success of a ferment depends on the quality of ingredients and how those ingredients have been treated. “If you’re making hot sauce, don’t use shit peppers,” warns Rich Shih, a fermentation expert based in Massachusetts. That means selecting produce that hasn’t been surface-treated with pesticides, coated in wax, or irradiated to extend its shelf life; organic produce tends to check these boxes, but an organic label isn't a guarantee of that. “I try to use ingredients that are very good quality,” agrees Mara Jane King. “They shouldn’t be covered in wax. Some chiles you might find have been covered in petroleum oil, or something to preserve them. A lot of people might run into issues with that and not even know.” All of these surface treatments deplete the population of available microbes essential for initiating fermentation.

If you’re using a brine, even the water you use is important. Mildly chlorinated, unfiltered water can inhibit microbial activity, so it’s best to use filtered or distilled water. At minimum, you should let your tap water sit out overnight in a fermenting container, uncovered, to let the chlorine evaporate before using it in a ferment.

Add Salt

Salt is helpful and, in most cases, essential to LAB fermentation. It inhibits the growth of undesirable microbes and provides conditions for salt-tolerant microbes to thrive. But what is the ideal salt concentration? For many applications, a salt concentration around 2 percent is sufficient. But in industrial settings, the concentration reaches as high as 10 percent (either as a brine concentration, or as a percent of the weight of the product); you often see such high salt concentrations with pepper fermentations for hot sauce, which can be left to age for months or years and then are eventually blended with vinegar, spices, and other seasonings to dilute the final salt level to something more palatable. Here are some general observations.

A low salt concentration:

• Does not suppress overall microbial activity as much, which facilitates faster fermentation.
• Is better suited to cooler temperatures, where microbial activity is slower and benefits from less suppression.
• Increases chances for spoilage, since harmful microbes are not as suppressed.
• Can compromise pectin cell wall strength, resulting in ferments that can be mushy, especially if fermented warm.

A high salt concentration:

• Significantly suppresses overall microbial activity, but minimizes risk of spoilage.
• Throttles fermentation, which can be helpful in warm conditions.
• Can ensure firmer, crisper products due to milder microbial degradation of pectin cell walls.

Ultimately, salt level is a matter of preference—highly dependent on the vegetable or fruit you’re using. But at a certain concentration, certain beneficial microbes such as P. cerevisiae just can’t ferment. Plus, a pickle at 10 percent salt concentration might taste aggressively salty.

Is there a lower limit for salt? Maybe not. Most fermenters would argue that some salt is integral to success. But in her travels to the mountainous regions of Southwest China, Mara Jane King witnessed a pickle fermented solely in discarded water from washing rice. “They pickle chiles and other vegetables in the rice water. It does not contain salt. It makes the whole thing a little risky and weird,” she says. But she also argues that rice water contains added starches and microbes to jumpstart fermentation; the pickles she tasted are evidence of sound technique.

Control the Temperature

Temperature has a profound effect on the rate and quality of LAB fermentation. In general, higher temperatures facilitate faster fermentation, while colder temperatures slow the process. At the same time, different temperatures favor differing populations of microbes.

In one experiment on kimchi conducted by the website Cook's Science, where I used to work (the article link no longer available), four identical samples fermented at four different temperatures (39°F, 50°F, 65°F, and 70°F) were analyzed for microbial composition. “The 39-degree sample contained almost entirely Leuconostoc species, while the 50-degree sample was a mixture of both Leuconostoc species and Lactobacillus species,” writes Anne Wolf. “The 65-degree and 70-degree batches were composed almost entirely of Lactobacillus species by the time they were analyzed.”

So what’s the ideal fermenting temperature? For most people—and most grandmas and grandpas—it’s room temperature—anywhere from 50°F to 70°F. In this range, a mixture of microbes thrives, tipping toward Lactobacillus at the high end. Low temperatures tend to favor sharper, more vinegar-like flavors due to the increased production of acetic acid; higher temperatures bring milder acidity and almost dairy-like notes. But again, choosing a temperature depends largely on what you're fermenting, how quickly you want your fermentation to proceed, and how broken down and soft you want your end product to be.

Timing and Monitoring

You’re in the home stretch. You’ve set up jars of plums, peppers, cucumbers, and cabbage—all destined to be pickles—and they’re bubbling away. How can you ensure a successful ferment? “Fermentation is all about maintaining, all about monitoring,” says Rich Shih. “You’ve invested time, energy, and money into this long process. You shouldn’t neglect it. You should care about it.” For Shih, that means checking your ferments daily and observing any differences from day to day. In some cases, it even means smelling and tasting the product as it matures. Mara Jane King recommends keeping a fermentation journal. “It’s a really simple thing to do, but it’s a really useful thing to have,” she says. “We would do it in the business to track every batch. But when things go wrong, or things go right, it’s nice to refer to the conditions, because conditions are always changing.”

When is a ferment done? From a food safety perspective, acidified foods must have a pH below 4.6, though many commercial producers look for a pH of 4.2 or lower. Technically, the bulk of lactic acid fermentation is complete within one to three weeks. At this stage, microbes have metabolized most of the sugars available in the ferment. So why do companies like Tabasco ferment their pepper mash for up to three years before processing and bottling? The answer lies in aging, a process by which more complex flavors develop as esters and other byproducts develop from acids over a much longer time scale. If you’ve ever had barrel-aged whisky, it’s the esters that impart fruity, spicy, or sweet notes.

In the end, the timing is up to you. If you prefer soft, broken-down sauerkraut, then ferment for longer. If you like your sauerkraut crisp and aggressively tangy, stop fermentation sooner.

Pickle Troubleshooting

Invariably, problems arise during fermentation. By far the most common issue is kahm yeast, an aerobic yeast that grows whenever a surface is exposed to oxygen. Kahm yeast presents as a thin, creamy white, opaque layer on the surface of a ferment. It tends to form in ferments exposed to open air, or when fermenting sweeter vegetables like carrots, beets, and peppers; it also forms more often in warm temperatures and in low salt concentrations.

Fortunately, kahm yeast is mostly harmless. In most cases of minor kahm growth, you can just mix the yeast right into the ferment and proceed. In other cases, the layer can grow quite thick, which can affect the flavor of your ferment, so scraping away or removing the layer is a better option.

To mitigate kahm yeast growth, it’s best to limit exposure to air as much as possible. Choose a narrow container, which minimizes the surface area exposed to oxygen. Use an airlock, if you can; if you can’t, try to cover the surface with plastic and weigh it down. Finally, some fermenters advocate sprinkling a layer of salt on the top, which severely throttles all microbial activity on the exposed surface.

After kahm yeast, the second most common problem is mold growth. Mold is a fuzzy fungus that can grow on the surface of a fermented food, and can be green, blue, black, even orange; it grows from mold spores present in the air and, in the presence of water, nutrients, and oxygen, it proliferates. Apart from its unsettling appearance, mold can be harmful if consumed. Some people are allergic to molds, and some molds produce mycotoxins, which can have severe acute and chronic health effects.

If you see mold growing on the surface of your ferment, you don’t always have to throw it out. You can still salvage your product. Sandor Katz recommends scraping off the top layer of mold as long as the growth isn’t extensive. According to this article, the filaments (or hyphae) that grow below the mold don’t appear to extend much below the top surface of the brine or mash, so scraping is often enough to remove it. Still, the best practice is to mitigate risk of mold growth from the jump. You can do that by ensuring an anaerobic environment, using fresher vegetables (which might have a lower concentration of mold spores), using the appropriate salt concentration, or fermenting in slightly cooler temperatures (not exceeding 70°F). But if you're ever in doubt, it's better not to risk your health. There’s no shame in starting over.

The Case for Slowing Down

Making a jar of lacto-fermented pickles channels an invisible world of microbes; it calls on processes that happen constantly both around us and in us. And just like sourdough baking, fermenting is subject to some measure of imperfection. You can try your best to provide optimal conditions for success, but you’re still at the mercy of chance—the particular collection of microbes living on the surface of a particular cucumber, at a particular moment in time and space, where conditions fluctuate. Probability, chance, chaos—these are the ideas that make fermentation wild. Each ferment can be unique in flavor and texture, right down to its final population of microbes. And we should embrace that uniqueness.

On the resurgence of fermentation in popular food media and dining in recent years, Mara Jane King is hopeful. “I feel like we’re waking up to it again,” she says. “We’re realizing that slow food is really real food, it’s human food. It’s cultural knowledge that is passed on from generation to generation. This is grandma’s food. People just know how to [ferment] food because it’s how we survive and be human and live in our cultural expression. Fermentation is really this amazing way that we communicate with our ancestors. We have the gift of these different techniques through all of the mistakes that people have made, and all of the experiments that people have done.”

For first time fermenters, King believes that the COVID-19 pandemic offers a unique opportunity. “It is really exciting to hear about all of these people in lockdown who are learning about sourdough for the first time or trying their hand at fermenting peppers for the first time. I feel like oftentimes we’re made to feel unimportant by the world that we live in today. But if we can connect with our process, if we can connect with our practice, and we connect with our mistakes, and then we find a way to share our successes as well—then that just puts us in the line of human history. And that makes us important again.”

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