Lactic Acid Fermentation


Lactic acid fermentation is really a simple affair on the surface. Human beings figured out long ago that if salt was added to vegetables and left to sit, their flavor would be enhanced and the vegetables preserved. That is the superficial view, but what’s happening on a microbial level is absolutely fascinating. Let’s dive straight into this magical microbial microcosm!

All fruits and vegetables are naturally covered in a world of microflora, a wide variety of microorganisms such as bacteria, yeast, and mold, all competing for energy. These microbes have evolved with the plants for millions of years, and they are there at the ready to break down the plants’ sugars upon the death of the plant. We can use these naturally occurring bacteria and yeasts to transform the host fruits or vegetables in a process referred to as wild fermentation. The skins of fruits are covered in wild yeasts, which, when the fruit’s skin is breached, will ferment the sugars in the fruit into primary alcohol, and eventually into vinegar (in the presence of a ubiquitous bacteria called Acetobacter). Vegetables are covered in lactic acid bacteria, which will ferment the vegetables’ sugars into lactic acid. Fruits and vegetables both have wild yeasts and bacteria on their skins to varying degrees, but yeasts typically dominate the fermentation of fruit, and bacteria typically dominate the fermentation of vegetables. These wild bacteria and yeasts play a catalyzing role in the fermentation process, and the fermenter’s ability to manipulate the environment in the crock to give the advantage to one organism or another greatly affects the outcome of any given ferment.

One of the most common and iconic vegetable ferments in the Western world is sauerkraut (fermented cabbage). Equally as ubiquitous in Korean culture and much of the East is the delicious and effervescent cabbage ferment known as kimchi. In fact, fermented cabbage was most likely the first lacto-fermented vegetable, and for good reason. Cabbage leaves are naturally covered in lactic acid bacteria, and are also full of sugars in the form of carbohydrates that are easily digested by bacteria. This makes cabbage a relatively easy vegetable to ferment, and a good entry point for beginners to the world of fermented food. Let’s examine the lacto-fermentation process through the fermentation of sauerkraut on a microbial level and find out what’s happening inside the crock.

Assorted jars of lacto-fermented vegetables and cabbages

Let’s say you’re making a batch of kraut. You shred a couple of heads of cabbage, massage them with salt at a measured rate, and pack the cabbage into a jar or crock, submerging it below the brine formed naturally by the liquid in the cabbage. Remember all those naturally present bacteria and yeasts? As soon as the cabbage is shredded, its sugars are exposed to the organisms present on the cabbage leaves, as well as the organisms present in the air, and they all begin to compete to eat those sugars at a rapid rate. With kraut, or any other lacto-fermented vegetable, you are trying to set the right circumstances for the lactic acid bacteria to thrive and outcompete the other microorganisms, allowing them to proliferate and acidify the ferment. The main way we do this is through the use of salt and temperature. By temporarily shifting the advantage to the halophilic (salt-tolerant) lactic acid bacteria, we are giving them an edge, just long enough for them to take hold and outcompete the other microorganisms. By fermenting at lower temperatures, we are also advancing the bacterial cause, as yeasts thrive in warmer conditions. Also get informed about understanding dehydrating.

Once the stage is set and the crock is closed, the lactic acid bacteria begin to work their magic. First, a species of bacteria called Leuconostoc mesenteroides asserts its influence and begins transforming the ferment by producing acids and lowering the pH of the ferment. This first phase, commonly referred to as the heterolactic or gaseous phase, is characterized by the dominance of this species of lactic acid bacteria. Leuconostoc is a heterofermentative bacterium, meaning it produces more than one by-product. As it digests the sugars from the cabbage, it produces mostly lactic acid and minor amounts of acetic acid (vinegar), carbon dioxide, and alcohol as by-products (there are also other by-products produced but these are the primary ones). As the Leuconostoc proliferates, it produces lactic acid in abundance and alters the environment of the kraut, making it more acidic and inhibiting the growth of other yeasts and bacteria. Leuconostoc are the flavor-building bacteria. You want these heterofermentative bacteria to dominate the early stages of the fermentation. Lower temperatures and lower salt content are both favorable to Leuconostoc, whereas higher temperatures and higher salt levels will favor the highly acid-tolerant Lactobacillus species.

This is an important distinction to note for home fermenters, because if you ferment at too high a temperature or with too much salt, Lactobacillus will dominate from the beginning, producing a singularly sour ferment. The other important note to mention for the home fermenter is that you can gauge what phase of fermentation your jars or crocks are undergoing based on whether gas (carbon dioxide, or CO2) is being produced. The production of gas assists in making the environment even more anaerobic, and thus favors the anaerobic organisms.

The Leuconostoc species are not as acid tolerant as other strains of lactic acid bacteria, so as the ferment gets more acidic, the Leuconostoc population dwindles. Much like the successive plants that colonize a disturbed forest, the initial bacterial species come in and alter the environment, making it more hospitable for the next successive wave of species to come in and occupy a different biological niche. After five to seven days, with the acidification under way, the microbial makeup of the ferment shifts and the acid-loving strains of bacteria, Lactobacillus brevis and Lactobacillus plantarum, begin to assert more influence over the ferment. We have now entered the second stage of fermentation, commonly referred to as the homolactic or nongaseous phase. These acid lovers are considered homofermentative bacteria, meaning they only produce one by-product: lactic acid (and lots of it!). They continue digesting the cabbage sugars and contributing to acidification, making it virtually impossible for any other organisms to live in the ferment. Other strains of Lactobacillus are commonly present during this stage, as well as other lactic acid bacteria such as Pediococcus. The acidification continues until the transformation is complete between twenty-one to twenty-eight days, producing a crunchy, tangy, and sour jar of goodness that will keep for years if refrigerated. At this point, if the fermentation is successful, the acidity will reach 1.6 percent and the pH will have dropped below 4.0, rendering the ferment acidic enough to be both well preserved and deemed “food-safe.” The last bacteria standing is always Lactobacillus plantarum, which is the only species able to live in such an acidic environment. This is typically when the ferment would be moved into refrigeration. If left to ferment further, the pH will continue to drop to between 3.4 to 3.6 or even lower, resulting in a very sour kraut.

Jars of pickled fermented vegetables on checkered cloth by window

There are a multitude of factors at play every time you make a batch of lacto-fermented kraut (or any other lacto-ferment, for that matter). Everything contributes to and determines the final outcome of the ferment. The most crucial—but also most controllable—factors are the amount of salt used, temperature of the fermentation space, and time allowed for fermentation. Other contributing factors include the freshness of ingredients, the temperature of the produce (you want produce as close to the fermentation temperature as possible), the season, the atmospheric conditions of the day, the atmospheric conditions in your kitchen, the growing practices of the farmer, the post-harvest handling, your handling of the produce, the cleanliness of your kitchen and utensils and fermentation vessels, how long it takes you to get the ferment salted and submerged under the brine after you’ve sliced it…and let’s not forget the microbes that you add to the ferment from your hands. Some people even take it as far as to ferment with the cycles of the moon! (We’ve made kraut on full moons, new moons, and everywhere in between, and have yet to notice a difference.) Another contributing factor is the cultural context from whence the ferment is made. Nearly every culture on the planet uses this ancient practice (or did at one time), but each place and people have their own methods, practices, and bacteria, or microbial terroir, all of which contribute to this global fabric of different flavors we call fermentation.

Colorful pickled vegetables in sealed jars at a market stall

As you can see, many factors make up the microbial culture of a crock of sauerkraut. By managing those factors over which we do have control, such as salt and temperature, we are giving our ferments the best possible chance for success. We encourage you to pay close attention to the produce you buy for your ferments—ask the producers at your local farmers’ market about their growing practices and when the produce was harvested (the fresher, the better). Hopefully the next time you pick up a jar of kraut, you will see deeper into the realm of the unseen little microbes responsible for making that sour and tangy ferment. You may also read about rehydrating.

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