Fermentation is responsible for many of our favorite foods. You probably know fermentation helps to make delicious products such as beer and cheese, but did you know soy sauce, pepperoni, sauerkraut, yogurt, bread, and even chocolate are also produced through fermentation? Read on to learn more about the important (and friendly) microorganisms inside our food.
What is fermentation?
To put it simply, fermentation is a bioprocess that utilizes microorganisms such as bacteria and fungi (yeasts and molds) to produce either alcohol or acid from carbohydrate sources. These metabolites preserve products and create unique flavors, textures and enhanced nutrition. Fermentation generally occurs in an anaerobic environment, meaning no oxygen is present.
To better understand fermentation, take a look at this video by TedED.
Fermentation may begin when naturally-occurring microbes (found in the air or on the food, itself) begin to break down food over time. You can actually try this at home by leaving out a glass of wine overnight. In the morning, you will find sour wine vinegar! This is likely how people in ancient times determined fermentation could help preserve their food. This is known as natural fermentation.
Today, we often use a controlled fermentation process to ensure consistency. Most bakers, brewers, or wine-makers have proprietary strains of yeast unique to their product. Yogurt makers also have specific bacteria strains they rely on.
Because natural microbiota can vary, manufacturers want to take control. They ensure consistency by controlling the exact type and amount of microbes used. They can also add different microorganisms at different times to produce unique flavors and textures. This helps to continually produce foods that customers know and love.
Why ferment food?
Without a refrigerator, how do you keep food from spoiling over long periods of time?
Yes, you guessed it: fermentation! Think about it, wine lasts much longer than grapes, pepperoni longer than fresh meat, and cheese much longer than milk.
Fermentation preserves many foods, including meats, fruits and vegetables. It also destroys natural toxins in food, thus keeping the product safe for us to eat for a longer period of time.
Did you know? Pasteurization was first used to remove spoilage bacteria from wine. Louis Pasteur discovered the microbes present in wine in the late 19th century and found that heat could kill unwanted bacteria. His methods were later applied to milk, for which it is now famous. Today, many beverages, including juices, are pasteurized to keep us safe from pathogenic bacteria.
Fermentation also adds value to food. The unique flavors and textures created in the process are highly prized by many consumers. Fermentation also creates carbonation, which people love to experience in champagne and beer. Finally, it increases nutrient bioavailability and digestibility of foods like legumes.
Microbiology of fermentation
Yeasts, such as Saccharomyces cerevisiae, are single-celled fungi. Yeast is used in products such as bread, wine, beer, vinegar, cocoa, and coffee. Yeasts are generally more hearty than bacteria and can survive in harsh environments.
Bacteria, such as Lactobacillus bulgaricus and Streptococcus thermophilus, are known as lactic acid bacteria, or LAB. They use lactose, the sugar commonly found in milk, to complete their fermentation duties and produce lactic acid. Another type of bacteria, Acetobacter spp., can produce acetic acid from alcohol. You may know acetic acid more commonly as vinegar. These acids are sour! They provide the characteristic tastes we know and love in items such as yogurt, cheese, vinegar, and sauerkraut.
There are two types of bacteria that ferment foods and each are used for a specific purpose. Homofermentative bacteria produce one type of catabolic reaction product, lactic acid. These are typically used in “starter cultures” and do the bulk of the fermentation. Heterofermentative bacteria produce multiple byproducts, such as carbon dioxide, ethanol (alcohol), aldehydes, lactic acid, and acetic acid. These bacteria are often used to contribute flavor and texture.
Believe it or not, molds are also used in food fermentation. Like yeast, molds are a fungus and are more adaptable than bacteria. You can see the characteristic hyphae in the photo below, which are used to feed and grow.
Aspergillus oryzae is a mold used in the production of soy sauce. This mold produces enzymes that break down carbohydrates and protein for later fermentation by bacteria and yeast.
Penicillium roqueforti is another type of mold used to ripen blue cheese. This mold is inoculated in the milk at the beginning of the process. Later, holes are cut into the cheese to allow oxygen into the center. Penicillium camemberti is used to make soft cheeses such as brie and camembert, which are surface-ripened cheeses.
Biochemistry of fermentation
Unless you have experience in biology or biochemistry, you may not be familiar with how cells actually go about producing the energy they need to survive.
When oxygen is present, microorganisms generally use a process called cellular respiration to survive. This process generates plenty of energy, known as ATP, that cells need to live and reproduce. The oxygen is important because it accepts electrons from the electron transport chain, ensuring the process is efficient and plenty of energy is produced.
However, when no oxygen is available the cells must get creative in ways to produce energy. This is where things get interesting.
In an anaerobic environment (without air), microorganisms must use different pathways to use the available energy sources. These different pathways result in desirable byproducts such as alcohol, acid, or flavor compounds. Because there is no oxygen available (to act as an electron acceptor in the electron transport chain), less energy is produced for the microorganism. Sugar is a key ingredient in fermentation reactions because it can act as an electron acceptor.
In yeast, this is process is known as alcoholic fermentation. See the diagram below for more info.
The resulting ethanol (alcohol) can even be further fermented to acetic acid (vinegar) by Acetobacter spp. or Gluconobacter bacteria.
Bacteria typically ferment in a process known as lactic acid fermentation. Lactic acid bacteria ferment lactose (milk sugar), resulting in the production of pyruvic acid and eventually lactic acid, as the energy cycle is completed. This acid is responsible for the tangy flavors in many fermented products.
Molds use a traditional aerobic pathway for growth. They typically utilize a wider range of energy sources, including starch, proteins, and lipids. Some types of mold may produce lactic acid or alcohol.
Molds undergo more of a “controlled spoilage” process as they break down fats and proteins to create new flavors, aromas, and textures. In fact, they are often responsible for the “stinky” cheeses you may love (or hate)!
If you’re still curious, check out this more technical video explaining the chemistry and energy cycles.
How does fermentation preserve food?
The main reasons fermentation increases product shelf-life is that it increases acid or alcohol content and decreases water activity. All microorganisms have basic requirements for life. They cannot tolerate harsh environments such as those with low pH, high alcohol content, or high salinity. When lactic acid bacteria produce lactic acid, for example, the pH is lowered, thus killing other microorganisms.
Did you know? This fluctuation in pH can also be used to the advantage of fermentation products like cheese. Cheesemakers may use a combination of different bacteria at different points in the production process, when pH is changed, to produce different flavors. These subsequent additions of bacteria are known as secondary microbiota.
Often, sugar and salt are added to fermented foods during processing. This also aids preservation by binding the available water microorganisms would need to survive.
Today, consumers in the US are moving beyond traditional fermented choices and turning to a wider variety of options. Kefir, kombucha, and kimche have recently become well-loved. Craft beer, wine, and distilling remain popular and are even becoming integrated into our food.
As consumers take more of an interest in their food and even experiment at home, we are likely to continue to see new innovations.
Have a question about how a specific fermented product is produced? Let me know below!
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Montville, TJ, Matthews, KR, and Kniel, KE. 2012. Food Microbiology: An Introduction Third Edition. ASM Press. doi: 10.1128/9781555817206.ch19