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Lactic Acid Fermentation: Using Fermentation to Make Food

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  • 0:07 Yogurt with a Side of Bacteria
  • 1:03 Lactic Acid Bacteria
  • 2:05 Lactic Acid Fermentation
  • 3:06 Lactic Acid Foods
  • 6:43 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

You eat food every day that is teeming with bacteria. It might surprise you to learn that many of them were intentionally put there. In this lesson, we will examine bacterial fermentation, required during the production of certain foods.

Yogurt with a Side of Bacteria

You have probably noticed a certain kind of commercial everywhere you look. I remember it starting with yogurt first, but now they can be found in all kinds of food. Probiotic bacteria are taking over the grocery stores, promising better digestion, improved immune function and a generally healthier you. There are probably some of you out there that are absolutely disgusted by the thought of eating bacteria. You cook food to kill the dangerous food-poisoning bacteria, so why would you intentionally eat yogurt packed with microbes?

Well, I hate to burst your bubble, but there were bacteria in your yogurt before the probiotics craze started - and not just in yogurt, but in many of the foods you might eat every day. In this lesson, we will examine a few foods that are dependent on bacteria and look at how the microbes contribute to the unique flavors and textures of these foods.

Lactic Acid Bacteria

When it comes to bacterial food production, flavor enhancement or preservation, the conversation has to start with the lactic acid bacteria. Lactic acid bacteria are a diverse group of bacteria that are able to ferment various sugars, producing lactic acid and other waste products. Lactic acid bacteria can be found in a wide range of environments, including decaying plant material, in association with animals and in unpasteurized milk products. In fact, lactic acid bacteria can be quite beneficial when they are found in the oral cavity, the intestinal tract or the vagina.

The lactic acid bacteria don't just produce acid; they produce a lot of acid - so much acid that it kills or inhibits the growth of other, potentially dangerous microbes that could make you sick. So if these species can protect you from microbial disease, can they also be used to protect foods from microbial spoilage? They can, and they have been for hundreds, if not thousands, of years.

Lactic Acid Fermentation

So, I mentioned that lactic acid bacteria ferment sugars and produce lactic acid during food production and preservation. Before we look at some of these foods, we should briefly touch on fermentation. Simply put, lactic acid fermentation is the anaerobic microbial breakdown of sugar, yielding energy in the form of ATP and releasing waste products, specifically lactic acid. The key point in this definition is that the process is anaerobic, or it must occur without any oxygen around.

Fermentation is not a very efficient way for organisms to obtain energy, so if a better way is available, like using oxygen to aid in the breakdown of sugars, it would use the oxygen instead of fermentation. Without fermentation, there is no lactic acid and the food production and preservation stops. So remember, we need lactic acid bacteria, a sugar nutrient source and a system with no oxygen for lactic acid fermentation to be successful.

Lactic Acid Foods

If you're a fan of Oktoberfest or always eat a traditional good luck dinner on New Year's Day, you're probably familiar with sauerkraut, which is the German word for cabbage. But have you ever thought about making your own sauerkraut? It can be an educational and delicious experiment. Sauerkraut is simply shredded cabbage fermented by lactic acid bacteria. All you need to make your own kraut is a ceramic crock, cabbage and salt.

Shred the cabbage, salt it and pack it tightly into the crock. It needs to be packed so tightly that any oxygen is quickly used up and you're left with an anaerobic system. Sound familiar? The crock remains anaerobic and the cabbage provides the sugar. What's missing? Lactic acid bacteria. That head of cabbage you bought at the grocery store is usually teaming with lactic acid bacteria, specifically Leuconostoc mesenteroides and Lactobacillus plantarum.

L. mesenteroides begins the fermentation and produces a lot of acid and other uniquely flavored waste products. Eventually, L. plantarum takes over the fermentation, dropping the pH so low that virtually no food spoilage microbes can survive. The kraut can become so acidic that as long as the batch remains anaerobic, it can remain edible and delicious almost indefinitely. Most home-fermented sauerkraut is made this way. Commercial kraut is often sterilized and engineered lactic acid bacterial cultures are added to ensure consistent quality and taste between batches.

We've already talked a little bit about yogurt and microbes. Yogurt is simply milk fermented by lactic acid bacteria. The two species most commonly used to make yogurt are Lactobacillus bulgaris and Streptococcus thermophilus. These lactic acid bacteria are commonly found on plant material. It is possible that yogurt originated when a cow's udder dragged across vegetation covered by these bacteria. Milking the cow resulted in a contaminated sample that quickly fermented into yogurt.

Specifically, the bacteria ferment lactose, which is the sugar found in milk, into lactic acid and other compounds with unique flavors. The lactic acid causes the milk proteins to clump together, giving the yogurt its characteristic thick texture. If you pulled out 1 gram of finished yogurt, you could find up to 100 million lactic acid bacteria, illustrating just how well these bacteria are able to thrive by fermenting just lactose.

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