Angela has taught college microbiology and anatomy & physiology, has a doctoral degree in microbiology, and has worked as a post-doctoral research scholar for Pittsburgh’s National Energy Technology Laboratory.
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.
So, we have sauerkraut with a side of yogurt, but I'm still hungry. Let's finish off our microbial meal with a big slice of sourdough bread. Not only will our bread contain lactic acid bacteria, but it will also contain yeast. So all we need to make sourdough bread is flour, water and microbes. The microbes, in this case Lactobacillus as our lactic acid bacteria and Saccharomyces or Candida as our yeast, are usually introduced by adding a starter culture from a sample of unbaked sourdough.
These starter cultures already contain microbes and are usually kept from spoiling by the presence of the lactic acid bacteria. You should already know what the Lactobacillus is doing: fermenting sugars while producing lactic acid and distinct flavors. What they can't do is make the bread rise. Enter the yeast, which metabolize the lactic acid and produce carbon dioxide gas to make the sourdough rise. Together, the microbes are responsible for the unique taste and texture of the sourdough bread.
It's time to review. Lactic acid bacteria are frequently needed to produce many of the foods you consume every day. Lactic acid bacteria are a diverse group of bacteria that are able to ferment various sugars, producing lactic acid and other waste products. The lactic acid and wastes can add flavors, be broken down to produce gases and even inhibit the growth of microbes that cause food to spoil. The lactic acid is produced by fermentation.
Lactic acid fermentation is the anaerobic microbial breakdown of sugar, yielding energy in the form of ATP and releasing various waste products, specifically lactic acid. In order for lactic acid fermentation to occur, you need a source of lactic acid bacteria, sugars and a completely anaerobic environment.
Some major foods that depend on lactic acid bacteria include sauerkraut, yogurt and sourdough bread. In each case, the bacteria ferment the sugars into lactic acid. The lactic acid bacteria provide flavor, and the acid prevents spoiling in sauerkraut. In yogurt, the lactic acid bacteria add flavor while the acid helps clump together the milk proteins, creating the unique texture of yogurt.
When making sourdough bread, the lactic acid bacteria again add flavor. This time, the lactic acid becomes a nutrient source for the yeast, which uses it to generate carbon dioxide. This gas is what makes the sourdough rise.
When you have finished this lesson, you should be able to:
- Define lactic acid fermentation
- Describe the benefits of lactic acid fermentation
- Identify foods which utilize this form of fermentation
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