Metabolism 101: Food and Anaerobic Respiration

I’m not sure how humans began using microbes (bacteria and fungi) in food production but it was well before we even knew what microbes are. Nevertheless, microbes are useful because they can undergo a process called fermentation. This process is similar to human anaerobic respiration but produces different end-products.

See, microbes are different from us and can actually thrive in anaerobic or oxygen lacking environments. Bacteria are quite simple and unlike our cells lack key components necessary for aerobic respiration. Therefore, they undergo fermentation which results in the production of byproducts such as ethanol (alcohol), carbon dioxide, or lactic acid. Although simply byproducts for the bacteria, humans taken advantage of them to make food.

So what are some of these commonly used microbes?

Saccharomyces cereviceae is a common strain of yeast. These little guys have been used everywhere from my old lab to wine, beer, and bread making. They convert sugars present in foods into ethanol and carbon dioxide. These sugars range from maltose in barley for beer, glucose and fructose in grapes for wine, or starch in wheat for bread. The alcohol gives beverages its distinct properties while the carbon dioxide helps develop taste and fizziness.  In terms of bread, the carbon dioxide makes the dough rise while ethanol evaporates during the baking process.

In the milk industry the well known probioticsLactobacillus bulgaricus and Streprococcus thermophilus, are commonly used. These little guys convert the milk sugar, lactose, into lactic acid, thus giving the final product that acidic taste. At the same time the acidity will alter the milk protein structure which gives yogurt its thick texture. The beauty of Lactobacillus is that it’s not necessary to use milk in order to culture them. Anything that’s got a sugar will do the job! That is the reason why you can make all of these non-dairy yogurts. All the benefits of probiotics without all the nasty saturated fat 😉

Lastly I wanted to mentioned kombucha which is fermented tea. In this case the fermentation involves SCOBY or symbiotic culture of bacteria and yeast. The exact composition of the culture varies but generally consists of Acetobacter sp. along with various yeasts such as Bretanomyces sp. and Saccharomyces sp. All in all, the sugar in the tea will feed the culture which will give off useful bi-products such as a bit of alcohol, acetic acid and gluconic acid. The gluconic acid is key for kombucha and is believed (although not well researched) to have liver cleansing properties.

La voila! Here are some of the common microbes we use in our food.

Metabolism 101: Fate of Lactic Acid

Recall the last time you’ve had an all out sprint. Remember that build up of pain in your legs that made you stop moving but then subsided? Well that’s lactic acid for you! As I’ve explained in the previous post, when we perform high intensity exercise we do not have enough oxygen to drive aerobic respiration. Therefore, our bodies are forced to switch to the inefficient process of anaerobic respiration and in turn lactic acid production.

That darn lactic acid! Well not quite… Lactic acid is more than just a waste product and isn’t necessarily bad. You must recall that shortly after high intensity exercise the lactate-associated pain subsides. Why? Our bodies have designed mechanisms to remove lactic acid from the muscle and actually convert it back into energy. How this occurs depends on the location of the lactic acid- actively contracting muscle, relaxed muscles, or the liver.

When oxygen becomes available, lactic acid within actively contracting muscles can be further oxidized to enter the Kreb’s Cycle and yield energy. After exercise, the lactic acid remaining in that muscle can be resynthesized into glycogen.

Alternatively, lactate can leave contracting muscles and enter the bloodstream through something called the monocarboxylate transport (MCT) protein. Using the bloodstream, lactic acid can travel to other muscles in the body or the liver.

If you think of biking, your leg muscles will be contracting while the arm muscles will be relatively relaxed. Therefore, lactic acid accumulated in the legs can travel to the arms where it can also be used for energy or synthesized into glycogen.

The liver though is where all of the magic happens. Here lactate can be converted back into glucose through a pathway called “The Cori Cycle”. This mechanism is somewhat wasteful however it does relieve muscles from lactic acid accumulation and pain. That can be quite useful if you’re being chased and need to run for as long as possible.

The Cori Cycle is the exact opposite of anaerobic respiration and involves lactic acid being turned into pyruvate and then into glucose through gluconeogenesis. I shall not bother you with details but it is important to note that the cycle consumes 6 molecules of ATP while anaerobic respiration only generates 2 ATP! Wasteful indeed! However, it’s still useful to make glucose as it has potential to yield much more ATP through aerobic respiration.

Et la voila! That is how lactic acid is removed from our bodies. Stay tuned on how we take advantage of anaerobic respiration in food production 😉 Yes… I’m talking beer 😛