Fermentation is the gift that keeps giving.

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Fermented foods have become the darlings of the foodie scene over recent years.

Beginning in the early 2000s, Noma started a trend for fermentation in fine dining restaurants that trickled down to us amateur cooks. Sometimes not always for the better; we can blame Noma for all the homemade kombucha we’ve been forced to drink.

For our ancestors, though, fermentation wasn’t a trend it was a matter of survival. They had to get through winter and fermentation was one of the methods they had to preserve food.

We don’t really need to preserve our food with fermentation anymore. We just go to the supermarket. But fermentation is still incredibly important, and recently trendy, because it doesn’t just preserve, it changes the character of food.

Often it makes food taste better. Often a lot better. If you’ve tasted raw cocoa beans you know the miracles that fermentation can perform.

Another very interesting way that fermentation can change food is by making it alcoholic. All alcoholic beverages begin with fermentation.

In fact, alcohol may have started our long history of fermentation. We have evidence for beer production dating back 13,000 years suggesting that a brew may have been our first fermented food.

There is some very fundamental biochemical science behind fermentation and, as our first fermented food, it seems appropriate to look at this science in the context of beer.

The Biochemistry of Fermentation: ATP and Respiration

Fermentation is simply a metabolic process that cells use to produce energy.

In fermented foods, the byproducts of microbial fermentation transform food, making them alcoholic or producing other compounds that make the food toxic to bacteria but still safe for human consumption.

To understand fermentation we are going to have talk about biochemistry. You have my apologies but I’ll keep it brief.

ATP and Cellular Energy

Fermentation is a part of a cellular process called respiration. The output of respiration is a molecule called adenosine triphosphate (ATP).

During respiration a molecule of adenosine diphosphate (ADP) and an organic molecule, glucose for example, are used to produce ATP.

The cell does this because ATP is used as a kind of energy currency. Cells use ATP in all sorts of biochemical reactions where it provides the energy to make those reactions possible. In the process it gets broken back down to ADP.

Respiration takes place in the mitochondria and a cell is in a constant cycle of importing ADP to the mitochondria, producing ATP and then exporting that ATP back to the rest of the cell.

Aerobic vs. Anaerobic Respiration

The reason we breath and transport oxygen to every cell in our body is that respiration works best in the presence of oxygen. This is why we also call breathing respiration.

When oxygen is present a cell will use a highly efficient set of chemical reactions that we collectively refer to as aerobic respiration.

Fermentation, on the other hand, is a different set of reactions that is used by cells in the absence of oxygen, it is a type of anaerobic respiration.

Fermentation is much less efficient than aerobic respiration, no organism lives on fermentation alone, but it does provides a fallback mechanism for cells to produce energy when there is little or no oxygen.

Although we normally associate fermentation with single celled organisms, like yeast and bacteria, it is also found in more complex organisms.

Human muscle cells, for example, can use fermentation chemistry when the blood stream cannot deliver enough oxygen for normal respiration. An ability that comes in useful when we are participating in strenuous activity.

Types of Fermentation: Lactic Acid vs. Ethanol

The interesting thing about fermentation is that there are different sets of reactions that you can call fermentation.

While all fermentation reactions produce ATP, different pathways can use different starting materials that yield different byproducts.

The figure below, apart from making your eyes bleed, shows the types of byproducts that can be produced during fermentation of glucose.

Types of fermentation are often referred to by their major byproducts and the two types that are important to foodies are lactic acid and ethanol fermentation.

Lactic Acid Fermentation

Bacterial lactic acid fermentation is used to produce things like yoghurt and sour cream, the bacteria using the lactose in milk as it’s starting material.

It’s also lactic acid fermentation that gives you sore muscles after strenuous exercise. The anaerobic fermentation of your muscle cells produces lactic acid from molecules of glucose.

Ethanol Fermentation Chemistry Explained

In ethanol fermentation glucose is converted to ATP. Importantly, though, it also produces ethanol and carbon dioxide as byproducts.

Ethanol fermentation is normally found in yeast, though some bacteria and molds can also produce ethanol during fermentation.

Beer and wine are famously produced with yeast using ethanol fermentation but another major food that utilises byproducts of ethanol fermentation is bread.

There is alcohol in bread, well before it is cooked anyway, the high heat of the oven burns off the alcohol, but the carbon dioxide, the other byproduct of ethanol fermentation, is what makes the dough rise and gives us our fluffy bread.

A Brief History of Beer Brewing

Okay, that’s a lot of chemistry. But now we know it we also know that we could pretty much throw some barley in a bucket of water and, after some time, get a fermented barley juice.

Much like when making a sourdough starter, natural yeasts in the environment would start fermenting free sugars found in the germinating barley and hey presto beer, or beerish, anyway.

It’s hard to say what was going on 13,000 years ago, but it’s likely this is how we got started making beer. I’m not sure how it would have tasted, but it clearly didn’t taste bad enough to deter our early ancestors.

By the time we get to 6000 BC humans in Mesopotamia were drinking beer and the Egyptians loved it. The Egyptians, in particular, elevated beer making into something of an art form.

Though it still wasn’t something we would recognise as a frosty brew now.

Beyond Fermented Barley Juice

Most of the beers we drink today were developed in Europe, which became the center of beer production after the Egyptians.

We know, from personal experience, that these beers are a lot more than just fermented barley juice.

What the Europeans did was to develop the art of beer making. Starting with the basic science of fermentation they extended a simple fermentation into a more complex process for making what we today recognise as beer.

Step-by-Step Science of Brewing Beer

Just like our ancestors 13,000 years ago, we still start with barley, or some other grain, and water.

The next step is to find some sugar. As we learnt above, yeast need glucose for ethanol fermentation. So, if we want our yeast to make ethanol we need to give it glucose.

We could just add some sugar, but barley is full of starch and starch is made of glucose (see the sugar and fries posts for more on starch). If we can break down some of this starch the yeast will have plenty of glucose.

Brewing Phase	Main Biological/Chemical Objective	Key Molecules or Enzymes Involved
Malting	Activate enzymes and begin breaking down barley cell walls	$\alpha$-amylase, $\beta$-amylase, $\beta$-glucanase
Kilning	Stop germination, develop color and flavor via Maillard reactions	Heat-sensitive enzymes, reducing sugars, amino acids
Mashing	Convert remaining starches into fermentable glucose (saccharification)	Amylase enzymes reacting with amylose/amylopectin
Boiling (Hops)	Inactivate enzymes, sterilize wort, and extract bitterness	Iso-$\alpha$-acids (from hops), phenolic compounds
Fermentation	Convert glucose to ethanol and carbon dioxide	Yeast (Saccharomyces cerevisiae), Pyruvate, ATP

Step 1: Saccharification and Starch Breakdown

Breaking down the starch in a grain before fermentation, a process called saccharification, is usually achieved using enzymes.

Historically, we’ve found these enzymes in a few different places.

In the east an enzyme secreted by Aspergillus oryzae, a mold that grows on cooked rice, is used to saccharify starches for fermentation. Usually for making sake, soy sauce and miso.

In Latin America women, following an ancient tradition, get starch-digesting enzymes from their saliva. To make a mildly alcoholic beverage called Chicha, the women chew ground corn and spit it into boiled corn to start saccharification.

Modern beer production, that needs something slightly more scalable, uses a process called malting that makes use of enzymes that are found in the barley itself.

Barley grains are germinated by soaking in cold water at around 18$\degree$C. As the new plants sprout, they start producing enzymes, such as alpha- and beta-amylase, that start converting some of the starch to glucose.

Step 2: Malting and Enzyme Activation

The precise amount of time, and starch breakdown achieved, is part of the art of beer making.

The more starch breakdown you allow the more alcoholic your final product will be because of the amount of fermentable glucose that is liberated.

It also affects the darkness of the beer as plenty of sugars means ample opportunity for Maillard reactions in subsequent steps (the same process that makes for an attractive sear on your steak).

The malt can also be held for a time between 60-80$\degree$C to maximise starch breakdown. This is the temperature range in which the starch-digesting enzymes operate most efficiently.

We’ve seen something similar when looking at the activity of pectin methylesterase when making french fries.

Step 3: Kilning

The malt is then put in a kiln and heated. Primarily this is to stop the germinating seeds from growing and consuming the sugars. But this stage is also an opportunity to influence some of the characteristics of the future beer.

The malt can be heated slowly to a low final temperature, somewhere between 50$\degree$C and 105$\degree$C, to preserve enzyme activity and keep colouring to a minimum. A technique used for beers like lagers and pilsners.

The malt can also be heated to a high temperature, 200$\degree$C or more, to kill all the enzymes and get a lot of colour from Maillard reactions. Malts for stouts and porters are treated this way.

A brewer can also use any temperature in between these extremes yielding a different flavour profile because of the variable amounts of Maillard reactions at a specific temperature.

Once kilned the malt can be kept for several months and for a specific beer multiple malts can be combined for the subsequent steps.

Step 4: Mashing and Wort Creation

Once the barley has been malted and kilned all that’s left to do is brew the beer.

To do this the malt is first dissolved in hot water to form a mixture called the ‘wort’ (a wonderfully medieval word) in a process known as mashing.

If they weren’t killed during kilning starch digesting enzymes will reactivate during mashing, if the temperature is right. So, this is an opportunity to further manipulate the relative levels of fermentable sugars and longer chain sugars that can add body to the final product.

Step 5: Boiling with Hops and Bitterness Extraction

After mashing hops, flowers from Humulus lupulus, are added to the wort and it is boiled. This inactivates any remaining enzymatic activity, pulls the bitter flavours out of the hops, kills any microbes present and concentrates and deepens the colour of the wort.

Step 6: Fermentation, Conditioning and Drinking

After boiling, the wort is cooled and yeast, or a mixture of different yeasts, is added and fermentation begins. The yeast breaking down the free glucose and producing ethanol and other flavours from the metabolic byproducts.

Finally, the beer is conditioned. This can involve many different processes designed to remove unwanted flavours, introduce carbonation, remove particulates and introduce additional flavours.

After this is done there is nothing left but the packaging and, the best bit, the drinking.

Conclusion

Anyone with even the slightest knowledge of brewing beer knows I have left an enormous amount out of this description. Brewing is a real art and I’ve merely brushed the surface.

But this isn’t a guide to brewing beer just a vehicle to introduce some of the chemistry of fermentation and a brief primer on the brewing process.

If you want to start brewing your own beer, which I may be forced to do if the Australian government doesn’t stop raising beer taxes, there are plenty of home brewing resources you can get your hands on.

Most importantly, in future posts I’ll look at other uses of fermentation in our food and that will be a lot easier now that we’ve got a good grip of the biochemistry behind fermentation.

So, after all that biochemistry, we probably all deserve a beer.