Hello everyone! Today I’ll be sharing about fermentation.

So what is fermentation? By definition, fermentation is a metabolic process that causes chemical changes of substrates and is carried out by microorganisms such as yeast and bacteria. Fermentation could be used to transform food, imparting complex flavors into food that may originally be plain and dull. It also helps to extend the shelf-life of these foods.

I’ve never really noticed this but fermented food are actually more commonly found in our daily lives than I originally thought! Prior to reading this chapter, when we mention fermented food, kimchi is the only dish that pops up in my mind. Other than that, there aren’t many other dishes which I would correlate the term “fermented” to right off the bat. But thanks to the workshop and readings, if someone were to ever ask me right now to name some fermented food (for whatever reason), I’ll definitely have a longer list of answers to give.

Some fermented food include: kimchi, miso, bread, yogurt, cheese, ketchup, soy sauce, vinegar, beer, wine, kombucha and even nata de coco! Never would I have thought that nata de coco is something that has been fermented. The word “fermented” makes me think of something sour, acidic and pungent – everything that nata de coco is not. How is that even possible?

Workshop

In week 4, we attended a Kombucha fermentation workshop held by Mr Ding Jie. Prior to this workshop, I have actually never heard of Kombucha, much less tasted it before.  If you’ve also been living under a rock like me and have never heard of Kombucha, it is actually fermented tea, and fermentation is carried out on tea (the substrate) by a symbiotic culture of bacteria and yeast (also known as SCOBY). The culture of bacteria consists of lactic acid bacteria (LAB) and acetic acid bacteria (AAB). LAB is responsible for the conversion of sugars into lactic acid, AAB is responsible for the conversion of sugars into acetic acid & cellulose while yeast is responsible for the conversion of sugars into ethanol. After fermentation, the sweet tea actually becomes slightly acidic, alcoholic (about 0.5%) and carbonated. There would be a visible layer sitting atop the liquid and that layer is actually called cellulose pellicle. Nata de coco is actually the cellulose pellicle of coconut water after fermentation!

The taste of the final Kombucha fermented can be affected by many factors, either intrinsic or extrinsic. Some intrinsic factors include the acidity and nutrient content whereas some extrinsic factors include temperature and gaseous exchange. A brief explanation on how each factor may affect the taste of the Kombucha:

Acidity of the culture would affect the fermentation process of Kombucha because the functioning of the yeast is pH dependent. If the pH is too extreme, it may end up deactivating or killing the yeast and fermentation would not occur.

Nutrient content refers to the amount of nutrients present in the culture for the microbes to feed on. Carbon and nitrogen sources such as carbohydrates serve as energy sources, which the microbes require to feed on and grow. If insufficient nutrients are available, the microbes may also be deactivated or killed.

Temperature affects the rate of Kombucha fermentation just like how it affects all other chemical reactions. The rate of fermentation actually follows the Arrhenius equation where every 10 Degrees Celsius increase in temperature leads to doubling of the rate of reaction. There is no rule that dictates how fast or slow the fermentation has to be. The decision on whether to store the tea at high or low temperatures should be based on how we want the final product to taste. Drawing a comparison to the fermentation of wine, winemakers prefer to ferment their wine at lower temperature since lower temperatures extends the duration of fermentation and allows more time for more aromatic molecules to accumulate. Thus, it may be the same for Kombucha.

Gaseous exchange refers to the flow of oxygen in and out of the jar containing the fermented food. For Kombucha, aerobic fermentation occurs first for about a week before it switches to anaerobic fermentation. The tea gets exposed to oxygen for the first week before it gets sealed in a tight jar where oxygen is not allowed to enter.

After making sure that we understand the basis of Kombucha fermentation, Mr DJ handed each of us a jar and a glass bottle, both filled with brown liquid.

 

The jar on the left contains Kombucha starter culture which is 5 days old. It smells sour, like apple cider. The glass bottle on the right is the final product and is ready for consumption (after addition of sugar).

Using the starter culture, we were given the chance to ferment our own batch of Kombucha! All we have to do is inoculate the starter culture, which is to add some brewed (and cooled) sweet tea to it and then let it ferment for a week under aerobic conditions.

A few important points to note:

  • Kombucha is a very “live” drink as it is largely dependent on the microbes present in the mixture. Kombucha prepared by different people would taste different, even different batches prepared by the same person may taste different!
  • Brewed tea needs to be cooled first before addition to the starter culture or else the high heat may kill the scoby. Lukewarm tea is also not recommended since lukewarm temperatures cause the mixture to be prone to contamination
  • To allow aerobic fermentation to occur, the jar should be covered with a breathable material such as a kitchen towel. This prevents unwanted material/microorganisms to enter the jar while allowing oxygen to flow in and out.
  • Sugar added to the fermented tea allows carbonation to occur, causingthe Kombucha to turn fizzy.

However, being the forgetful person that I am , I left my Kombucha totally sealed and unattended to for a few days. When I finally remembered about it, it already looked like this

The cellulose pellicle formed should have been wet, smooth and clear but mine was not smooth and had lots of air bubbles on it. Not sure if it is still safe to drink but fermenting my own Kombucha was definitely an interesting experience for me.

Videos

Here are some interesting facts I’ve learnt while watching the videos about fermentation:

  • Fermented food may improve our immune system and reduce risks of some metabolic diseases caused by diets
  • Consumption of fermented food introduces the live microbes into our digestive system, interact with our cells and support our microbiota
  • Century eggs are fermented not by yeast/bacteria but by chemicals. The eggs are covered in salt, sodium carbonate, tea, ash, quicklime and rice husks. Sodium carbonate and calcium oxide in quicklime reacts to form sodium hydroxide, the main chemical involved in fermentation of century egg. NaOH digests the egg white proteins and the peptides attach themselves to water to form the gel-like texture of the century egg.
  • Lead oxide (toxic and fatal if swallowed) is involved in the fermentation of century egg
  • Growth of fungi, bacteria and mold on cheese sounds disgusting but these are harmless and are essential to give the cheese complex flavors
  • Milk is white but cheese is yellow because the draining away of water from the milk causes the concentration of beta-carotene
  • Lactic acid bacteria produces lactic acid which is responsible for fermentation of food such as vegetables
  • Addition of salt to cabbages removes water due to osmosis and causes the cabbage to be softer
  • Kimchi has fruity notes attributed to geranylacetone & beta ionone and a buttery note attributed to 2,3-butanedione
  • Presence of gut microbes could cause changes in brain chemistry and affect psychological stress levels

That is all for this chapter. Thank you for reading!