Content

The second chapter Brewing and Fermentation covers the chemistry of coffee, tea and beer brewing, and fermentation of food and beverages.

Brewing

To continue with my previous post, the Coffee Roaster workshop introduced the brew parameters of coffee. This week, with the online videos and readings, I deep-dived into the science of coffee brewing.

There is a ‘Goldilocks’ set of brew parameters to ideally get a good cup of coffee. In general, there are 6 brew parameters:
1) Water quality – this includes water hardness, acidity/pH, cation composition. All of these can influence the overall result of the coffee extract. As we know that there are many compounds in coffee, each with their own pKa and related chemical properties such as solubility.

2) Temperature – it represents the average kinetic energy of water molecules. The higher the temperature, the more able these molecules may penetrate the interstitial (between ground coffee particles) and interstitial (within the ground coffee particles) spaces of the coffee grounds. This also affects the solubility of compounds, affecting the overall flavour profile of the coffee. The recommended brewing temperature for hot coffee is 91 – 94 degC.

3) Particle Size/Grounds Size – the smaller the coffee grind size, the higher surface area for the extraction of compounds to take place, leading to higher concentration of compounds

4) Brew ratio – this is referred to the final coffee grounds to water ratio by mass. This affects the overall concentration of the compounds in the final cup. Espresso

5) Pressure – the higher the pressure from the espresso machine, water is better able to penetrate into the intrastitial spaces of the coffee ground, extracting the flavour compounds better.

6) Time –  time depends on the method of brew and the 5 other brew parameters above. The longer the coffee grounds are immersed in/exposed to water, less soluble compounds may be extracted over longer periods of time.

There are other brew parameters but will not be discussed in detail here. These brew parameters may be applicable for tea brewing as well.

According to ‘Chapter 15: The Brew – Extracting for Excellence’, if coffee is extracted at too low temperatures, the acidity and sweetness will remain the same while the aroma (volatile compounds) and bitterness (phenylindanes, chlorogenic acid, lactones, etc.)  will be lacking.

There are many details that a chemist would greatly appreciate and understand the intricacies shared in the readings that others might not fully grasp or understand.

My favourite beverage is cold brew, due to its ideal non-bitter, non-astringent taste if prepared and stored properly. It has the taste similar to iced americano, but without the bitter aftertaste. By immersing coffee grounds in cold water, it lacks the heat to extract some less polar compounds such as coffee oil. Thus, a longer extraction time was required for the flavour compounds to diffuse out of the coffee grounds. As mentioned in the reading, cold brew coffee “emphasizes on body, sweetness, and chocolate notes, and have a syrupy characteristic.” Long brewing times and improper storage conditions will lead to oxidized flavours in the cup, making it more sour/acidic.

Reading and learning about coffee makes me understand the western coffee menu better, like those in Starbucks or at cafes.

This led me to think about the preparation method of Chi Cha San Chen, a Taiwan bubble tea chain in Singapore. They use the espresso method of pressure extraction of tea leaves for their drinks, leading to a fresh cup of extracted tea. An interesting fact is that white tea, green tea, oolong tea and black tea can be prepared from the same harvested tea leaves. Their difference lies in the preparation process, namely drying, bruising, roasting and fermentation and this brings me to the next topic, fermentation.

Fermentation

Fermentation is defined in 2 ways:
(1) the microbial conversion of glucose to ethanol or;
(2) the natural browning reaction catalyzed by enzymes from the tea leaves.

The fermentation of wine, beer, tea, kimchi, kombucha, etc. refers to the first definition of fermentation, the more commonly known definition. The second definition refers to the preparation process of tea: withering, rolling, fermentation and roasting. Withering removes moisture and concentrates the compounds in tea leaves; rolling bruises and damages the tea leafs and allow oxidation of the compounds to occur; fermentation transforms the polyphenols of green tea leaves into more complex polyphenols in black tea and lastly; the roasting process, to stop the enzymatic activity and ends the tea preparation process.

The primary fermentation (1) is the ‘fermentation’ process everyone knows it to be. The microbial conversion of glucose or sugar in presence of oxygen into ethanol and carbon dioxide.

Workshop

In Week 4, the fermentation workshop focused on kombucha, a fermented tea. The workshop was conducted by Ding Jie, founder of Starter Culture and a NUS MSc Food Science alumni.

He introduced the concept and science of fermentation by microorganisms, and it is used a lot, including miso, natto, sourdough, and even tempeh! Interesting 🙂 He went in-depth about fermentation microbiology, the type of culture used, the aerobic and anaerobic air exchange, and chemical changes during fermentation.

In this workshop, he focused on the fermentation of kombucha. He explained that lactic acid bacteria (LAB) and acetic acid bacteria (AAB) converts sugars to lactic acid, and acetic acid and cellulose (pellicle) respectively, providing that sour taste in kombucha. Yeast was added to convert sugar to ethanol.

Carbonation drops, unfermented kombucha in the jar, fermented kombucha in the bottle.

Both the jar and bottle were brought home for the next step. After putting a kitchen towel over the jar, it was left to sit for a week. The porous kitchen towel allows oxygen to enter the jar and allow the yeast to start doing its work and ferment. Over the week, I observed the primary fermentation of kombucha, the aerobic process occuring in the jar.

He went on to share the chemistry and application of the general fermentation process, in yoghurt, Chinese red rice wine, hot sauce, mead and amazake. Fermentation sure is widely used in food and beverages!

Pictures taken over a week, to observe the fermentation of kombucha, with the cellulose pellicle building on the surface of the tea. 

The secondary fermentation process, or the anaerobic process, occured in the bottle. Carbonation drops were added to make the fermented kombucha in an airtight bottle to make it fizzy. The carbonation drop, composed of glucose and sucrose, was added to allow the anaerobic process to begin. When first opened, the pressurised water vapour in the headspace above the bottle suddenly condensed, forming a cool misty smoke effect 🙂

Next week: Baking & Cooking! We are making pancakes on Valentine’s Day for our workshop! Can’t wait 🙂