First Experiment (Brian)
Finally! Here we come to the most exciting part of the project......trial experiment! Recall a bit on the aim of my project: To compare the effect of eggshells, salt and baking soda on lowering the acidity of pour-over coffee. Although I got the recommended recipe for making pour-over coffee from the National Coffee Association of U.S.A., I am not so sure about the amount of additive added for acid neutralization in coffee. So, I would use 5g as a testing point for this trial.
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| The material used in the trial experiment |
From left to right,
- Water boiler and electronic heater (with temperature control function)
- Rwandan ground coffee (medium roasted)
- Salt
- Funnel and collector
- Eggshells
- Baking soda
- Measuring spoons and cup
- Scissor
- Buffer / calibrating solution (pH 6.86)
- pH meter
- Marker
- Filter paper
Experimental procedures
pH meter calibration
Before kicking off the experiment, I try to calibrate the pH meter as the instruction given by the producer with the buffer solution at room temperature.
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| pH calibration |
Controlled set-up
- Measure 0.2L of tap water. Extra 20mL of water is used to pre-wet the filter paper.
- Pre-wet the filter paper before boiling the water
- While boiling the water, transfer 15g ground coffee powder into the funnel using the measuring spoons.
- When water temperature reached 93˚C, start pouring water into the funnel in a spiral manner
- Stop pouring water when coffee powders are immersed with water and wait for 30s
- Keep pouring water into the funnel slowly
- Measure the pH of fresh hot coffee once filtration is finished.
- Observe, smell and taste the final product!
Video clip of the controlled experiment
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| pH measurement of the coffee |
Experimental set-up
It is almost identical to the controlled set-up excepted that 5g of different additives (eggshells, salt and baking soda) are added separately before the brewing process.
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| Add 5g eggshells |
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| Add 5g salt |
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| Add 5g baking soda |
Result analysis and reflection
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| In this trial, I have made 6 cups of coffee in total |
A table showing the pH measurement and my observation
| Independent variables | Dependent variable / pH | Color | Flavor |
| 5g eggshells | 5.28 | Deep brown | Same as control |
| 5g salt | 4.99 | Very salty | |
| 5g baking soda | 8.80 | Dark brown | Blend / slightly salty |
| Control | 5.07 | Deep brown | Slightly sour, pleasant aftertaste, little bitter, coffee aroma |
| 5.12 | |||
| 5.22 |
About the flavor...
In terms of the flavor of the control, I quite appreciate the taste of coffee that I made, but I think some may find it too flat. For the coffee brewed with salt and baking soda..... it is undesirable! The salty taste of both salt and baking soda completely blocks the aroma and pleasant taste of coffee! Gosh! That means 5g of additives, especially salt and baking soda, is too much to make a cup of tasty coffee.
Hypothesis support?
Back to my hypothesis, it seems that baking soda is more effective in lowering the coffee acidity, compared to that of eggshells. Strangely, there is no effect for salt on coffee acidity! For eggshells, it is hard to tell whether it is effective in reducing coffee acidity in this trial.....
After doing the trial, there are still a lot of question marks in my mind:
- Why the pH of coffee brewed with salt is even lower? What's wrong?
- Are eggshells effective in lowering coffee acidity?
- Why the color of coffee brewed with baking soda is darker?
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| Left: control, Right: 5g baking soda added |
To answer these questions, I need to find out the error or other limiting factors that affect the flavor and pH of coffee in this study as well!
Factor 1: Cooking technique
Since this is my first time making pour-over coffee, the cooking technique definitely is a huge factor affecting both flavor and pH significantly.
Under-extraction2
Hundred of flavor compounds are extracted when hot water contacts with the ground coffee. Therefore, the time of coffee brewing affects critically the type and amount of flavor compound. In general, acidic compounds are extracted earlier than bitter compounds. So, longer the brewing time, bitter and stronger the coffee taste. In my trial, I cannot balance the water flow and pouring force properly. This may cause a shorter time of coffee extraction and leads to blend and acidic coffee.
Uneven mixing
I add the additives on top of the coffee powder and I didn't stir them when I started pouring water into the funnel. Next time, I could possibly mix coffee powder and additives well before putting into the funnel.
Factor 2: Properties of water3
Water Temperature
Although I boil the water to the recommended temperature (93˚C), it dropped significantly when I pour water into the funnel. Lower temperature favors slower extraction of volatile and acidic compounds.
Water Hardness
Tap water contains minerals like magnesium and sodium, which will affect the extraction of flavor compounds in coffee powder. Tap water in Hong Kong is pretty soft that favors the extraction of the acidic compounds,
Factor 3: Properties of other ingredients
Coffee powder
Longer roasting time results in the darker and less acidic coffee beans. This time, I choose medium roasted ground coffee, which should be more acidic and taste weaker than darker coffee beans.
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| Medium roasted coffee powder from Rwandan |
Baking soda
From my observation during the experiment, water drips more slowly when baking soda is added. This might also explain why the color of coffee becomes darker: with longer extraction time, more flavor compounds should be extracted out.
Eggshells
The basic chemical compositions of eggshells are mainly calcium carbonate, which has very low water solubility. Even if it can form carbonic acid in the water, the pH should be very similar to coffee. So, I believed that they have very little or even no effect on coffee acidity.
Factor 4: reliability of pH measurement
There is a huge temperature difference between coffee pH measurement and calibration. I measure pH when coffee is still hot around 60˚C while pH calibration is conducted under room temperature (25˚C). The pH of a sample may vary under different temperatures. Under higher temperatures, water undergoes auto-dissociation into hydrogen ion and hydroxide ion more readily according to Le Châtelier's principle.1
H2O ⇌ H+ + OH- (→ Shifting)
Based on the definition of pH, a higher concentration of free hydrogen ion leads to a decreasing pH.
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| Relationship between pH and temperature of natural water Retrieved from https://www.fondriest.com/environmental-measurements/wp-content/uploads/2014/02/watertemp_ph1.jpg |
Since temperature keeps dropping when coffee is cooling down, pH fluctuates significantly which affects the accuracy and reliability of pH measurement.
Possible follow-up experiments and improvements
The aim of this study is to find out the most effective additive that could lower coffee acidity while at the same time it has a minimal effect on coffee flavors. So, my possible future plan is to find out the optimum amount of baking soda added. I could make pour-over coffee with 5 different amounts of baking soda ranging from 1 - 5g.
There are some possible ways to minimize errors in this study:
- Stir while pouring water / pre-mix the additives and coffee before brewing
- Break down eggshells into smaller pieces to increase the surface area
- Measure the pH when it cools down to room temperature
- Start pouring water from the center to immerse all the ingredients for brewing. Then, pour water gently in a spiral manner (Suggestion from my friend, who is a Bristeia)
Date of Experiment: 17-18th March 2020
By TSE Chun Hei
UID: 3035470955
References
- Greene, M. (202, January 6). The Science of Coffee Brewing. Retrieved March 20, 2020, from https://prima-coffee.com/learn/article/coffee-basics/intermediate-brewing-principles/intermediate-brewin
- Perfect Daily Grind. (2018, May 25). How to Accentuate (or Reduce) Acidity When Brewing Coffee. Retrieved March 20, 2020, from https://www.perfectdailygrind.com/2018/04/how-to-accentuate-or-reduce-acidity-when-brewing-coffee
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