First experiment (Michael)
Here is my first trial experiment of my project! Numerous organic acids are presented in a cup of coffee (e.g. quinic acid, critic acid ...), and everyone have their own preference in coffee acidity. The following experimental procedures, designed based on the recipe shown on the coffee package and the operation manual of the pH meter, aim to find out the relationship between the water temperature and the pH value of coffee that may help you to brew coffee with a perfect acidity. (With my supervisor's permission, the following experiment was conducted in laboratory.)
Experimental setup and procedures
Objective
Find out the effect of water temperature on pH value of drip coffee
Hypothesis
The extraction of efficiency of the acidic compounds inside the ground coffee will increase with higher water temperature brewing as predicted by Arrhenius equation.
Hypothesis
The extraction of efficiency of the acidic compounds inside the ground coffee will increase with higher water temperature brewing as predicted by Arrhenius equation.
Sample
Maxwell House Drip Coffee (Blue Mountain Flavour) with coffee filter bag
Apparatus
Beakers, conical flasks, stir bars, heat plates (with stirring function), thermometers, pH meter.
Materials
Tap water, phosphate buffer solution (pH = 7) and the ground coffee sample.
Procedures
1. Calibrate the pH meter with phosphate buffer solution (pH=7)
2. Boil 200 mL water to 35oC with a conical flask, stir bar, thermometer and heating plate
Figure 1: The setup for boiling water
3. Pour water over the filtration bag provided by the sample coffee (contains 10g of ground coffee), and collect the coffee (filtrate) to a beaker
Figure 2: The ground coffee and filter bag
4. Cool down the coffee product to room temperature with water bath
5. Measure the pH value of the coffee product with pH meter
6. Repeat step 2 to 5 with water at 0 oC, 70 oC and 100 oC
Experimental data
Figure 3: The final coffee products (From left to right are the coffee products brewed with 100, 70, 35 and 0 oC water respectively)
Figure 4: The pH readings from those coffee products (Top left = coffee brewed with 0 oC water; top right = coffee brewed with 35 oC water; bottom left = coffee brewed with 70 oC water; bottom right = coffee brewed with 100 oC water)
Amount of water added (mL)
|
Amount of ground coffee (g)
|
Temperature of water (oC)
|
pH*
|
200
|
10
|
0 (ice water)
|
6.0
|
200
|
10
|
35
|
5.9
|
200
|
10
|
70
|
5.8
|
200
|
10
|
100 (boiling water)
|
5.7
|
*The pH meter used in this experiment has a built-in function which can automatically correct the pH deviation due to the temperature difference of the sample solution. (Shown as ATC on the screen of the pH meter)
Figure 5: The relationship between the water temperature and the pH values of coffee
From the experimental data, it seems that the extraction efficiency of the acidic compounds in coffee increases with the water temperature (smaller pH value = more acidic). This agrees with the assumption made in the beginning. Also, to approximate the relationship between pH and water temperature, linear regression was applied, and it gave out the following equation: pH of coffee = -0.003(water temperature) + 6.0028.
Possible errors
Uneven brewing
As the brewing was conducted manually, it is difficult for me to make sure that the way I brewed for these four sample were actually the same. Beside the water temperature, this resulted in an another variable for the brewing efficiency.
Inaccurate measurement of amount of water
Since I used the makers on the conical flasks to measure the amount of water and those makers can provide an approximation of the volume only, the volume of water I added to brew coffee may not be 200 mL.
Drop of water temperature
As there was a significant temperature drop of water during the brewing step, the water temperature throughout the brewing step may not be consistent with the reported temperature.
Low filtration flow rate for 0 oC water sample
Drop of water temperature
As there was a significant temperature drop of water during the brewing step, the water temperature throughout the brewing step may not be consistent with the reported temperature.
Low filtration flow rate for 0 oC water sample
During the brewing step with 0 oC water sample, an unexpected low filtration flow rate was observed. This increased extraction time may also resulted in an another variable for the brewing efficiency.
Further improvements and possible experiment
- Record the water temperature right at the points that start adding and finish adding water, and take their average as water temperature data
- Replace the filter bag with formal coffee filter to see if there are any improvement for the low flow rate in 0 oC water sample
- Measure the water volume using one single beaker/ conical flask for all four samples
- Use the similar setup and the online reference to compare the caffeine content in 0 oC water and 100 oC water brewed coffee
References
- Acids. Coffee Chemistry. https://www.coffeechemistry.com/chemistry/acids
- Arrhenius equation. Chemistry Libretexts. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/Modeling_Reaction_Kinetics/Temperature_Dependence_of_Reaction_Rates/The_Arrhenius_Law/Arrhenius_Equation
- Extraction of caffeine (experiment). Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Ancillary_Materials/Laboratory_Experiments/Wet_Lab_Experiments/Organic_Chemistry_Labs/Experiments/3%3A_Extraction_of_Caffeine_(Experiment)
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