Senior Lab - Week 3

Equipment and Materials

Equipment

• Kill-a-Watt energy meter
• Coffee grinder
• Brush
• Electric kettle
• Timer
• TDS meter
• Digital balance (± 0.1 g)
• Graduated cylinder (100 mL)
• Pour-over carafe
• Filter papers
• Beakers (250 mL, 500 mL, × 3)
• Kimwipes
• Warming plate

Materials

• Roasted beans from Week 1 (open one valve bag)
• Water (≥ 2.5 L per group)

Background

Week 3 completes the experimental matrix. You will vary water temperature and extraction time, build a quantitative model of extraction, and run the pH time-study. By the end of the session you will have enough data to specify all operating conditions for your client and calculate the full 600 g process design.

Note: Parts 3A (temperature) and 3B (pH time study) share a brew. As soon as the 94 °C brew in Part 3A is complete, immediately move the carafe onto the hot plate set to “warm” and begin the pH sampling protocol from Part 3B.

Part 3: Temperature + pH + Extraction Time

3a: Effect of Water Temperature

Use the grind size that produced the closest match to your target TDS/PE window on Day 2. Hold brew ratio and extraction time constant (use the natural drip time, same as Day 2). Vary only water temperature: 50 °C and 94 °C.

1. Baseline TDS.
   Measure TDS of tap water (three readings; record mean ± standard deviation). This is your blank baseline.

2. Brew.
   For 50 °C and 94 °C, brew and record contact time and mass of brewed coffee. Let the sample cool to room temperature, then take three TDS readings (mean ± SD). Calculate PE.

   

   Beautiful coffee bed

3b: Post-Brew pH Time Study

Begin this part immediately after the 94 °C temperature brew from Part 3A. Place that carafe on the warming plate and begin sampling.

1. Take pH measurements.
   As soon as brewing completes, withdraw a ~10 mL sample and record the time as t = 0. Allow the sample to cool before testing and record the pH.

2. Continue for an hour.
   Every 5 minutes for 60 minutes, withdraw a fresh ~10 mL sample, recording pH and time.

3c: Extraction Time Study

This final part builds a TDS-vs-time extraction curve and is the basis for estimating \(k_{eff}\). Use medium grind and 94 °C water.

1. Prep.
   Weigh grounds to your target brew ratio mass. Heat 94 °C water.

2. Brew again!
   Begin a brew (your last one yippee!).

   

   Pouring over the pour over

3. Take TDS measurements.
   Every minute for ten minutes, withdraw a ~15 mL sample into a labeled small beaker. Continue pouring or allow steeping as needed to maintain water contact through 10 minutes. Do not remove the filter until 10 minutes have elapsed. 4. Allow each sample to cool to room temperature and measure TDS (three readings per sample; record mean ± SD).

4. Weigh.
   Weigh the total brewed coffee at the 10-minute endpoint.

   

   Carafe with brewed coffee

5. Analyze.

   Plot \([H⁺]\) vs. time and fit a linear trendline. The slope is the average rate of \(H⁺\) generation (mol L⁻¹ min⁻¹). Extrapolate the fit to find t at which pH = 4.8. Report this as your maximum recommended hold time. Also compute \(k_{eff}\) at 50 °C by repeating the linearized plot analysis using your temperature brew data from Part 3A, treating the 50 °C brew TDS at its natural contact time as a single point with a short extraction time, or if time permits, brew a second extraction time series at 70 °C. Then apply the Arrhenius equation to estimate \(E_a\).