Senior Lab - Week 2

Equipment and Materials

Equipment

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

Materials

• Roasted beans from Week 1 (open one valve bag)
• Water (≥ 1.5 L per group)

Background

Week 2, and the focus shifts from roasting to brewing. Your client still wants that 600 g pour-over hitting 1.2–1.4% TDS and 18–22% PE, but now your manager is asking how efficiently you can get there. Grind size becomes your main control. It sets particle size, shapes extraction, and even determines how much energy your grinder uses. This week, you’ll dial in grind size while quantifying what it does to extraction, particle distribution, and energy use.

Part 2: Heat Capacity + Grind Size + Energy

2a: Specific Heat Capacity of Water

The Kill-a-Watt meter was used during Week 1 roasting; today you use it to characterize heat transfer in the kettle. This value is needed to calculate brewing energy in Part 2C.

1. Measure the water.
   Measure 200 mL of cold tap water into the kettle. Record the exact mass and initial temperature T₀.

   

   Water

2. Heat up the water.
   Plug the Kill-a-Watt meter into the wall; plug the kettle into the meter. Reset the meter to zero. Start the kettle to your desired \(T_{water}\). Each time the energy display increments by 0.01 kWh, record the current temperature and cumulative energy. Continue until the water reaches Twater.

3. Repeat and analyze.
   Repeat with 400 mL of water. Set this water aside at 94 °C for the grind size brews in Part 2B. For each data point, compute \(Q_{elec}\) and m\(\Delta\)T to find your experimental \(C_{p,exp}\).

2b: Grind Size

Run three pour-over brews, one at each grind size, holding all other variables constant at the baseline condition: \(T_{water}\) your chosen Rbrew, and extraction time equal to the time at which the last drop falls through the filter naturally (do not stop early or wait after dripping ceases). Record this time for each brew; it should be approximately the same across all three if you pour consistently.

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

2. Brew 1: Coarse grind.
   Plug the grinder into a Kill A Watt meter and reset it. Since the grinder uses such little energy, it may not be possible to measure the energy consumption in kWh. However, you can estimate it with the instantaneous power (in W) being drawn by the grinder and multiply by time to get energy. Collect ~10–20 g of ground coffee and perform a sieve analysis using stacked sieves (largest to smallest), shaking for a fixed time and recording the mass retained on each sieve to obtain a particle size distribution by mass fraction.

   Weigh the remaining grounds to your target brew ratio mass. Heat water to \(T_{water}\). Brew and record contact time. Weigh the brewed coffee. Let the sample cool to room temperature, then measure TDS. Wipe the probe with a Kimwipe between readings and take three readings; record mean ± SD. Calculate PE.

   

   Coarse vs fine grind

3. Brew 2: Medium grind.
   Repeat the same procedure with a medium grind, including energy measurement and sieve analysis.

4. Brew 3: Fine grind.
   Repeat the same procedure with a fine grind, including energy measurement and sieve analysis.

5. Analyze.

   Plot TDS and PE as separate labeled column charts with error bars representing the standard deviation of your three TDS readings. From the sieve data, plot particle size distributions (mass fraction vs. sieve size) for each grind. Compare energy consumption across grind sizes. For each chart, discuss the trend in terms of the mass transfer flux equation. Quantify the relative change in TDS from coarse to fine (i.e., report (\(TDS_{fine}\) − \(TDS_{coarse}\) ) / \(TDS_{coarse}\)× 100%). Identify which grind size, if any, falls within the target TDS and PE windows. This will inform your final operating condition selection.

2c: Process Energy Summary

Using your experimental Cp, calculate the energy consumed to heat the brewing water for a 600 g batch: \(Q_{brewing}\) = \(mass_{water}\) × \(C_{p,exp}\) × (\(T_{water}\) − \(T_{room}\)). The roasting energy was measured by the Kill-a-Watt during the Day 1 roast. Sum the roasting, grinding, and water heating energies to find the total amount of energy required, the energy per gram brew. Use local energy costs to find the operational cost.