Lab 4: Energy of Brewing

Duration: 1.5 hours

Equipment and Materials

Equipment

• Roaster / Popcorn popper
• Coffee grinder
• Brush
• Timer
• Metal bowl
• Digital balance
• Pour over carafe
• Filter paper
• Kill-a-Watt Energy meter
• Graduated cylinder
• Beaker
• Thermocouple or thermal camera

Materials

• Green coffee beans (≥ 40 g per group)
• Water (≥ 600 mL per group)

Background

Energy is required to convert green coffee beans into a drinkable beverage. In this experiment, the most relevant form of energy transfer is the conversion of electrical energy into thermal energy. When an electric kettle is plugged in, electrical energy carried by moving electrons passes through a resistive heating element. Due to electrical resistance, collisions within the element convert electrical energy into heat. As the electrical current increases, the rate of these collisions increases, resulting in greater heat generation.

The quantity of heat transferred to a substance is given by:

\[ Q = m C_p \Delta T \]

where \[Q\] is the heat energy, \[m\] is the mass, \[C_p\] is the specific heat capacity at constant pressure, and \[\Delta T\] is the change in temperature. This equation represents the amount of energy required to raise the temperature of a given mass by a specified amount.

In this lab, we will be finding the value of water’s specific heat capacity experimentally, and calculate the amount of energy required during the brewing process, from roast to brew.

Part 1: Roasting + Brewing

1a: Roasting

1. Measure the green coffee beans.
   Weigh approximately 40 g of green coffee beans.

Green coffee beans

2. Set up for measurements. Plug in the Kill-a-Watt meter into the wall, reset it, and then plug in the roaster into it. Prepare the thermocouple or thermal camera to record the temperature of inside the roaster.

Kill-a-Watt meter

3. Roast the coffee beans.
   Roast the green coffee beans in the roaster to produce a light roast, until first crack. Record the temperature, visual observations, and energy consumption every 30 seconds from start to finish of the roast. Be sure to note down when first crack is observed.

Temperature distribution after roasting

1b: Grinding

4. Grind the roasted coffee beans.
   Plug the grinder into the Kill-a-Watt meter and reset it. Grind all the light roasted beans to a coarse grind. Since the grinder uses such little energy, it may not be possible to measure the energy consumption in kW-hr. However, you can estimate it with the instantaneous power (in W) being drawn by the grinder. Then multiply by time to get energy!

Grinder with beans

1c: Heating water

5. Prepare the kettle.
   Plug the electric kettle into the Kill-a-Watt meter and reset it. Pour approximately 200 mL of water into the kettle. Prepare the thermocouple or thermal camera to record the temperature of inside the kettle if your kettle does not show temperature. Begin the kettle to boil to 95 degrees Celsius. Record the time, temperature, and energy consumption every time the the energy reading changes by 0.01 kW-hr.

   Mass of water: ____________________ g

Pouring water into the kettle

6. Repeat.
   Repeat step 3 but with 400 mL of water. When finished, do not discard the water. Keep the water in the kettle at that temperature.

   Mass of water: ____________________ g

1d: Brewing

7. Prepare your pour over.
   Place a filter paper in the top part of the pour over apparatus. Decide on an appropriate brew ratio for the 400 mL of water, and weigh out the corresponding amount of grounds.

   Mass of grounds: ____________________ g

Filter with grounds

8. Brew!
   Decide on an appropriate extraction time. Then, begin brewing by pouring the water from the kettle over the grounds in a circular motion. Obviously, the pour over apparatus itself does not use any energy. How nice!

Pouring in a circular motion

Part 2: Analysis

You should create a report that includes the following:

• In a software of your choice, use the electric kettle data for both masses to prepare plots of (i) temperature vs. time, (ii) cumulative energy in kW-hr vs. time, and (iii) cumulative energy vs. temperature. What trends do you observe?
• Prepare a table with columns for mass, overall T, overall total energy, and \[C_p\]. Enter the electric kettle data for the two masses and calculate \[C_p\] accordingly. Report your measured heat capacity in terms of J/g C. How closely dp your calculated values correspond to the established specific heat capacity for water?
• Use your roasting data to create a plot of (i) temperature vs. time and (ii) energy in kW-hr vs. time.
• Prepare a column chart that compares the energy usage for the kettle, grinder, and roaster. Note that your energy usage should be normalized on a mass basis! That is, the dependent axis should be in units of energy per gram.

In your discussion, you should answer:

• What trends do you see in your temperature and energy measurement vs. time for heating water? How does your experimental value differ (in % difference)? What might account for this discrepancy?
• At what time and temperature did first crack occur? Add any additional notes on how roast color changed with time.
• During roasting, why may the temperature and energy usage vary in time in the way it did? What effect do you think it had on the reactions occurring during roasting?
• Find the current price of energy rates in our area. How much did the energy you used overall cost?