Lab 2

Equipment and Materials

Equipment

• Popcorn popper/roaster
• Coffee grinder
• Brush
• Timer
• Mesh colander
• Metal bowl
• Digital balance
• Graduated cylinder
• Drip brewer
• Filter paper
• Beaker
• Electric kettle, or hot plate and thermocouple

Materials

• Green coffee beans (≥ 60 g per group)
• Water (≥ 900 mL per group)

Background

A unit operation is any step in a process in which a physical or chemical change occurs. In a coffee roastery, several unit operations take place, such as roasting and grinding the beans. A drip brewer combines multiple unit operations (heating, extraction, and filtration) within a single device.

For every unit operation, mass must be conserved. In a drip brewer, a fixed amount of water enters the system, but a smaller mass of brewed coffee exits because some water remains trapped in the moist spent grounds and some is lost as released gases or vapor. Therefore, to design a process that produces a fixed amount of coffee, we must analyze how mass flows through the system.

Roasting

During roasting, only green coffee beans enter the process. The exiting streams are:

1. Roasted coffee beans
   
2. Chaff, the thin outer skins that flake off during roasting
   
3. Escaping gases, including water vapor, carbon dioxide, and other volatile organic compounds (VOCs)

Brewing

Similarly, during brewing, the input water exits the system as brewed coffee, moist spent grounds, and evaporated steam. For simplicity, we will neglect the mass lost to evaporation and ignore dissolved coffee solids in the brewed coffee.

Coffee grounds absorb a finite amount of water. To quantify the amount of water retained in the spent grounds, we define the absorption ratio:

\[ R_{\text{abs}} = \frac{\text{mass of water absorbed into coffee grounds}} {\text{initial mass of dry coffee grounds}} \]

By performing a mass balance, the amount of drinkable coffee produced can be determined if the mass of input water, the mass of dry grounds, and the absorption ratio are known. In this experiment, the absorption ratio will be determined experimentally by varying the amount of coffee grounds while keeping the amount of input water constant.

Part 1: Roasting + Brewing

1a: Roasting

1. Measure the green coffee beans.
   Weigh approximately 60 g of green coffee beans and record the volume they occupy in a graduated cylinder.

   Mass of green beans: ____________________ g

   Volume of green beans: ____________________ mL

2. Roast the coffee beans.
   Roast the green coffee beans in the roaster for approximately 18 to 20 minutes to produce a light roast. Monitor the color of the beans during roasting and listen for first crack. The total roasting time may vary.

   Time spent roasting: ____________________ min

3. Weigh the chaff and roasted beans.
   Allow the beans to cool, then transfer them to a mesh colander. Collect the chaff that falls through the mesh and brush out any remaining chaff from inside the roaster. Weigh the chaff and roasted beans separately. Finally, transfer the roasted beans to a graduated cylinder and record the new volume.

   Mass of roasted beans: ____________________ g

   Mass of chaff: ____________________ g

   Volume of roasted beans: ____________________ mL

1b: Brewing

4. Grind the roasted coffee beans.
   Grind all roasted beans to a coarse grind.

5. Weigh the parts of the drip brewer.
   Weigh the entire empty drip brewer with the cord draped on top. Then weigh the empty glass carafe and the empty brew basket with a dry filter paper inside.

   Mass of drip brewer before brewing: ____________________ g

   Mass of carafe: ____________________ g

   Mass of basket + filter paper: ____________________ g

6. Prepare a brew.
   Weigh approximately 300 g of cold water and add it to the drip brewer. Weigh approximately 30 g of coffee grounds and place them in the filter basket. Begin brewing.

   Mass of cold water: ____________________ g

   Mass of grounds: ____________________ g

7. Measure the coffee brewed.
   After brewing is complete, weigh the entire drip brewer, then weigh the glass carafe containing the brewed coffee and the filter basket containing the moist spent grounds.

   Mass of drip brewer after brewing: ____________________ g

   Mass of carafe with brewed coffee: ____________________ g

   Mass of basket + filter paper + spent grounds: ____________________ g

8. Repeat.
   Repeat Step 6 for two additional brews using the same mass of water, but with 20 g and then 10 g of coffee grounds.

Part 2: Analysis

Create a report that includes a process flow diagram, mass balance calculations, a scatterplot, before and after photos of your roast, and a discussion of your observations.

Process Flow Diagram

Using software or paper, sketch a process flow diagram for roasting and brewing coffee.

• Start with green coffee beans and end with a final beaker of brewed coffee.
  
• Represent each step with a labeled rectangle.
  
• Draw arrows for all material streams entering and leaving each step, including waste streams.
  
• Label each stream with the corresponding masses from your first brew.

Finding the Absorption Ratio

In Excel, use your experimental data to plot mass of water in the brewed coffee against mass of the dry coffee grounds. Generate a line of best fit and determine the negative of the slope, which is equal to the absorption ratio. Include this plot in your report.

Before and After

Include photos of the coffee beans before and after roasting. Calculate and report the percent volume change.

Discussion

In your discussion, address the following questions:

• What numerical value of \[ R_{\text{abs}} \] did you obtain?
  
• If you start with 600 g of cold water and 50 g of coffee grounds, how much drinkable coffee would you expect to produce?
  
• How much water was lost to steam during brewing, if this loss is non-negligible?
  
• How much mass of the initial green beans was lost during roasting?
  
• What fraction of this mass corresponds to chaff, and where does the remaining mass go?
  
• Would you expect these results to be the same for light and dark roasts?