Lab 3: The pH of Coffee
Duration: 3 hours
Equipment and Materials
Equipment
- Roaster / Popcorn popper
- Coffee grinder
- Brush
- Timer
- Metal bowl
- Digital balance
- Drip brewer
- Filter paper
- pH strips
- Graduated cylinder
- Beaker
Materials
- Green coffee beans (≥ 120 g per group)
- Water (≥ 1200 mL per group)
Background
Imagine you carefully roast and brew high quality coffee beans. You taste it and think it is the best coffee you’ve ever had. Excited, you text your friend and invite them over. The L is not running this weekend, so it takes them a while to arrive. When they do arrive, they taste the coffee, but their response is a polite “I guess it’s okay…”. Confused, you taste it again, and you agree… now it tastes mediocre. What happened?
A key thing to realize about coffee is that the taste of a given brew will change with time. While mass transfer stops once you separate the grounds from the brew, two other processes continue to occur:
- The escape of volatile organic compounds (VOCs) from the liquid to the gas phase. These comprise the wonderful aroma you smell while brewing. Eventually, the VOCs are depleted, the coffee aroma dissipates, and the taste suffers.
- Chemical reactions. Although it seems like the coffee is just sitting there, a variety of complicated reactions are taking place within. Some of these reactions generate new VOCs, others consume them. In particular, several reactions produce additional acidic molecules. Some acidity is to be desired, but too much makes the taste sour.
Recall that pH is a measure of the acid concentration in a solution, defined as pH = -log[H+]. The initial pH of coffee upon brewing depends on several factors such as variety of beans, how they were roasted, and pH of the water used. With time, it is expected that as the hydronium ions are generated in reactions, the pH will decrease.
A natural questions that arises is, “How fast does the chemical reaction happen?”. The coffee pot is effectively a batch reactor, so you can use pH data to estimate the rate of reaction as
\[ \text{rate} = \frac{d[H^+]}{dt} \approx \frac{\Delta[H^+]}{\Delta t} \approx \frac{[H^+]_{\text{final}} - [H^+]_{\text{initial}}}{t_{\text{elapsed}}} \]
Note that the rate has units of moles per liter per unit time and that it is a rough estimate because we approximate the time derivative (an “engineering aproximation”!).
Part 1: Roasting + Brewing
1a: Roasting
Measure the green coffee beans.
Weigh approximately 120 g of green coffee beans.
Green coffee beans Roast the coffee beans.
Roast half the green coffee beans in the roaster to produce a light roast, until first crack. Set these aside and let the roaster cool for 5 minutes. Then, roast the remaining 60 grams to a dark roast.