At Fission and Bloom, we love digging deep into the science of coffee roasting and how each step in the process influences the final cup. One of the most fascinating aspects of coffee roasting is the effect of temperature, especially at the early stages of the roast, on the flavour of the brewed coffee. In this experiment, I tested how changing the turning point temperature at the start of the roast affects the taste profile. The results were surprising, showing a pronounced effect on both the acidic and sweet/bitter portions of the brew.

In addition to the evolution of acidity and bitterness during roasting, temperature plays a key role in shaping these flavours. As roasting temperatures increase, the chemical reactions responsible for generating acids and bitter compounds accelerate. At lower roasting temperatures, acids such as citric and malic acids, which contribute to a bright, pleasant acidity, are preserved, leading to a more vibrant cup. However, as the temperature rises, these acids degrade, and bitter compounds like chlorogenic acid lactones and phenylindanes begin to form, intensifying the bitterness. Prolonged exposure to higher temperatures not only reduces acidity but also enhances bitterness, leading to a darker, more intense flavour profile. The challenge for roasters is to carefully manage temperature to balance acidity and bitterness, ensuring the coffee delivers a pleasing sensory experience without overwhelming bitterness.

The Roasting Experiment

I roasted three batches of 150 grams of speciality coffee beans, keeping the roasting profile consistent for each batch, except for the turning point temperature at around 42 seconds into the roast. The turning point refers to the moment when the temperature probe measuring the beans’ temperature starts to increase after initially cooling due to the introduction of the beans into the roaster (see the figure below for the changing bean temperature with time).

• Sample 1 had a turning point temperature of 81.64°C.

• Sample 2 had a turning point temperature of 68.01°C.

• Sample 3 had a turning point temperature of 72.82°C.

After this initial temperature shift, I aimed to match the rest of the roasting profiles as closely as possible. However, there were some slight differences in temperature at 2 minutes 15 seconds:

• Sample 3 reached 137.27°C, the highest temperature at this point.

• Sample 1 measured 127.59°C.

• Sample 2 recorded 122.97°C.

All three samples were roasted to an end temperature of 198°C with a total roast time of 9 minutes.

Considering the differences in the three curves between times 0 minutes and 2 minutes, Sample 2 and 3 have similar temperatures at around 40 s (remember, all three samples start from room temperature at time 0 min), but Sample 3 increases to the highest temperature by 2 minutes or 2:30; and Sample 1 has similar temperatures as Sample 2 at 2:30, but starts at a higher temperature than Sample 2 from 40 s. That is, its temperature increases the most rapidly to 40 s, but the rate in temperature rise quickly steadies to meet with the temperature of Sample 2 by 2:30.

Brewing Method: The V60 Pour-Over

To keep the brewing process consistent across the three roasts, I used the V60 pour-over method. Here’s how I approached it:

1. I brewed 15 grams of coffee, ground to a medium fine setting.

2. After warming the filter and vessel, I poured 50 ml of hot water over the grounds and let the coffee bloom.

3. After around 40 seconds, I added another 50 ml, collecting the first 100 ml of the brew, which represents the more acidic portion of the coffee.

4. I then poured two more rounds of 50 ml of hot water, collecting the remaining sweet and bitter portion of the brew, which amounted to 100 ml.

I repeated this process for each sample and separated the first and second portions of each brew to assess the differences in flavour.

The Tasting Results

Acidic Portion (First 100 ml)

• Sample 1: The acidic portion was mellow and smooth with some dark flavours. The sourness was balanced, sitting towards the back of the mouth. The grapefruit notes were softened, more like a juice than a concentrate.

• Sample 2: This sample was strikingly more sour than Sample 1, with a concentrated grapefruit and lime flavour. It tasted tart, slightly green, and a little dry.

• Sample 3: The sourness was more astringent, felt at the front of the mouth. It was less intense than the other two, with darker flavours coming through.

Sweet/Bitter Portion (Final 100 ml)

• Sample 1: This portion had roasty flavours with hints of brown sugar. It was mellow, with a subtle woodiness.

• Sample 2: Sample 2 presented a sweetness, with flavours of brown sugar and a touch of malt.

• Sample 3: Darker, more roasty flavours dominated, with notes of dark sugar, molasses, chocolate, and tobacco.

Combining the Portions

After tasting each portion separately, I combined the acidic and sweet/bitter portions of each sample into one cup. This revealed even more interesting results:

• Sample 1: The combined cup had a balanced acidity and smoothness.

• Sample 2: The cup was very acidic and tart, with a mild sweetness but a slightly dry finish.

• Sample 3: The combined cup was roasty, with a noticeable dryness lingering at the end.

Key Takeaways: The Influence Bean Temperature Early in the Roast

This experiment shows that even small differences in bean temperature at the start of the roast can have a significant impact on the coffee’s flavour. Sample 2, with the lowest turning point temperature and rise in temperature to 2:30, produced a coffee that was much more acidic and tart, while Sample 1, with the highest turning point, yet a similar temperature to Sample 2 by 2:30, was smoother and more balanced. Sample 3, though not as sour as Sample 2, had darker, roastier flavours, perhaps owing to the maximum temperature reached within the first 2 minutes.

It’s important to note that although I aimed to control the roasting variables as much as possible, other factors that are not represented by the bean temperature could have played a role in the flavour differences. Also, remember this result is applicable to only these beans, with their size distribution, density, water content, post-harvesting processing method, content of flavour-compound precursors, etc. This highlights the complexity of roasting and how multiple factors—temperature, airflow, and roasting time—all come together to shape the final cup.

Conclusion

Temperature adjustments at the start of the roasting process can lead to strikingly different flavour profiles in the brewed coffee. Whether you’re looking for bright acidity, balanced sweetness, or roasty bitterness, small changes in the roasting approach can unlock a wide range of flavour experiences. As always, experimenting with different variables allows you to truly tailor your coffee to your personal taste preferences.