Why Coffee Tastes Sour at First – And How It Evolves in the Cup

At Fission & Bloom, we’re fascinated by the alchemy that turns a roasted coffee bean into a complex, aromatic brew. One of the most intriguing parts of the process? The changing flavour profile that unfolds second by second as you brew.

You may have noticed that a quick brew or an under-extracted cup can taste sharply acidic—while longer extractions bring balance and, if pushed too far, bitterness. Why does this happen?

Let’s break down the science behind the sequence of flavours in your cup. I have covered this topic in earlier posts, but here I introduce new information, like the compounds responsible for the flavours.

The Early Stage: Bright and Sour

At the start of brewing, water quickly pulls out the most soluble compounds. These include small organic acids—such as citric, malic, and acetic acid—which are highly water-soluble and tend to sit near the surface of ground coffee particles.

This is why the first few seconds of brewing release mostly acidic-tasting compounds, leading to a sharp, bright, or sour note—particularly noticeable in light roasts and fast brews like espresso.

How much acidic-tasting compounds there are in a coffee could be determined more by the roaster than the individual brewing the coffee, while the brewer can control how the acidity is perceived (however, at the end I propose one way for the brewer to modify acidic content). These acidic-tasting compounds can be burned off during the early part of the roast, and their content manipulated using varying roast temperatures and times. I will adjust the acidity of a coffee by changing a number of variables in the roast, ensuring that a coffee will be suitable for an individual giver her or his tastes and brew methods.

Middle Phase: Sweetness and Balance

As extraction continues, the brew starts to pull in larger, more complex molecules that contribute body, sweetness, and aromatic complexity. These include:

• Chlorogenic acid lactones, which are smoother derivatives of the acids and contribute bitterness and balance.

• Maillard reaction products, like melanoidins, that add body and sweetness.

• Aromatic volatiles, including pyrazines and thiols, that bring out the roast character and depth.

At this point, the acidity is still present, but it is increasingly balanced by sweetness and complexity. This is the sweet spot—where the best brews live.

Late Phase: The Bitterness Sets In

If you continue brewing past the optimal extraction point, the water starts to draw out harder-to-extract, more bitter compounds, such as:

• Phenylindanes, derived from further breakdown of chlorogenic acids

• Tannins and other polyphenols, which add astringency and dryness

This phase can overwhelm the cup, muting acidity and sweetness while pushing it towards harshness or unpleasant bitterness.

Why Does Sourness Fade Over Time?

Interestingly, the sour flavours don’t disappear—they are simply perceived differently. As more compounds are extracted, our palate recalibrates. Bitterness and sweetness mask sourness, and certain compounds (like melanoidins and lipids) buffer the acidity, making it feel smoother and less aggressive.

The science of flavour perception confirms this: interactions between taste modalities (sweet, sour, bitter) and retronasal aroma can suppress or enhance each other, a phenomenon known as cross-modal suppression.

Extraction in Action: A Visual Curve

Here’s a simplified model of how flavour compounds are extracted over time:

This extraction curve illustrates how different flavour compounds dissolve over time during brewing. This dynamic helps explain why underextracted coffee tastes sour, properly extracted coffee tastes balanced, and overextracted coffee tastes bitter.

• Acids (sour): Peak early and fade quickly

• Sugars (sweet): Build gradually to stabilise the brew

• CGA lactones: Contribute balance and mid-roast complexity

• Phenylindanes (bitter): Appear late and can dominate if overextracted

This progression underpins why brew method and timing are so critical. A fast pourover may highlight acidity. A well-tuned espresso balances acidity with body. An overlong French press steep may lose the brightness entirely.

Brewer’s Tip

To highlight brightness, brew light roasts with short contact times and higher grind coarseness. For balance, aim for medium roast profiles with controlled flow rates and a target total dissolved solids (TDS) of 1.25–1.35% for filter brews.

If using a pour-over brew method like the V60 or Hario Switch that allow you to separate the brew liquid from the grounds, you could experiment by removing a small amount of the liquid during the early part of the extraction. Pour ~50 ml of hot water over the grounds, remove the extracted coffee liquid, and continue the brew process. Depending on the coffee, what you could have is a less acidic but sweeter cup. If you prefer more acidity, you can add a portion of the 50 ml volume back to the brew or all of it.

In the cup, time is chemistry.

And at Fission & Bloom, we design our roasts to hit that perfect point—where acidity sings, sweetness hums, and bitterness stays quietly in the background.