Resolving the Bitterness Paradox

Given the flow profile of the SSC Brewer, where about 70% of the flow takes place in the last 1/3 of the brew time, we could expect the coffee to be more bitter than traditional brewers. This is because it’s generally understood that bitter compounds extract later in the brew. Experience has proved that this is not the case, with many people reporting a less bitter cup. Why?

Two Main Contributors to Bitterness

During roasting, chlorogenic acids – which account for up to 8% of the composition of unroasted coffee beans - degrade into chlorogenic acid lactones and then further into phenylindanes, as the roast level moves into medium and dark.

Lactones and phenylindanes are two major contributors to the bitterness in coffee.

Typically lactones give a pleasant, rounded bitterness; adding complexity.

Phenylindanes give a strong, lingering bitterness with a metallic or ashy edge. They are easily detectable, with a sensory threshold of about ~0.1 µg/mL (compared to caffeine with ~200 µg/mL). So tiny quantities can significantly affect the taste of a brew.

The concentration of phenylindanes is almost negligible in light roasts but rises sharply in the medium–dark range.

Bitterness In Light Roasts

Even though they are easily tasted, phenylindanes are at such low levels in light roasts that they are not an issue.

Any undesirable bitterness in a brew with light roasts is likely to be associated with over extraction. Light roasts retain more chlorogenic acid than dark roasts — sometimes 3–4 times as much. When these acids are over-extracted (e.g. from too fine a grind, slow drawdown, or long contact time), they degrade into quinic and caffeic acids, both of which taste bitter and sour/astringent.

This bitterness is sharper and more astringent than the deep, dry bitterness of dark roasts (which comes from phenylindanes).

Solubility in Water

After acids and salts the chlorogenic acid lactones are among the first bitter compounds to dissolve and end up in our cup. They have good solubility in water (while acids and sugars have very high solubility). Lactones contribute to the balance being sought in a well brewed coffee. We will always have them in our brews.

This means that the SSC Brewer will have as much or as little lactones as any other brewer, as the skew towards a higher volume flow rate at the end of the brew will not have an impact – lactones are not late extraction compounds. They tend to appear in the first half of a pour-over extraction (first 60–90 seconds in a 3 min brew).

Late Extraction

Phenylindanes are late extraction compounds. They are much less soluble in water than lactones.

Counter-intuitively, water that has already dissolved other compounds is actually better at extracting phenylindanes than clean water. This happens because clean hot water is highly polar and initially dissolves polar compounds first—organic acids, caffeine, and sugars extract readily in the early brewing stages. As these polar solutes accumulate in the water, they disrupt water's structured hydrogen-bonding network, fundamentally changing the solution's chemistry.

This disruption creates a more favorable environment for dissolving hydrophobic compounds like phenylindanes. Since phenylindanes are partly hydrophobic and aromatic, they resist extraction into pure water. However, once the brewing water becomes loaded with polar solutes, the altered solvent environment can more easily accommodate and stabilize these less-polar molecules. This explains why phenylindanes extract disproportionately during the late phases of traditional brewing methods—during long drawdowns, immersion methods, or over-extraction—when the water is already saturated with other coffee compounds. The solution essentially becomes a better solvent for hydrophobic molecules as it becomes "dirtier" with polar solutes.

Pure Percolation

This “dirty” water fact is a major reason that we don’t get a bitter tail end effect when brewing coffee with the SSC Brewer, despite the bias of volume flow towards the end of the brewing process. 

In the SSC Brewer, the water sits above the coffee bed without creating a coffee slurry. Aside from sugar and acid dissolving and transporting through diffusion, the water stays pretty clean – and therefore unable to easily extract phenylindanes.

Accelerating Flow Rate

It’s likely that there is a secondary factor limiting the extraction of phenylindanes – the increasing flow rate towards the end of the brew.

A longer contact time of the water molecules in the coffee bed helps with the slower extraction of the larger phenylindane molecules. With a typical SSC Brew the water dwell time in the latter part of the brew has dropped to around 15 seconds.

Conclusion

The SSC Brewer's unique design achieves what seems paradoxical: despite having about 70% of its flow occur in the final third of brew time, it can sometimes produce a less bitter cup than traditional brewers. This is made possible by two key mechanisms working in tandem.

First, the brewer maintains clean water above the coffee bed rather than creating a slurry, which means the water remains highly polar throughout the brewing process. This clean water environment is inhospitable to phenylindanes due to their hydrophobic nature, preventing their extraction even during the high-flow late stage. Second, the accelerating flow rate in the latter part of the brew reduces contact time to around 15 seconds, giving these slower-extracting molecules insufficient opportunity to dissolve.

By functioning as a pure percolation device, the SSC Brewer avoids the "dirty water effect" that features in traditional pour-over methods or immersion brewing. The result is a brew that extracts desirable compounds while leaving a lot of the harsh phenylindanes behind in the coffee bed—challenging conventional wisdom about extraction dynamics and demonstrating that water chemistry and contact time are important factors when it comes to managing bitterness with medium and dark coffees.