All the Random Variables in Pour-Over Coffee

Many brewers experience this:

Same recipe, same dripper, same grinder — different cup.

This often leads to claims that pour-over coffee is “mysterious” or “unpredictable.”

From an engineering perspective, these are not mysteries — they are unaccounted variables.

What Are Random Variables?

In pour-over coffee, random variables are factors that:

• Occur in every brew
• Are difficult to observe or control
• Have a real impact on extraction

They are not uncontrollable — just often ignored.

Variable 1: Flow Rate Is Never Constant

Flow rate changes continuously due to:

• Coffee bed compaction
• Fines migration
• Filter paper deformation
• Air exchange instability

This means early and late extraction phases behave very differently.

Variable 2: Filter Paper Geometry

Stacked filter papers often vary in:

• Fold angle
• Elastic rebound
• Wall contact area

Small geometric differences alter internal flow paths.

Variable 3: Unpredictable Fines Behavior

Fines migrate, accumulate, and sometimes suddenly clog pathways.

This explains why drawdown often slows dramatically near the end.

Variable 4: Air Exchange

Water cannot flow unless air can escape.

Restricted air paths cause pressure imbalance and unstable extraction.

Why Experience Sometimes Fails

Experience assumes system stability.

When the system itself fluctuates, technique alone cannot guarantee consistency.

From Mystery to Engineering

Consistency is achieved not by endless technique adjustment, but by:

• Reducing structural uncertainty
• Stabilizing flow paths
• Constraining variables

What seems like “mystery” is often just unexplained science.

— Vihi Coffee Research Lab