Agitation, turbulence, and channeling

How movement of water through coffee grounds affects extraction uniformity, flavor, and potential defects.

Article 5 of 12 in Brewing Physics & Extraction Theory/

Agitation, Turbulence, and Channeling

Agitation:
Manual or mechanical stirring of coffee during brewing.
Purpose: Breaks up crusts or clumps, promotes uniform extraction.
Examples: Stirring a French press after bloom, swirling AeroPress.
Turbulence:
Natural water movement during pour-over or espresso.
Small-scale eddies improve contact between water and coffee particles.
Prevents stagnant zones where under-extraction may occur.
Channeling:
Occurs when water finds preferential paths through the coffee bed.
Leads to uneven extraction: some areas over-extracted, others under-extracted.
Causes: uneven tamping, coarse clumps, bed cracks, high flow rate.
Key Implications:
Proper agitation reduces channeling and improves flavor balance.
Controlled turbulence is beneficial; excessive disturbance can over-extract.
Channeling is most critical in espresso due to short contact time and high pressure.
Best Practices:
Even distribution of grounds.
Bloom and stir in pour-over for uniform saturation.
Avoid cracking the bed or over-tamping.
Monitor flow rate to prevent fast water paths.
Outcome:
Well-managed agitation and controlled turbulence lead to consistent extraction, balanced flavor, and fewer defects.