Advanced Coffee Making Lecture at Assembly Coffee London

Summary

The lecture argues that better coffee comes not just from adjusting strength or measuring extraction, but from making extraction more even across coffee particles. The speaker presents this as a way to reduce harsh or generic flavors while making a coffee’s distinctive qualities clearer and sweeter.

Some claims—especially the idea that coffee is rarely over-extracted—are presented as the speaker’s developing interpretation and experiments, not as settled scientific consensus.

1. Strength and extraction are different

• Strength refers to the concentration of dissolved coffee in the drink, commonly measured as TDS. Filter coffee is typically much weaker than espresso.
• Flavor intensity is not the same as strength: a coffee can have an intense flavor while still being weak or mild.
• Strength is relatively simple to adjust:
  • Use more coffee to make a brew stronger.
  • Add water to make it milder.
• Bitterness can sometimes be a response to excessive strength. The speaker suggests diluting the coffee and tasting again: if the bitterness remains, it may come from something other than strength.

2. Extraction yield and taste

Extraction yield is the proportion of the dry coffee dose that dissolves into the drink. The speaker associates:

• Low extraction with sour, vegetal, salty, or nutty notes, and a cup that seems disappointing or incomplete.
• Excessive extraction with dryness, astringency, dustiness, or an empty, powdery quality.
• Good extraction with sweetness, balance, aroma, nuance, and transparency.

The speaker emphasizes sweetness as a useful target. In their experience, more even and improved extraction often makes coffee taste sweeter.

A refractometer can provide a useful average extraction measurement, but it cannot show how evenly the individual particles extracted.

3. Uneven extraction as the central problem

The lecture’s main explanation is that a brew contains particles of different sizes, and they do not extract equally:

• Fine particles have a large surface area relative to their mass and can extract quickly.
• Coarser particles are harder for water to penetrate. Water may extract their outer layers while leaving their centers less extracted.
• The resulting cup can combine under-extracted and more-extracted flavors. An average measurement may conceal this mixture.

The speaker argues that improving uniformity has a “double benefit”: it removes unwanted flavors and allows more of the coffee to contribute desirable ones. More even extraction can therefore make a coffee seem sweeter, fuller, more aromatic, and more distinctive.

4. Sources of irregularity

The speaker describes several stages that can introduce variation and make coffee harder to extract evenly:

• Growing and sourcing: Regional lots may combine farms, altitudes, varieties, soils, and growing conditions. Smaller lots may show a more distinctive character, though the speaker does not claim they are automatically better.
• Processing: Harvesting, sorting, fermentation, and especially uneven drying can change how beans extract.
• Shipping and aging: Heat, humidity, water activity, and age can introduce differences.
• Roasting: The speaker criticizes drum roasters as difficult to control uniformly and argues that uneven development between the inside and outside of a bean can create inconsistent solubility.
• Blending: Coffees with different densities or solubilities may not roast or extract alike, particularly if mixed before roasting.
• Grinding: Particle-size variation is presented as a major source of unevenness and one that baristas can often influence directly.

5. Grinding and extraction

The speaker describes experiments in which different particle-size fractions were brewed separately in hot water. In the reported results:

• Fine particles extracted quickly.
• Medium particles took longer to approach similar extraction.
• Larger particles remained less extracted, even after a much longer steep.

The speaker interprets these results as evidence that water penetrates only a limited distance into a particle during typical brewing. They estimate roughly 100 microns in the particular experiment, while stressing that coffee and roast differences may change the result. They argue that larger particles can lower average extraction because their interiors remain relatively untouched.

The lecture also challenges the common assumption that rising extraction necessarily leads to over-extraction. The speaker’s position is that uneven extraction can make attempts to increase the average taste worse, because some parts may already be over-extracted while others remain under-extracted. In their view, more uniform extraction creates more room to raise extraction successfully.

The practical target is to grind finer and increase extraction only until the brew reaches its best point. If grinding finer causes flow to stall or extraction to fall, the brewer has passed the useful maximum for that setup and should back off.

6. Practical recommendations

General brewing

• Wet all grounds promptly and evenly.
• Stir or agitate when appropriate to help all particles become wet. The speaker says brief stirring does not automatically cause over-extraction.
• Watch for bubbles appearing later in brewing; the speaker treats them as a sign that some grounds may not have been wetted initially.
• Do not grind so fine that the brew becomes choked or passes its extraction maximum.

Pour-over and filter coffee

• The speaker favors flat-bed brewers over conical ones, reasoning that water has less incentive to favor one route through the coffee.
• Keep grounds away from the sides of the brewer.
• Avoid an excessively high water level, which may encourage water to pass through the filter rather than the coffee bed.
• Keep pouring height and agitation consistent.
• For batch brewers, keep water flowing and the level just above the grounds rather than flooding the basket. The speaker says batch brewing may require a coarser grind because it lacks pressure to push water through a fine bed.
• Weigh the water actually dispensed by batch machines rather than trusting the displayed amount. Take repeated measurements and use an average.
• If practical, sieve out particles larger than about 500 microns and regrind them where the grinder permits.

Immersion brewing

• Pour water in quickly enough to mix the grounds, then stir.
• Avoid creating a sustained swirl. The speaker suggests that spinning can make particles move and extract unevenly.

Espresso

• The speaker recommends VST baskets and a flat tamper, arguing that a curved tamper may contribute to uneven flow.
• Keep the grinder hopper filled, since the speaker says a changing depth of coffee in some grinders may change the particle distribution.
• Distribution methods remain under investigation. The speaker reports that a distribution tool and tapping the basket appeared similarly effective in tests, but says consistency across baristas still needs testing.
• A prototype “lateral tamping” tool was described as promising in preliminary tests, but the speaker explicitly says it was still being evaluated.

7. Other advice from the Q&A

• Water: Experiment with different water recipes and mineral combinations; the speaker describes this as something best learned through controlled tasting.
• Resting coffee after roasting: Use taste rather than a fixed number of days, since different coffees degas differently.
• Water temperature: Use water as hot as possible while the coffee still tastes good.
• Grinder alignment: The speaker recommends aligning EK43 burrs when shots run too quickly or the grinder produces an inconsistent particle mix.
• Research: The speaker says Barista Hustle’s experiments are intended to move quickly and are not always formally peer-reviewed. They describe the findings as preliminary and open to further testing.

8. Education and Barista Hustle

The speaker briefly promotes Barista Hustle’s online education plans, including courses on extraction, espresso, filter coffee, and coffee-business finances. They describe the courses as detailed, updated in response to student questions, and part of a shift from publishing primarily free blog material to offering paid online education.

Speakers and sources mentioned

• Main lecturer: Unnamed in the subtitles; addressed once as “Matt.”
• Audience members: Several unnamed attendees ask questions during the lecture.
• George Howell / George Howell Coffee: Cited for the “clean or dirty window” analogy for transparency in coffee.
• Jeremy Challenger of Proof Rock: Cited for an analogy comparing drum roasting to placing a fragile vase on a high shelf.
• Barista Hustle: The lecturer’s organization and the source of the experiments and courses discussed.
• Other named organizations or products: VST, Onyx, UC Davis, and the Specialty Coffee Association (SCA).