| # | Description | Filter | Grind | Disp. Time | TDS (ppm) | PE (%) |
|---|---|---|---|---|---|---|
| 1 | Aeropress – Gentle Pressure | Paper | Medium-fine | 4:53 | 803 | 1.204 |
| 2 | Aeropress – Strong Pressure | Paper | Medium-fine | 4:10 | 732 | 1.098 |
| 3 | Aeropress – Gentle, Double Grounds | Paper | Medium-fine (×2) | 4:57 | 1070 | 1.605 |
| 4 | Aeropress – Coarse Grind | Paper | Coarse | 4:58 | 545 | 0.818 |
| 5 | Aeropress – Medium Pressure | Paper | Medium-fine | 4:28 | 690 | 1.035 |
| 6 | Aeropress – Metal Filter | Metal | Medium-fine | 4:40 | 781 | 1.172 |
| 7 | French Press | Metal mesh | Medium-fine | 4:05 | 1120 | 1.680 |
| 8 | No Filter (Decanted) | None | Medium-fine | 4:05 | 1330 | 1.995 |
Lab 6 Report
Results & Discussion
All eight brews used 85 °C water, a 15 g : 225 g coffee-to-water ratio, and a 4-minute pre-extraction soak. TDS values originally recorded in ppt (Trials 3, 7, 8) were converted to ppm (×1000) for consistency. Percent extraction (PE) was calculated using the formula below, which simplifies to PE = TDS × 0.015 % at the fixed 1:15 brew ratio used throughout:
\[ \text{PE} (\%) = \frac{\text{TDS (ppm)}}{1{,}000{,}000} \times \frac{\text{brew mass (g)}}{\text{coffee mass (g)}} \times 100 \]
The full trial summary is presented in Table 1. TDS ranged from 545 ppm (Trial 4, coarse grind) to 1330 ppm (Trial 8, no filter), and PE tracked TDS directly given the constant brew ratio.
Effect of Applied Pressure on Flow Rate, TDS, and PE
The first manipulation was applied pressure. Trials 1 and 2 used identical conditions (medium-fine grind, paper filter, 225 g of 85 °C water, 15 g coffee), except that Trial 1 used a gentle, slow press and Trial 2 used a forceful one. As Figure 1 shows, higher pressure directly shortened dispensing time (from 293 s to 250 s), and TDS fell with it: from 803 ppm to 732 ppm, and PE from 1.205 % to 1.098 %. The positive slope of the trend line across all six Aeropress trials confirms this general relationship: longer contact during dispensing yields slightly more dissolved solids.
This behaviour is consistent with Darcy’s Law, as a higher applied pressure gradient (ΔP) across the filter bed increases volumetric flow rate, shortening the time each parcel of water spends in contact with the grounds. With less contact time per unit volume, solute transfer is incomplete and TDS is lower. The effect is moderate because the 4-minute pre-extraction soak already drives the majority of diffusion-driven extraction. The pressing phase primarily controls the draining rate, not total extraction yield.
Effect of Ground Mass and Grind Size
Doubling the grounds bed (Trial 3) and coarsening the grind (Trial 4) both perturb the Darcy resistance of the packed bed, though in opposite ways. Figure 2 makes both effects immediately visible, where Trial 3 is the second-highest TDS of any Aeropress brew (1070 ppm), while Trial 4 is the lowest across all eight trials (545 ppm).
For Trial 3, the denser puck provides more solute mass per unit volume of water, driving up concentration despite a minimally changed dispensing time (4:57 vs. 4:53 in Trial 1). This suggests the paper filter, not the grounds bed, was the primary flow-limiting bottleneck at this pressure, so doubling the bed length had little hydraulic effect while still increasing extraction yield.
Coarse grinding (Trial 4) had the opposite outcome: TDS dropped to 545 ppm despite a near-identical dispensing time (4:58). A coarser grind reduces total particle surface area and increases inter-particle void space, both lowering the rate of solute diffusion out of the particle interior. Because the dispensing time is unchanged, the low TDS reflects reduced extraction efficiency rather than faster flow, since the water passes through at the same speed but picks up less along the way.
Variable with the Most Pronounced Effect on TDS
Before moving to filtration, it is worth summarising the relative magnitude of each variable’s effect. Figure 3 reinforces what Figure 2 already suggests: the filtration trials (5–8) dominate the range of outcomes, compared to the pressure and grind-size effects seen in Trials 1–4.
Pressure variation across Trials 1–2 produced only an ~8–9 % change in TDS. Grind size (Trials 1 vs. 4) produced a ~32 % change. Filtration method, by contrast, spans from 690 ppm (Trial 5, paper) to 1330 ppm (Trial 8, no filter), which is a ~93 % difference. Filtration is therefore the dominant variable. The paper filter physically retains micro-fines, colloidal coffee particles, and emulsified lipids, species that contribute substantially to TDS readings but would otherwise pass freely into the cup. Removing this colloidal fraction compresses TDS far more than any adjustment to pressure or grind size within the ranges tested.
Effect of Filtration Method and Microscopy
Stepping through Trials 5–8 in order of increasing filter permeability — paper → metal → French press mesh → none — reveals a hierarchy in both TDS and PE, visible in Figure 2 and Figure 3 above. Paper (690 ppm, PE = 1.035 %) gives the cleanest, least concentrated brew; no filtration (1330 ppm, PE = 1.995 %) gives the richest (but not the tastiest!). The ~640 ppm spread across just this one variable is larger than any other single manipulation in the experiment.
The microscopy observations show this as well. In the French press and unfiltered samples, a pipetted drop shows irregular, angular coffee colloids (fragments of cell-wall material, oxidized phenolic aggregates) alongside perfectly spherical oil droplets indicative of emulsified lipids. For the finest colloidal particles, Brownian motion is detectable. A paper-filtered brew, viewed under the same conditions, appears nearly transparent with only a few colloids, confirming that the filter physically excluded a large amount of this fraction. The ~91 ppm TDS gap between the metal-filter and paper-filter Aeropress brews (Trials 5 and 6) is therefore attributable not purely to dissolved small molecules, but in large part to the colloidal and lipid load that a metal filter permits to pass and a paper filter does not.
Taken together, the results show that filtration operates on a fundamentally different mechanism than pressure or grind adjustment. Pressure and grind size modulate the rate and completeness of solute diffusion out of the grounds, which are continuous, physically predictable processes governed by surface area and contact time. Filtration, by contrast, sets a hard size threshold, where particles either pass the filter or they do not. This discrete, categorical action on the colloidal fraction explains why its effect on TDS is much greater than that of flow rate and surface area.
Appendix: Raw Data
| # | Description | Filter | Grind | Coffee (g) | Water (g) | Disp. Time | TDS (ppm) | PE (%) |
|---|---|---|---|---|---|---|---|---|
| 1 | Aeropress – Gentle Pressure | Paper | Medium-fine | 15 | 225 | 4:53 | 803 | 1.204 |
| 2 | Aeropress – Strong Pressure | Paper | Medium-fine | 15 | 225 | 4:10 | 732 | 1.098 |
| 3 | Aeropress – Gentle, Double Grounds | Paper | Medium-fine (×2) | 15 | 225 | 4:57 | 1070 | 1.605 |
| 4 | Aeropress – Coarse Grind | Paper | Coarse | 15 | 225 | 4:58 | 545 | 0.818 |
| 5 | Aeropress – Medium Pressure | Paper | Medium-fine | 15 | 225 | 4:28 | 690 | 1.035 |
| 6 | Aeropress – Metal Filter | Metal | Medium-fine | 15 | 225 | 4:40 | 781 | 1.172 |
| 7 | French Press | Metal mesh | Medium-fine | 15 | 225 | 4:05 | 1120 | 1.680 |
| 8 | No Filter (Decanted) | None | Medium-fine | 15 | 225 | 4:05 | 1330 | 1.995 |
| Notes: | ||||||||
| All brews: 85 °C water, 4-minute pre-extraction soak. Trials 1–6: Aeropress. Trial 7: French press. Trial 8: no-filter decant. Grind: medium-fine for Trials 1–3, 5–8; coarse for Trial 4. |