Science
Extraction science: solubility, diffusion and total dissolved solids
A deeper look at what 'extraction' physically means, beyond the practical sour/bitter heuristic.
Solubility and diffusion
Extraction is driven by diffusion: soluble compounds move from inside the coffee particle into the surrounding water, driven by a concentration difference. Grind size affects how far compounds must diffuse; smaller particles have a shorter path and dissolve faster, which is why grind size has such a large effect on extraction speed.
Total dissolved solids (TDS)
TDS measures the concentration of dissolved material in a brewed coffee, usually as a percentage, and is one input into calculating extraction yield (the percentage of the original dry coffee mass that ended up dissolved in the cup). Measuring TDS requires a refractometer; without one, taste remains a perfectly valid way to judge a brew.
Limitations of these metrics
TDS and extraction yield are useful for diagnosing and repeating results, but they do not capture everything about a cup's quality — two brews with identical TDS can taste quite different depending on which specific compounds extracted. Treat these numbers as one tool among several, not a complete description of taste.