Brewing control chart

Two-dimensional chart (TDS on y-axis, extraction yield on x-axis) defining the ideal zone of well-extracted coffee (TDS 1.15-1.35%, EY 18-22%). Developed by E.E. Lockhart (MIT, 1950s), standardised by SCAA. Diagnostic tool to identify under-extraction (low EY) or over-extraction (high EY).

Background & Context

The Brewing Control Chart is a graphical tool developed by coffee scientist Ernest Lockhart at MIT in the 1960s and later adopted and refined by the Specialty Coffee Association. It maps the relationship between extraction yield (EY, the percentage of coffee mass dissolved into the brew) on the X-axis and Total Dissolved Solids (TDS, the strength of the final cup) on the Y-axis, defining a central 'ideal' zone — the 'Golden Cup' — that represents the most palatable combination of both variables according to trained panel tests. For filter coffee, the SCA ideal zone is: TDS 1.15–1.35% and EY 18–22%. The chart visually shows that it is possible to be within the ideal strength range but still under-extracted (sourness, lack of complexity) or over-extracted (bitterness, astringency), and vice versa. A refractometer is required to measure TDS and calculate EY. The Brewing Control Chart has been criticized by some contemporary coffee researchers (including Scott Rao and Jonathan Gagné) for being developed primarily on commercial-grade, dark-roasted coffees and not necessarily representing the optimal zone for modern specialty light roasts — which often taste best at higher EY values (up to 24%) that fall outside the original 'ideal' zone.

Practical Use

To use the Brewing Control Chart practically: measure your coffee's TDS with a refractometer after brewing, then use the formula EY = TDS × (water used / coffee dose) to calculate extraction yield. Plot both on the chart. If you are in the ideal zone and the coffee tastes bad, the chart is telling you the issue is in the coffee quality, not the brewing. If you are outside the zone but the coffee tastes great, trust your palate — the chart is a guide, not a law.

Related Terms

Related terms: TDS — one axis of the chart. Extraction yield — the other axis. Brew ratio — the main control variable for chart navigation. SCA — the organisation that maintains the standard.