The Coffee Brewing Control Chart

Overview

Published in 1957 by Dr. E.E. Lockhart at the Coffee Brewing Institute (CBI), this foundational paper is perhaps the most important scientific document in the history of coffee brewing. It introduced the Coffee Brewing Control Chart, a visual and mathematical tool that defines the relationship between the strength of coffee, the extraction yield, and the brewing ratio.

The Problem

Before Lockhart's research, coffee brewing was largely treated as an art with subjective guidelines. There was no standardized way to measure what made a "good" cup of coffee, nor was there a mathematical model to explain how the amount of coffee grounds relates to the final cup profile.

The Methodology

Lockhart conducted extensive consumer preference panels to determine the ideal sensory profile of brewed coffee. He then used scientific instruments (such as dehydrating brewed coffee to measure the remaining solid mass) to quantify the objective parameters of those preferred cups.

He established three core variables: 1. Strength (Total Dissolved Solids - TDS): The percentage of the brewed coffee liquid that is actually coffee material. 2. Extraction Yield (EY): The percentage of the dry coffee grounds that was dissolved into the water. 3. Brew Ratio: The ratio of coffee grounds to water used.

Key Findings

Lockhart discovered a direct, linear mathematical relationship between these three variables.

$$ \text{TDS} = \text{EY} \times \frac{\text{Dose}}{\text{Yield}} $$

Through consumer testing, he identified the "Ideal" or "Optimum" balance box on the chart: * Ideal Strength (TDS): 1.15% to 1.35% (later expanded slightly depending on regional preferences). * Ideal Extraction Yield: 18% to 22%.

To hit this ideal box reliably, Lockhart's chart demonstrated that a brew ratio of approximately 55 grams of coffee per liter of water (roughly 1:18) was the most mathematically sound starting point.

Legacy and Impact

The Specialty Coffee Association (SCA) adopted Lockhart's findings to establish the "Golden Cup Standard." Over 60 years later, the Brewing Control Chart remains the universal standard taught to baristas and coffee professionals worldwide, serving as the foundational logic behind tools like the Brew Ratio Calculator.