Unlike wine where the winemaker has complete quality control up to the time of bottling – coffee requires that the consumer use a bit of their own craftmaship during preparation. There are a number of variables that can ultimately ruin a cup of coffee – temperature, weight, extraction time, etc.
Luckily, much of guess work in preparing a good cup of coffee has already been investigated by the Coffee Brewing Center (CBC) during the 1960’s. Led by Dr. Earl Lockhart, his research has become thee body of knowledge in truly understanding the physics and science behind coffee brewing.
Before discussing the fundamentals its important to define some basic brewing terminology, namely: strenght, extration and brew formula. Together these three variables make up the bulding blocks of what creates aroma, taste and body in coffee.
Strenght refers to the measure of solubles present in the beverage and usually expressed as a percentage of the flavoring material to water.
Extraction or sometimes referres to as “solubles yield” – refers to the amount of solubles extracted from the bean itself and also expressed as a percetage.
Brewing formula refers to the ratio of water used per quantity of coffee.
Brewing Control Chart
Much of Dr. Lockhart’s work at the CBC was dedicated to the creation of the “Coffee Brewing Control Chart” which is still in use today. In essence, the brewing control chart provides a graphical representation of strength, extraction and brew formula in an easy to read format.
By measuring the soluble coffee flavoring content in brewed coffee relative to brew formula, the CBC was able to graphically represent “solubles yield” given the coffee to water ratio. As such, the work has provided the industry with the framework in which to discuss and compare coffee quality.
To determine any one of these variables simply select a brew ratio (depicted as diagonal lines on the chart) and beverage strenght or extraction can easily be extropolated. Well discuss this in greater detail below, but for now, one important thing to remember is that, according to the CBC, the best cup of coffee is obtained when approximately 18-22% of the flavoring material is extracted from the bean. As such, this infsion would represent a beverage with a corresponding strenght of 1.15-1.35%. When both these factors are met, the beverage is said to be at its “Optimum Balance” and is depicted as a grey box in the center of Brewing Control Chart.
|SCAA Brewing Control Chart|
|Click for larger view (pdf )|
Strenght (solubles concentration)
The amount of flavoring material contained in a beverage will vary depending on brewing method used. Typically a drip cup of coffee contains roughly 1.2% flavoring material and 98.8% water. Whereas, a typical espresso will contain on average a flavoring concentration of 1.8-2.2% and ~98.2% water. Making coffee is a potent flavoring material.
Lucklily, strenght measurements can be made realatively easily using a number of different instruments. The most common include the use of hydrometers, conductivity, brix and and moisture microwave. Since their relatively cheap, TDS or – total dissolved solids – are the most commonly used in the industry. For all practical purposes, a TDS meter is just a customized conductivity meter that relates the amount of coffee flavoring material based on its conductivity across a coffee infusion. For example, a reading of 1800 TDS refers to corresponding flavoring material content of 1.8% and 98.2% water on the Brewing Control Chart.
|Figure 1: Strenght|
|Click for larger view|
Extraction (solubles yield)
Approximately 28% of the organic and inorganic material in a coffee bean is water soluble. The remaining 72% of the material consists on insoluble cellulose which serve as important structural components for the bean. Upon the addition of hot water much of the soluble material readily dissolves to create important flavor and aroma compounds in the cup.
As shown in Figure 2, a 20% extraction (shown at the bottom of the chart) indicates that 20% of the soluble flavoring material was dissolved in water. For example, if 10 grams of ground coffee was used in brewing, the bean would have lost 2 grams of soluble material to water. Because the rate at which these soluble compounds dissolve vary, changes to grind, water temperature, brew time, coffee weight and brew equipment will invariably produce vastly different beverages. For example, extractions below 16% produces a beverage with a weak peanut-like flavor, while extractions over 24% producing over-extracted bitter charateristics. Well discuss this subject in other sections later.
For now, the key is to brew coffee within the “Optimal Balance” having 1.15-1.35% coffee flavoring material.
|Figure 2: Extraction|
|Click for larger view|
Although it may seem intimidating at first, using the Coffee Brewing Control Chart will provide for the basic groundwork in which to ensure consistent beverage preparation.
Lingle, T. The Coffee Brewing Handbook. SCAA