Time’s Relationship to Extraction

Hot water removes flavoring solubles effectively and rapidly. At the beginning of the brew cycle, the resulting extract appears dense and dark caramel in color. As extraction continues, the extract becomes less concentrated and lighter in color. Near the end of the brew cycle, the extract is pale and appears almost like water.

Extraction occurs rapidly at first, and then slows. Initially, hot water contacts the grounds, displaces gases and quickly washes away most of the coffee solids on the exposed surface of the grounds. After that first phase, the bean fibers slowly absorb water and swell, hot water drives off additional carbon dioxide and volatile aromatics, and coffee solids dissolve into the brewed liquid.

Analysis of brews during thirds of the brewing cycle

First third: the extract has the best flavor with the least bitterness and astringency. It has the highest solubles concentration than at any other point in the brewing cycle. It has the lowest pH value, which translates to the greatest acidity.

Second third: the extract has average solubles concentration three times lower than the first third. Exhibits a decrease in acidity almost five times the level of the first third.

Third: the extract becomes impalitable. The desirable solubles have been exhausted, and only the solubles that produce bitterness and astringency dominate. This point – the point at which bitterness and astringency dominate – is referred to as over-extraction.

Temperature’s Effect on Taste

Second to grind, temperature has the greatest influence on the brew’s taste attributes. Sensory studies show that both acidity and body increase when coffee is brewed at higher temperatures. By contrast, lower temperature brews correspond with bitterness and astringency. The rate at which components in coffee is dissolved in water is not only dependent upon its natural characteristics, but also varies depending upon the temperature of water during extraction.

Higher extraction temperatures result in faster extraction rates because most compounds are more soluble at higher temperatures. Temperature affects flavor as well because the relative solubility of various compounds changes at different temperatures. For this reason, we brew coffee with water hot enough to achieve a temperature of 195-202. Temperatures below 195 may result in sour-tasting coffee. Temperatures above 202 may result in coffee that is bitter, astringent, acrid or too sharp.

Measuring Temperature

It is important to measure the temperature within the coffee slurry, and not simply the temperature of the water before it contacts the grounds. Various factors can lead to brewing temperatures well below the water’s initial temperature. For instance, manual drip cones and Chemex brewers lose a lot of heat to the atmosphere during brewing. Ground and brewing vessels act as heat sinks and absorb heat from the brewing water. This affect is less so if the brewing vessel is pre-heated.

Turbulence’s Influence on Brewing

Turbulence causes coffee particles to separate and thereby allow for more uniform flow of water through the past them. There are two distinct sources of turbulence: agitation by an outside force (e.g., flowing water from a pouring kettle) and the release of gases from coffee grounds. Turbulence caused by agitation can accelerate extraction and improve the uniformity of extraction. Without good turbulence, the water will fail to uniformly extract flavoring materials from all portions of the coffee bed. In this case, some portions of the coffee bed are under-extracted while other sections are over-extracted. The following are three aspects to turbulence.

1. Wettability

For extraction to occur, the coffee particles must absorb water. Wetting of coffee particles also leads to enhance uniformity of extraction. As a rule, each ounce of coffee will absorb two ounces of water (i.e., each gram will absorb 2.086 ml). Non-uniform wetting causes inconsistent extraction. it tends to create a channeling of the water flow; some portions of the coffee bed are over-extracted, due to excessive water flow, while other portions deliver under-developed flavors because the coffee particles weren’t fully extracted. This problem can be readily apparent with equipment that uses relatively short brewing times, particularly single-up brewers.

2. Control of the Bed Height

The coffee bed must be level. In gravity-fed brewing systems, it must measure 1 to 2 inches in depth. If these conditions are not met, the resulting brew will be under- or over-extracted and inconsistent. Beds that exceed 2 inches slow down the cycle and channel the water flow, creating off-flavors.

3. Water Feeding

For uniformity of extraction, the brewing water must lift and separate each coffee particle. Correct water feed depends upon the brew basket and method of water distribution. Feeding water to the ground coffee must be done uniformly and gently to ensure that the entire area of the coffee bed receives equal treatment. When manually feeding water, pour it slowly, and in a circular fashion, to guarantee complete and uniform wetting.

Utilizing Agitation

The following are simply two examples of using agitation to improve the uniformity of extraction:

Dunking the bloom of a French press to ensure that all the grounds are submerged in the slurry, rather than leaving some grounds floating at the top of the bloom where they may have less contact with the brewing liquid.

Stirring the slurry of a pourover to prevent grounds from sticking “high and dry” on the wall of the filter. These high and dry grounds extract less than grounds situated lower in the filter because they experience less turbulence during the drawdown and less contact time with the liquid.

Outgassing

The coffee bean’s structure is a porous, honeycomb-like matrix of chambers. The chambers are made of cellulose fiber coated with soluble solids and oils. These chambers trap carbon dioxide and volatile aromatics under pressure. (Some experiments have measured CO2 at 87% and volatile aromatics at 13%.)

Grinding coffee fractures some of these chambers, and results in the release of CO2 and aromatics. Studies have shown that nearly 50% of CO2 trapped within coffee beans is released within 5 minutes of grinding. The addition of hot water to the coffee bed drives much of the remaining CO2 from the grounds (because CO2 is not very soluble in hot water). This CO2 mixes with the brewing liquid as it is released and creates turbulence and the bloom seen on top of the brewing slurry.

Outgassing at the beginning of the brew cycle can improve the evenness of extraction by:

• Lifting and separating coffee particles
• Creating a more uniform concentration gradient throughout the coffee bed