Coffee seeds have many different compounds inside them. Using water, our goal is to extract just the right amount of them to get a cup that is tasty, balanced, and not too strong or weak. Extraction yield is communicated as a percentage and refers to how much of the coffee seed ends up in the finished product (herein referred to as “the cup”).
About 30 percent of roasted coffee is water soluble; target extraction ranges between 18 and 22 percent. Finding the balance between extracting enough of coffee’s soluble compounds to deliver complex flavor, yet not going so far as to overextract, is challenging—and where the barista sets herself apart from the home brewer. This process is tied to a number of variables. Here are a few basics to keep an eye on.
VARIABLES TO CONSIDER:
Freshness: Freshly roasted coffee necessarily undergoes a period of aging that allows it to degas. Gases from the roasting process eventually exit the pores of the bean, making it more permeable. Coffee that has not degassed sufficiently will be less permeable and difficult to extract.
Water type: Water quality varies greatly depending on geographical origin, local water treatment, and in-house filtration. Calcium, alkalinity, pH, and sodium—as well as total dissolved solids (TDS)—impact how soluble compounds are extracted from coffee, so it’s important to know what’s in your brewing water.
Brew ratio: The ratio of coffee grounds to water; or, how much coffee is used for a given quantity of water. This may be expressed in units of either grams or ounces. A 1:16 coffee-to-water ratio is commonly recommended as a starting point.
Grind size: Grind size and burr setting directly correspond to surface area. Generally, increased surface area means increased extraction potential. A whole bean has very little surface area compared to a bean that is broken up, which is why we brew with ground coffee. The smaller, or finer, the grind size, the more surface area there is and the easier it is for water to extract flavor. The opposite is true for larger, or coarser, grind sizes. In addition to drastically affecting the coffee’s solubility, grind size also affects flow rate (think water flowing through boulders versus sand), which will in turn affect water contact time.
Temperature: Hot water extracts coffee flavor faster than cold water. That’s why cold brewing takes hours while hot brewing takes minutes. However, too-hot water can burn coffee and lead to overextraction. The industry generally considers the ideal brewing temperature to be between 195 and 205 degrees Fahrenheit. Temperature stability throughout the length of the brew cycle is desirable for replicable results. Just as baking foods at different temperatures will result in drastically different results, different brew temperatures will extract coffee differently.
Contact time: The length of time water and grounds are interacting. Too short = underextracted; too long = overextracted.
Bloom: The initial wetting of the grounds. This phase prepares particles for extraction of solubles, both by helping to release CO2, and by allowing subsequently added water to move more freely through the bed of grounds (an even pre-wetting should result in a more even extraction).
Strength/(TDS): TDS refers to the solids dissolved in the cup, which does not include crema or oils formed on the surface. This is measured with a refractometer. TDS translates to concentration of dissolved coffee solids in the cup and is described in terms of strength. The higher the concentration of TDS, the stronger the cup. The lower the concentration of TDS the weaker the cup. In practice, strength is determined by how the coffee feels in your mouth. Strong cups feel thick or heavy while weak cups feel thin or watery.
Agitation: This is the word we use for creating turbulence in the coffee (with a spoon or with the water stream from the kettle). Agitation directly relates to the rate of extraction, so this variable should be kept consistent from brew to brew.
Flow rate: the rate at which water flows through coffee.
—Jennifer Haare is the director of training and staff development at Ipsento Coffee.