Coffee extraction

Key concepts for coffee extraction include:

Extraction Yield

Extraction yield refers to the solubles dissolved during brewing. This is often expressed as a percentage of the coffee's mass. It is also known as solubles yield or simply extraction. The extraction yield percentage describes the mass transferred from the grounds to the final beverage, expressed as a percentage of the initial mass of the grounds. It is given by the following equation:

$${\displaystyle Y={\frac {M_{2}\cdot t}{M_{1}}}}$$

where ${\displaystyle Y}$ is the extraction yield expressed as a percentage, ${\displaystyle t}$ is the total dissolved solids expressed as a percentage of the final beverage, ${\displaystyle M_{1}}$ is the mass of the grounds in grams, and ${\displaystyle M_{2}}$ is the mass of the final beverage in grams. For example, an extraction yield of 20% can be obtained by brewing 18 grams of coffee, yielding a 36-gram final beverage with a ${\displaystyle t}$ of 10%.

Total Dissolved Solids

Expressed as a percentage of the mass of the final coffee beverage, or, alternatively, in parts-per-million (ppm).

Strength

Also known as solubles concentration, most closely associated with Total Dissolved Solids. A higher concentration of solubles is associated with a stronger beverage, and lower concentration is associated with a weaker, or more "watery," beverage.

Brew ratio

The brew ratio describes the ratio of the mass of coffee grounds to the volume of water used. This may be expressed in grams per litre. The following identity describes the relationship between strength and brew ratio.

$${\displaystyle {\frac {t}{V}}={\frac {M}{V}}\times {\frac {t}{M}}}$$

where ${\displaystyle t}$ is the total dissolved solids expressed as a percentage of the mass of the grounds, ${\displaystyle V}$ is the volume of the water used, and ${\displaystyle M}$ is the mass of the grounds. The strength of a coffee beverage can therefore be defined as the product of the brew ratio and the extraction percentage.

External images
SCAA brew chart (American)[1]
SCAE brew chart (European)[2]
NCA brew chart (Norwegian)[3]

Brewing control chart

During brewing, different solubles may be extracted, depending on factors such as extraction time – some compounds are extracted only in the initial stages of extraction, and some only in the later stages – and temperature.

A diagram called a brewing control chart can be used to visualize a coffee beverage's degree of extraction and strength. In it, degree of extraction is plotted on the horizontal axis, and strength on the vertical axis. The optimal ratio between extraction and strength is represented by a rectangle in the center of the chart – within that range, coffee is neither over- nor under-extracted, and is neither too strong nor too weak. The beverage can also be represented as a point along a diagonal line on the brewing control chart. Along this line, extraction and strength are directly proportional.

An extraction yield of 18% to 22% and a strength of 1.15% to 1.35% is considered ideal for American coffee beverages. For Norwegian beverages, the ideal strength is considered to be 1.30% to 1.50%. For European coffee beverages, the range is considered to be 1.20% to 1.45%.

These ratios can be applied across most brewing methods, with the exception of the espresso method. Espresso is commonly much stronger, with far more variability in extraction – dark roasts often taste much stronger than medium roasts (standards are often based on medium roasts), and achieving an equivalent strength requires the use of a lower brewing ratio for darker roasts.

Commonly, an extraction yield of 18% to 22% is desirable for traditional coffee beverages. In 1952, Dr. E. E. Lockhart – head of the Coffee Brewing Institute at MIT – published research suggesting that a 20% extraction yield was ideal. This research has subsequently been reproduced by the Specialty Coffee Association of America (SCAA).

Yields of under 18% are considered under-extracted, or under-developed – desirable compounds have not been sufficiently extracted. The resulting beverage is "unbalanced," and often associated with a predominantly sour taste – acids are extracted early in the brewing process, while balancing compounds such as sugars and bitter substances are extracted later.[4]

Yields of over 22% are considered over-extracted and are often associated with a predominantly bitter taste, as bitter compounds are extracted after acids and sugars have largely dissolved. However, in certain situations, yields surpassing 22% can be desirable, and absent the characteristic bitterness.[5]

Yields depend primarily on temperature, brew time, and grind size, and in a complex way on method. Yield is inversely proportional to grind size – a smaller grain size produces more surface area, and faster extraction – and increases roughly linearly with brewing time. Thus, for a given temperature and grind size, an ideal brewing time can be determined. A French press is associated with a large grain size in conventional brewing, and a comparatively long brew time, commonly 3–4 minutes. Filter coffee is associated with a smaller grain size and shorter brew time. Espresso is associated with extremely fine grain size and short brew time, often lasting only 20–30 seconds.

Caffeine is extracted relatively early in the brewing process, so higher yields do not result in significantly more caffeinated coffee, and often lead to over-extraction.

Brewing methods

Once the ideal yield has been reached, the grounds must be removed from the beverage, halting further extraction. For this reason, coffee in is not typically left in the brewing chamber of a French press after a desirable extraction has been achieved. Percolators are commonly considered less-than-ideal for proper coffee extraction, due to a design flaw that causes them to pass coffee through the basket of grounds multiple times, making them prone to over-extraction.

Coffee may be over-extracted to achieve the desired strength while minimizing the amount of ground coffee required. However, this often results in a bitterer, less full-bodied beverage.

Temperature

Proper water temperature – a subtler but still important component of proper extraction – can improve the degree to which desirable solubles are extracted. The most commonly recommended brewing temperature for traditional coffee beverages is 91–94 °C (195–202 °F), which maximizes extraction of desired compounds and minimizes extraction of undesirable compounds.[6] Note, the recommended temperature range is slightly below the boiling point of water (100 °C or 212 °F at standard pressure). When brewing, it is commonly not recommended to pour water into coffee grounds immediately after it has reached the boiling point, and instead to briefly let it come off the boil. Heat loss during brewing may also occur – in the manual pour-over method, for example, the mixture of coffee grounds and water, or "slurry," is prone to heat loss, and high temperatures may be difficult to maintain.[6]

The impact of transient temperature – the temperature of the final coffee beverage after brewed is finished – is not as significant. If coffee is heated to boiling point only very briefly, the taste will be not be significantly affected – however, the longer it is kept at a high temperature, the more the taste is impacted.

Strength refers to the amount of dissolved coffee solids per unit of coffee liquid volume. An espresso, specifically a ristretto, is the strongest of coffee drinks, meaning that it takes the same amount of coffee solubles and dissolves it in significantly less water. For example, a 20% extraction of 15 grams of espresso in a double shot (60 g, weight of espresso) yields 3 grams per 60 g, or 5% strength, which is 3–4× typical brewed coffee strength.

Adding water to dilute a drink after brewing affects strength but not yield (as the grounds are no longer present) – an Americano differs from an espresso simply by being weaker, more dilute (American preparation may also eliminate the crema).

Stronger coffee (assuming the same extraction yield) is not more bitter than weaker coffee, but simply has more solids. This is reflected in being darker and in thicker mouthfeel (oilier), though mouthfeel also depends on suspended solids (very small grinds, so-called "fines"), particularly in French press as opposed to filter.

Unlike yield, desired strength varies measurably between regions, and, assuming proper (20%) yield, strength is determined by brewing ratio – how much ground coffee is used for a particular drink.[7] Coffee above the ideal may be considered "too strong", while coffee below the ideal may be considered "too weak", but individual tastes vary significantly.

The usual ratio of coffee to water for the style of coffee most prevalent in Europe, America, and other Westernized nations (evident in publications such as textbooks on coffee and instruction manuals for drip-brew machines) is between one and two tablespoons of ground coffee per six ounces (180 millilitres) of water; the full two tablespoons per six ounces tends to be recommended by experienced coffee lovers. A more exact weight ratio of 1:17 coffee and water is also used in publications.

The ideal brewing ratio is determined by which brewing ratio line in the control chart most intersects the ideal rectangle, or passes through the center (the subtle distinctions about exactly which line is ideal is generally considered too minor to comment, and changes brew ratio by only a small amount). This ideal ratio makes it most likely that one will land in the rectangle and near the center, and gives the widest room for error.

Extraction depends in subtle and complex ways on brewing method. Simplest is directly mixing the grounds and water, as in cupping, press pot, and vacuum brewing. Note that in Turkish coffee, the grounds are very finely ground and not removed from the water – this yields essentially 100% extraction of the solubles (over-extraction by Western standards), and suspension of the remaining insoluble parts of the grounds in the water. Significantly more complicated is brewing via soaking a column of grounds, as in filter, espresso, or percolation – in this case the water soaks and moves through the column of grounds, and (if there is water on top, as in espresso or percolation), a gradient develops over the column.