The Fundamentals of Coffee Brewing Exemplify the Processes of Extraction and Dilution.
The preparation of any coffee beverage, regardless the method, involves simultaneously the processes of extraction and dilution.
Extraction is the “removal of essence” or flavoring material, referred to as soluble solids, from roasted and ground coffee through its contact with water. Immersion, filtering and percolation are the common methods practiced from the beverages earliest beginnings.
Dilution can be considered as the “coffee to water ratio” in the brewing process. The larger commercial brewers actually use a bypass for diluting the extract during brewing. This after the fact dilution allows a shorter extraction process designed to minimize the effects of over extraction and still bring the beverage into the proper strength to the most favorable taste.
The properly brewed beverage will contain approximately 1 to 1.5% coffee essences and the remainder water.
Simply put the flavor of coffee is derived from aromatic gases (aroma), liquids (taste) and solids (body). Gases trapped in suspension in the water will evaporate from the surface to give a coffee its aromatic properties. The liquid fraction is composed of soluble organic and inorganic substances inherent in the roasted coffee. Their combined effect contributes to the coffee’s taste. Solids that are insoluble or un-dissolved fiber components are suspended in the liquid coffee beverage. Some of his material is called colloids composed mainly of lipids and other oils. This is the source of a perceived body and depth to a brew.
Under normal brewing conditions approximately 20 to 25% of roasted coffee ground’s weight is removed. The remaining fraction is insoluble cellulose materials left in the filter basket.
The coffee brewing process has three distinct phases:
Wetting: First the cellular structure is softened and expanded allowing access to the solubles within the cells. Trapped gases within the cells is expanded by the heat of brewing water during the wetting stage will afford exit through the expanded pores of the compromised cellular structure.
Extraction: After wetting, the most highly soluble organic substances (principally sugars) and some inorganic minerals are adsorbed into the heated water and flushed from the bean fiber. Oils and colloids and some fines are rinsed along with the dissolved solids extract.
Over-extraction: Hydration will continue to degrade cellular structure and allow the removal of additional less soluble materials from the grounds that have a somewhat sour and more bitter taste.
The Basics of Brewing
Brewing is not rocket science and requires only common sense and attention.
The basic formula: A good cup of coffee can be brewed with 3 to 3.2 ounces of coffee grounds for a twelve-cup brewer (64 fluid ounces). Unfortunately some home brewers don’t have an adequately designed brew basket to hold the larger amount but it’s important to get as close as you can or use slightly less water.
Match the grind to the brewer type or adjust your grind to taste. Too coarse produces a thin, weak, flavorless brew. Too fine a grind produces an acrid, bitter and over extracted brew. Paper filters and screen filter may require a different grind for best results.
Home brewers should cycle in four to six minuets for a full carafe. Obviously a brew cycle ceases after the lesser amount of water is used when a half batch is exhausted. Half the grounds, half the water and half the brewing cycle may change the taste of your coffee. Home brewers are designed for full batches only.
Some coffees are more flavorful than others and if you are disappointed in a weak flavor of a particular coffee use a little more grounds or a little less water. A finer grind will make for a stronger brew but may result in a bitter over-extraction.
The correct water temperature for proper extraction is approximately 200F. Home brewers are a compromise of price and quality. Too low a temperature will always produce an under extraction and signals a trip to the appliance store. Too high a temperature is rarely the case. Nevertheless the result is the same as leaving a pot on the burner for an extended period of time, resulting in the decomposition of the organic molecules contributing to an acerbic (sour and bitter) taste and therefore adversely affecting the flavor. You will recognize the smell as soon as you walk into a mini-mart.
The thermos bottle was invented for holding hot coffee for half an hour or more
Coffee can be brewed in several different ways, but these methods fall into four main groups depending upon how the water is introduced to the coffee grounds.
Brewed coffee, if kept hot, will deteriorate rapidly in flavor, and reheating such coffee tends to give it a “muddy” flavor, as some compounds that impart flavor to coffee are destroyed if this is done. Even at room temperature, deterioration will occur; however, if kept in an oxygen-free environment it can last almost indefinitely at room temperature, and sealed containers of brewed coffee are sometimes commercially available in food stores in America or Europe, with refrigerated bottled coffee drinks being commonly available at convenience stores and grocery stores in the United States.
Electronic coffee makers boil the water and brew the infusion with little human assistance and sometimes according to a timer. Some such devices also grind the beans automatically before brewing.
Boiling was the main method used for brewing coffee until the 1930s and is still used in some Nordic and Middle Eastern countries. The aromatic oils in coffee are released at 96C (205F), which is just below boiling, while the bitter acids are released when the water has reached boiling point.
The simplest method is to put the ground coffee in a cup, pour in hot water and let it cool while the grounds sink to the bottom. This is a traditional method for making a cup of coffee that is still used in parts of Indonesia. This method, known as “mud coffee” in the Middle East owing to an extremely fine grind that results in a mud-like sludge at the bottom of the cup, allows for extremely simple preparation, but drinkers then have to be careful if they want to avoid drinking grounds either from this layer or floating at the surface of the coffee, which can be avoided by dribbling cold water onto the “floaters” from the back of a spoon. If the coffee beans are not ground finely enough, the grounds do not sink.
“Cowboy coffee” is made by heating coarse grounds with water in a pot, letting the grounds settle and pouring off the liquid to drink, sometimes filtering it to remove fine grounds. While the name suggests that this method was used by cowboys, presumably on the trail around a campfire, it is used by others; some people prefer this method. This method is still used in certain situations in Finland, Norway and Sweden, which have the highest consumption of coffee per-capita, but filter brewing is the standard method there today.
The above methods are sometimes used with hot milk instead of water.
Turkish coffee (aka Greek coffee, Arabic coffee, etc.), a very early method of making coffee, is used in the Middle East, North Africa, East Africa, Turkey, Greece, the Balkans, and Russia. Very finely ground coffee, optionally sugar, and water are placed in a narrow-topped pot, called an cezve (Turkish), kanaka (Egyptian), briki (Greek), džezva (Štokavian) or turka (Russian) and brought to the boil then immediately removed from the heat. It may be very briefly brought to the boil two or three times. Turkish coffee is often flavored with cardamom, particularly in Arab countries. The resulting strong coffee, with foam on the top and a thick layer of grounds at the bottom, is drunk from small cups. The pot is sometimes referred to as an ibrik in the West, in the mistaken belief that it is the Turkish language name for the pot.
A cafetière, or French press, is a tall, narrow cylinder with a plunger that includes a metal or nylon mesh filter. The grounds are placed in the cylinder, and boiling water is then poured into it. The coffee and hot water are left in the cylinder for a few minutes (typically 4–7 minutes) and the plunger is pushed down, leaving the filter immediately above the grounds, allowing the coffee to be poured out while the filter retains the grounds. Depending on the type of filter, it is important to pay attention to the grind of the coffee beans, though a rather coarse grind is almost always called for. A plain glass cylinder may be used, or a vacuum flask arrangement to keep the coffee hot; this is not to be confused with a vacuum brewer–see below.
Coffee bags are less often used than tea bags. They are simply disposable bags containing coffee; the grounds do not mix with the water, so no extra filtering is required.
Malaysian coffee is often brewed using a “sock,” which is actually a simple muslin bag, shaped like a filter, into which coffee is loaded, then steeped into hot water. This method is especially suitable for use with local-brew coffees in Malaysia, primarily of the varieties Robusta and Liberica which are often strong-flavored, allowing the ground coffee in the sock to be reused.
A vacuum brewer consists of two chambers: a pot below, atop which is set a bowl or funnel with its siphon descending nearly to the bottom of the pot. The bottom of the bowl is blocked by a filter of glass, cloth or plastic, and the bowl and pot are joined by a gasket that forms a tight seal. Water is placed in the pot, the coffee grounds are placed in the bowl, and the whole apparatus is set over a burner. As the water heats, it is forced by the increasing vapor pressure up the siphon and into the bowl where it mixes with the grounds. When all the water possible has been forced into the bowl the brewer is removed from the heat. As the water vapor in the pot cools, it contracts, forming a partial vacuum and drawing the coffee down through the filter.
The AeroPress is a device invented in 2005 that combines steeping and pressure. Hot water is poured onto ground coffee, similarly to a French press, but soon after the coffee is forced through a paper microfilter using pressure. This filter allows a finer grind and removes more of the sediment than the stainless steel mesh filter of a French press.
Drip brew coffee, also known as filtered or American coffee, is made by letting hot water drip onto coffee grounds held in a coffee filter surrounded by a filter holder or brew basket. Drip brew makers can be simple filter holder types manually filled with hot water, or they can use automated systems as found in the popular electric drip coffee-maker. Strength varies according to the ratio of water to coffee and the fineness of the grind, but is typically weaker than espresso, though the final product contains more caffeine. By convention, regular coffee brewed by this method is served in a brown or black pot (or a pot with a brown or black handle), while decaffeinated coffee is served in an orange pot (or a pot with an orange handle).
A variation is the traditional Neapolitan flip coffee pot, or Napoletana, a drip brew coffee maker for the stovetop. It consists of a bottom section filled with water, a middle filter section, and an upside-down pot placed on the top. When the water boils, the coffee maker is flipped over to let the water filter through the coffee grounds.
The common electric percolator, which was in almost universal use in the United States prior to the 1970s, and is still popular in some households today, differs from the pressure percolator described above. It uses the pressure of the boiling water to force it to a chamber above the grounds, but relies on gravity to pass the water down through the grounds, where it then repeats the process until shut off by an internal timer. Some coffee aficionados hold the coffee produced in low esteem because of this multiple-pass process. Others prefer gravity percolation and claim it delivers a richer cup of coffee in comparison to drip brewing.
Indian filter coffee – This apparatus is typically made of stainless steel. There are two cylindrical compartments, one sitting on top of the other. The upper compartment has tiny holes (less than ~0.5 mm). And then there is the pierced pressing disc with a stem handle, and a covering lid. The finely ground coffee with 15-20% chicory is placed in the upper compartment, the pierced pressing disc is used to cover the ground coffee, and hot water is poured on top of this disk. Unlike the regular drip brew, the coffee does not start pouring down immediately. This is because of the chicory, which holds on to the water longer than just the ground coffee beans can. This causes the decoction to be much more potent than the American drip variety. 2–3 teaspoonfuls of this decoction is added to a 100–150 ml milk. Sugar is then sometimes added by individual preference.
Another variation is cold-brewed coffee, sometimes known as “cold press.” Cold water is poured over coffee grounds and allowed to steep for eight to twenty-four hours. The coffee is then filtered, usually through a very thick filter, removing all particles. This process produces a very strong concentrate which can be stored in a refrigerated, airtight container for up to eight weeks. The coffee can then be prepared for drinking by adding hot water to the concentrate at a water-to-concentrate ratio of approximately 3:1, but can be adjusted to the drinker’s preference. The coffee prepared by this method is very low in acidity with a smooth taste, and is often preferred by those with sensitive stomachs. Others, however, feel this method strips coffee of its bold flavor and character. Thus, this method is not common, and there are few appliances designed for it.
The amount of coffee used affects both the strength and the flavor of the brew in a typical drip-brewing filtration-based coffee maker. The softer flavors come out of the coffee first and the more bitter flavors only after some time, so a large brew will tend to be both stronger and more bitter. This can be modified by stopping the filtration after a planned time and then adding hot water to the brew instead of waiting for all the water to pass through the grounds.
A variation on the moka pot with the upper section formed as a coffee fountain.
Espresso is made with hot water at between 91 °C (195 °F) and 96 °C (204 °F) forced, under a pressure of between eight and nine atmospheres (800–900 kPa), through a lightly packed matrix, called a “puck,” of finely ground coffee. It can be served alone, often after an evening meal, and is the basis for many coffee drinks. It is one of the strongest tasting forms of coffee regularly consumed, with a distinctive flavor and crema, a layer of emulsified oils in the form of a colloidal foam standing over the liquid.
The moka pot, also known as the “Italian coffeepot” or the “caffettiera,” is a three-chamber design which boils water in the lower section and forces the boiling water through coffee grounds held in the middle section, separated by a filter mesh from the top section. The resultant coffee (almost espresso strength, but without the crema) is collected in the top section. These pots usually sit directly on a heater or stove. Some models have a transparent glass or plastic top.
Various types of single-serving coffee machines force hot water under pressure through a coffee pod composed of finely ground coffee sandwiched between two layers of filter paper or a proprietary capsule containing ground coffee. Examples include the pod-based Senseo and Home Café systems and the proprietary Tassimo and Keurig K-Cup systems.
Proper brewing of coffee requires using the correct amount of coffee grounds, extracted to the correct degree (largely determined by the correct time), at the correct temperature.
More technically, coffee brewing consists of dissolving (solvation) soluble flavors from the coffee grounds in water. Specialized vocabulary and guidelines exist to discuss this, primarily various ratios, which are used to optimally brew coffee. The key concepts are:
* Extraction: Also known as “solubles yield” – what percentage (by weight) of the grounds are dissolved in the water.
* Strength: Also known as “solubles concentration”, as measured by Total Dissolved Solids – how concentrated or watery the coffee is.
* Brew ratio: The ratio of coffee grounds (mass, in grams or ounces) to water (volume, in liters or half-gallons): how much coffee is used for a given quantity of water.
In brief, ideal coffee is widely agreed to be most easily achieved with a brewing ratio of 55 g/L (55 grams of coffee per 1 L of water) in American standards, to 63 g/L in Norwegian standards, yielding approximately 14–16 grams of coffee for a standard 240 ml (8 oz) cup.
These guidelines apply regardless of brewing method, with the following exceptions: espresso is significantly different (much stronger, and more varied extraction), and dark roasts taste subjectively stronger than medium roasts (standards are based on medium roasts; equivalent strength requires using a lower brewing ratio for darker roasts).