Brewing coffee should be considered from two aspects: physical and chemical. The physical aspects of brewing coffee include the effects that water will have on the coffee grounds themselves (such as expansion), the amount of solids dissolved (as the result of temperature, stirring, etc.) and the physical actions that take place as the water passes over the amount of time the water is in contact with coffee grounds. The chemical aspects include the actual flavors, tastes and aromas of coffee that are dissolved into solution. These will result in the flavor attributes of the coffee liquid that will be appreciated (or rejected) by consumers.

Brewing procedures differ in their use of pressure, methods of combining water, grounds and methods of separating grounds from the liquid. Brewing methods can be roughly divided into four categories:

1. those in which the grounds are never separated from the liquid (e.g., Turkish, Greek, Russian and Middle Eastern preparation);
2. those in which the coffee and water are put into contact for a period of time, and then the liquor quickly separated (e.g., French press, vacuum pot);
3. drip brewing in which the water is passed over the coffee grounds in a continuous stream; and,
4. techniques using pressure (e.g., espresso).

These methods mainly differ in the amount of physical action that is involved. All brewing procedures have the same goal of extracting solids from the coffee into a liquid. And only after the substances present in roast coffee are dissolved into a liquid can they be served and enjoyed.

Physical Action of Brewing

During the brewing process, ground coffee goes through several physical processes that allow the conversion of solid into liquid. This process is summarized as follows:

Initial Contact and Penetration

• The water initially penetrates the coffee bed. At this point, some gases (including carbon dioxide) are displaced from the particles, the particles are moisturized and the bed of coffee is mixed. if the coffee has been freshly roasted, a considerable amount of carbon dioxide (a by-product of the sugar-browning that takes place during roasting) must be displaced before any water penetrates the coffee particle. Water does not take the path of least resistance through the bed of coffee as might be expected. Instead, it spreads throughout the bed and occupies an ever-increasingly wide pattern. This phenomenon is referred to as hydrodynamic dispersion. The degree of hydrodynamic dispersion, responsible for the even wetting of the coffee bed, is a function of any external force exerted (including pressure in the case of espresso), the geometry of the pore system within the coffee particles, interactions between water, the coffee, viscosity and density of the liquid. The evenness of wetting also affects how quickly the entire bed of coffee is saturated with water. The rate of particle expansion is a function of the density of the particle (e.g., dark roasts are lighter in density and have more porosity), grind particle size (e.g., smaller particles are saturated more quickly) and the quality of water. Ions and compounds present in the water used for brewing especially affect the latter.
• The first material to be dissolved is the soluble material on the surface of the coffee particles. How much of this material is available will partially depend upon how recently, and how finely, the coffee was ground. In freshly ground, recently roasted coffee, this is often gaseous carbon dioxide. The more volatile aromas – those released immediately after grinding – will potentially dissolve into solution if they are still present.
• The hot water begins to penetrate the coffee particles.
• The particles swell.
• Water-soluble substances within the particles dissolve and, at higher temperatures and/or pressure, normally insoluble substances can dissolve.

Diffusion into Liquid

• The dissolved substances diffuse to the surface of the particle. This is the slowest aspect of the brewing process and will determine the rate of extraction. During this phase of brewing, the movement of dissolved substances to the surface of the coffee particle is affected by both physical aspects, such as particle expansion, and coffee bed agitation (which is greater in the case of drip brewing than in French press brewing), and by chemical aspects of brewing water.

Dissolving into the final coffee liquor

• The mass of compounds now dissolves from the coffee particles into the surrounding solution, through a process known as convective mass transfer. As substances dissolve, the liquid immediately adjacent to the coffee particle becomes denser and more concentrated.
• The denser liquid further diffuses into the less dense liquid that is moving through, or present in, the bed of coffee.
• The degree of convective mass transfer that occurs is affected by the properties and states (gas, liquid or solid) of the substances it contacts, the flow, and the pore system through which it takes place. In the case of drip brewing, the mass transfer continues at a greater rate, since the flow of water through the bed ensures that the coffee liquor does not become over-saturated with solids.

In short, brewing washes the various solutes out of the coffee and then diffuses the particles throughout the brewing liquid. The degree of extraction is a function of a) the amount of time the water is in contact with coffee particles, b) the temperature of the water (195-205 degrees Fahrenheit) and c) the law of mass action, which can be stated as: the extraction rate decreases as the concentration of solids increases. The highest percent of solutes are extracted earliest in the brew cycle. Water with too many dissolved solids (a high TDS) already present will not have as great a tendency to pick up coffee flavor substances. This is why the previous discussion of Total Dissolved Solids is important to understand, as it is a major issue in determining the quality of water for brewing. During the initial contact and penetration phase of coffee brewing, the rate at which the water penetrates the bed particles is affected by water’s structure – a function of both temperature and TDS.

Along with this physical action of extracting coffee solids into solution, there are some chemical aspects of water quality, which affect the flavor and rate of extraction.

Chemical Aspects of Brewing

When the solvent (water) passes through the solute (coffee) at a certain temperature, the following processes take place on a chemical level:

• The most polar compounds, including ionic salts and acids, are dissolved into solution. how much, and which ones are extracted, depends primarily upon solubility of the particular compound and temperature of the water. The temperature of the water must be high enough to overcome the ionic bonding and the tendency of water molecules to orient themselves around the charged particles. It has often been observed that when brew water is not hot enough, the acidity of brewed coffee is muted. In some cases, acids and bases combine to produce a neutral salt and water. The presence of certain ions in the brewing water will affect this process and the resulting flavor.
• Polar compounds which are covalently bonded, such as sugars and other carbohydrates, are then extracted. Again, adequate heat is needed to dissolve the molecules from their sites on the coffee particle. The presence of certain minerals ions (especially calcium) in the water will affect this process and later the extraction process to a degree where minute changes in mineral content can have a pronounced flavor change in the coffee that is brewed.
• Larger molecules which are not as soluble – including bitter tasting compounds such as chlorogenic acids and trigonelline, and compounds contributing to mouth feel such as complex carbohydrates and oils – begin to dissolve into solution.
• Other solid suspensions, gasses (in the case of espresso) and liquids are extracted into solution. Solubility of solids increases with temperature. The amount of solids extracted into the liquor will depend upon the method of brewing, ranging from paper filtration (least amount of suspended solids) to Turkish brewing (greatest amount of suspended solids). Solubility of gases decreases as temperatures increases, but under pressure, such as during the espresso method of brewing, gasses will dissolve.
• Other than water, liquids of different miscibility, including oils and waxes, combine as the result of both temperature and pressure. Because oil and water do not emulsify easily, oils, which contain many aromatic substances, are retained to a certain degree in the grounds.
• At extremely high temperatures, normally insoluble substances, such as cellulose and other complex carbohydrates, will hydrolyze and suspend in the liquid.

The chemical processes can be correlated to sensory effects, the taste of the brewed coffee. For example, from a gustatory standpoint, the first substances extracted consist of the ionic substances and include stronger acids, salts and bases. This will result in perceptions of sour and salt. The most polar and volatile aromas will also be initially extracted.

The sugars and carbohydrates extracted in this second stage modulate the effects of the ionic substances previously extracted. The amount of the polar compounds extracted will depend upon the available heat and the amount of substances already dissolved. If the solution is saturated, either because of the amount of solids in the water or the amount of polar substances already extracted, fewer of these polar compounds will be extracted, resulting in under extraction.

The other substances that will be extracted, including oils and dissolved cellulose (the complex carbohydrates that form the cell wall of the coffee bean), which affect the perception of body, depend upon the dwell time (the amount of contact between coffee and water), temperature and (in the case of espresso) pressure. The oils contain many aromas. Too much hydrolysis of complex carbohydrates will take place if excessive temperatures or pressure, coupled with extended dwell time, occurs.

The swelling of individual coffee particles is partially a function of the amount of bicarbonate atom present. This is measured as total alkalinity (along with other ions). Too much alkalinity can slow the progress of water through coffee, which may cause over extraction. Water with higher total alkalinity causes the particle to expand more quickly.

[insert figure 8 – page 20 – water quality handbook]