12.1.1 Definition of Extraction
Extraction is a mass transfer process in which specific components are selectively separated and transported between substances of different phases. It generally occurs through interactions between phases such as solid–liquid or gas–liquid systems. Coffee extraction is a representative example of a solid–liquid extraction process.
The target substance to be extracted is referred to as the solute, while the substance that selectively extracts the solute is called the solvent. For instance, if caffeine is extracted from coffee using water, caffeine is the solute and water is the solvent. The phenomenon in which the solute is extracted and uniformly dispersed into the solvent is called dissolution, and the resulting homogeneous mixture is referred to as a solution.
However, for a system to be classified as a solution, the solute must be completely and uniformly mixed at the molecular level. The solute must be dispersed as molecules or ions such that no particles are visible, no sedimentation occurs over time, and phase separation (such as oil and water) does not take place. Furthermore, the concentration must remain uniform at any arbitrary point within the system. Only under these conditions can it be defined as a true solution. Otherwise, it is classified as a colloid (e.g., milk), a suspension (e.g., muddy water), or a general mixture. Coffee is not a pure solution, but rather a complex mixture of solution, colloid, and suspension.
In addition, the term “dissolved” is only appropriate when the solute is dispersed at the molecular or ionic level to form a uniform phase. If this condition is not met, the system should instead be described as suspension, dispersion, or emulsion. In coffee extraction, substances that are insoluble in water are also carried into the beverage. In such cases, the correct term is not dissolution but entrainment. Therefore, coffee extraction is a complex process involving both the dissolution of water-soluble compounds and the entrainment of insoluble materials.
12.1.2 Key Variables of Extraction
The extraction process is governed by solubility, partition coefficient, concentration gradient, and extraction equilibrium.
Solubility
Solubility refers to the maximum concentration of a solute that can be dissolved in a given amount of solvent under specific temperature and pressure conditions. In simple terms, it indicates how much of a substance can dissolve in a solvent.
Solubility is highly sensitive to variables such as the nature of the solute and solvent, temperature, and pressure. Therefore, it is essential to specify the conditions under which solubility is defined. In general, the solubility of solids increases with temperature, whereas the solubility of gases decreases with increasing temperature.
For example, cocoa powder does not dissolve easily in cold water or milk. However, when hot water or milk is used, dissolution becomes significantly easier. Similarly, in coffee extraction, it is well understood empirically that higher temperatures facilitate the dissolution of coffee compounds.
In contrast, gas solubility behaves oppositely. Carbon dioxide dissolves in water to form carbonic acid and is used in carbonated beverages. However, CO₂ does not readily dissolve at ambient conditions, which is why carbonated drinks are produced under pressures of 2–4 atmospheres. At 25 °C, the solubility of CO₂ in water is approximately 1.45 g/L, whereas at 90 °C (typical coffee extraction temperature), it decreases to around 0.36 g/L. The sensory detection threshold for carbonation is approximately 1.2 g/L. Based on this, it can be concluded that CO₂ present in coffee grounds cannot dissolve into the beverage at concentrations sufficient to produce a perceptible carbonation sensation. Even if trace amounts dissolve, they remain below the detection threshold.
Partition Coefficient
The partition coefficient describes how a solute distributes itself between two immiscible phases at equilibrium. It represents the ratio of concentrations of the solute in each phase.
For example, caffeine is soluble in both water and chloroform. In a liquid–liquid extraction system, caffeine initially dissolved in water can migrate into chloroform. This principle allows for the selective extraction of caffeine from aqueous solutions.
Concentration Gradient
The concentration gradient refers to the spatial rate of change in concentration. It is the driving force for diffusion and determines the rate at which substances move.
This relationship is expressed by Fick’s First Law of Diffusion:
J=−D⋅∇C
where
J is the flux of the substance,
D is the diffusion coefficient,
∇C is the concentration gradient.
Everyday examples include the spreading of perfume molecules in air or the dispersion of espresso when poured into water to form an Americano. In all such cases, substances move from regions of higher concentration to lower concentration.
Extraction Equilibrium
Extraction equilibrium is not merely the point at which no further dissolution occurs. Rather, it is a state in which diffusion, dissolution, concentration gradients, and chemical potential collectively reach a balance, such that there is no net movement of solute.
This can be interpreted as a state where the chemical potential of the solute is equal across phases, resulting in no further spontaneous change. In reality, solute molecules continue to move; however, the rate of extraction is equal to the rate of re-deposition into the original phase. This condition is known as dynamic equilibrium, and it appears as though extraction has ceased.
To summarize these concepts, consider a system in which a component C is extracted from a solid A using a liquid B. Here, B is the solvent and C is the solute. The process typically involves three stages: solvent penetration into the solid, dissolution of the solute, and transport of the solute into the liquid phase.
The most critical factor is the concentration gradient. At the moment the solvent first penetrates the solid, the concentration difference is at its maximum, resulting in the highest extraction rate. As extraction proceeds, the concentration gradient decreases but continues to drive diffusion. Convection, induced by temperature or density differences, can further accelerate this process. Extraction continues until equilibrium is reached, beyond which no net transfer of solute occurs.
12.1.3 Types of Extraction Methods
Extraction methods based on fundamental physicochemical principles include leaching, percolation, maceration, Soxhlet extraction, and decoction. Modern extraction technologies include supercritical fluid extraction, microwave-assisted extraction (MAE), and enzyme-assisted extraction (EAE).
12.1.3.1 Percolation
Among various coffee extraction methods, percolation is one of the most widely used. The term originates from the Latin percolare, meaning “to filter” or “to percolate.” It refers to the process in which a liquid solvent passes through a porous medium under gravity or external pressure, extracting soluble compounds along the way.
The mechanism of percolation can be divided into four stages:
- Wetting – The solvent contacts the particle surface and spreads due to surface tension.
- Penetration – The liquid infiltrates the porous structure via capillary action; pore size distribution is a critical factor.
- Dissolution and Diffusion – Soluble compounds dissolve and diffuse according to concentration gradients. Insoluble materials may be transported via entrainment.
- Drainage – The extracted solution moves downward with the flow.
A key risk in percolation is channeling, where the fluid preferentially flows through certain pathways, leading to uneven extraction. This results in both under-extracted and over-extracted regions, ultimately degrading overall quality.
12.1.3.2 Leaching
Leaching refers to the process of extracting soluble substances from a solid using a liquid solvent through physicochemical interactions. It is a broad and inclusive term encompassing processes driven by dissolution and diffusion.
Since percolation also involves a liquid passing through a solid to extract components, it can be considered a form of leaching.
12.1.3.3 Maceration
Maceration is a static extraction process in which a solid is immersed in a stationary liquid for an extended period, allowing extraction to occur solely through diffusion. Typically conducted at low or room temperatures without agitation, it relies entirely on diffusion-driven mass transfer.
A common example is placing a green tea bag in cold water for an extended time. Maceration is also a subtype of leaching.
12.1.3.4 Immersion
Immersion simply refers to the act of placing a solid into a liquid. It does not inherently imply extraction. For example, saying “I immersed green tea” only means the tea was placed in water, not that extraction is occurring.
To indicate extraction, the correct expression is “immersion brewing.” The French press is a representative example of immersion brewing.
12.1.3.5 Steeping
Steeping refers to an extraction process in which a solid is immersed in a warm liquid for a relatively short period. It contrasts with maceration in both time and temperature.
Steeping typically involves higher temperatures, shorter extraction times, and may include agitation, resulting in faster extraction rates compared to maceration.