Mouthfeel is the tactile sense that is derived from physical sensations in the mouth during and after ingestion. These sensations are created by density, viscosity, surface tension and other physical and chemical properties of what is being ingested. The mouth features a network of free nerve endings, which respond to touch and light pressure, as well as thermal, chemical and mechanical stimulation. The characteristic feel of a food or beverage is often one of its most important aspects in assessing quality. Features such as firmness, softness, juiciness or oiliness can be measured in the mouth in the same way they can be measured by the finger.

In coffee, the tactile sensation in the palate comes from both the undissolved liquid material (fatty oils) and the undissolved solid material (sediment) that remain suspended in the beverage after it is brewed. In addition to creating the overall mouthfeel, these suspended materials also contribute to the flavor of the beverage through the formation of brew colloids.

Fatty Oils

Green coffee beans contain between 7 and 17 percent fat, which is produced by the coffee shrub and stored in its seeds to provide sustenance for germination. Typically, the fat of a plant becomes oil above room temperature and are often used as cook oils. Coffee oil is a mixture of triglycerides and is similar in composition to butter or cottonseed oil.

Coffee oils play a subtle but important role in the overall presentation of its flavor. First, they reduce the surface tension of the water as the oil droplets are suspended in the beverage. This gives the coffee a smooth or creamy texture. Secondly, the oils carry other flavoring compounds. The fats in coffee are also the prime carriers of foreign flavoring compounds that can taint the flavor of coffee. Finally, hydrogenation and oxidation of the fats are responsible for part of the major flavor changes that occur during the staling process, similar to butter that turns rancid when left in a warm, moist environment.

Sediment

Sediment is undissolved solid material and comes from two sources. First, a small amount of bean fiber washes off the surface of coffee particles, and remains suspended in the beverage. Gravity pulls these micro-fine particles to the bottom of the cup. Secondly, the remaining amounts of undissolved solid materials are insoluble proteins. These proteins come from amino acids present in the green beans. They are formed as the amino acids combine to form larger molecules during the roasting process. Ultimately these protein molecules become so large they are no longer water-soluble. The result is coffee “tars” that often build up into a dark, oily residue on the surface of coffee brewing equipment.

Brew Colloids

The oil and sediment suspended in the coffee beverage combine to form coffee brew colloids, which are oily in nature. They help give coffee its texture as a beverage, just as dust and water vapor combine in the atmosphere to form clouds. Brew colloids absorb and adsorb other flavoring compounds, which contributes much to the synergism of coffee’s flavor.

When absorbing, the brew colloids adhere to thin layers of aromatic compounds, causing the gaseous materials to remain trapped in the beverage until swallowed. When adsorbing, the colloids act as buffering agents to produce a less acid cup, which can be detected by both taste and pH measurements.

We can remove some of the brew colloids by passing the coffee through a filter. But some small colloids can pass through most filter papers. Continued heating also breaks down the stability of brew colloids, and gravity separates them into an oily film on the surface of the brew and sediment on the bottom of the cup. Consequently, coffees that are directly heated for any length of time go through flavor change that is the result of the breakup of brew colloids.

Body vs Strength

Body is the measure of the tactile sensation perceived by the nerve endings in the mouth responding to the insoluble material suspended in the coffee brew. Body should be differentiated from strength, which is an intensity measure of the amount and type of soluble material is present. Strength gives coffee its taste characteristics, whereas body gives coffee its mouthfeel characteristics.

Body can be described based on the coffee’s fat content and the hardness of the bean fibers (the degree of splintering during grinding). Coffee with a very low fat content and the presence of hard or non-brittle bean fiber would be described as watery or thin. Coffee with a moderate fat content and some bean fiber splintered off during grinding would be described as smooth or light. Coffee with a relatively high fat content and some splintered bean fiber would be described as creamy or heavy.