The trigeminal nervous system is not as familiar to most people, but it has been the subject of much recent study in terms of how it affects flavor. These nerve endings exist in both the nasal passages and mouth, and give thermal sensations and pain. At a less intense level, they cause sensations of pungency and tingling. They affect the perception of olfactory and gustatory perception, especially in terms of intensity of flavor. Many of the substances present in coffee can cause this sensation, such as aromatic mercaptans, citric, phosphoric and quinic acid. However negative this sounds, trigeminal sensations contribute to the satisfaction of many foods, including coffee.
The phenomenon of astringency is a special case. Astringency refers to a puckering, drying or rough feeling in the mouth that is the result of a lack of saliva. The compounds responsible for this sensation precipitate proteins, especially those in saliva. While most often thought of as a tactile (i.e., physical) sensation, it has a direct effect on perception of tastes and aromatics.
When considering water for brewing, seek to balance all of these aspects into a satisfying beverage. Some aspects present in water, such as salts, will have a direct effect and can throw the beverage out of balance. Other water constituents affect what is extracted from the solid coffee particles, ensuring extraction (of sugars), over-extraction (of bitter components) or neutralizing (alkalinity combining with acid) flavor attributes.
