One of the less pleasant of all tastes, but nevertheless so important form the therapeutical point of view: bitter. Let’s have a look at the nature of this taste and of its secondary tastes: acrid-bitter and bitter due to cyanogenetic glycosides.
The bitter taste is one of the most common among drugs, especially the vegetal ones. It is naturally unpleasant (children, for instance, naturally tend to refuse it): this probably happens because bitter is a taste typical of toxic substances, like alkaloids and glycosides.
The substances that are responsible of the bitter taste are numerous and belong to different chemical classes: alkaloids, terpenes, flavonoids and some essential oils. The bitter receptors family (TAS2R) consists of more than 20 receptor subtypes and can detect more than one hundred substances that are completely uncorrelated from a chemical point of view. [Masé, McDonald]. The very act of tasting bitter substances stimulates a whole series of reaction within the human body. The action of the bitter taste in the body is complex: the bitter taste receptors are widespread in the body and located in several body districts (oral cavity, stomach, bowels, placenta, respiratory system, …): this means that bitter substances are able to affect a lot of physiological functions.
Even if the bitter taste is considered in se slightly warming in some traditions (e.g., TMM), bitter drugs have usually a definitely cooling effect, because they are able to “clear” heat moving it downward (they are indeed evacuating); some of them are clearly cold (for instance, opium, one of the coldest herbs at all).
Obviously, the nature of a taste is something different from the action of a herb with the same taste: the whole nature of a plant, indeed, is determined by different components, each of them contributing with its own characteristics: for instance, some bitter herbs are decidedly hot because of their content of essential oils (that are hot on their own).
The bitter taste dries (that is, it removes dampness), is firming and hardening (since it dries moisture, that instead softens), is purifying, opens and removes obstructions, and it is evacuating. Because of its purifying and evacuating functions, bitter treats the heat in excess, draining it downward. Bitter herbs, so, are essentially purifying and evacuating. The bitter taste is typical of those drugs that are called alteratives in western herbalism. Most part of the herbs that, according to the TCM, clear fire and heat, free the bowels, dry moisture and descend Qi counterflow are bitter. They are usually used to treat heat patterns, constipation, stomach and abdomen fullness due to obstruction and dampness, and coughing and vomiting due to counterflow Qi. According to the TCM, cold herbs are used to treat Heat syndromes, while in Fire syndromes, where the heat is far more intense, cold herbs are not sufficient and so bitter herbs are used instead, since they are able to drain the whole excess heat downward, driving it out of the body.
The bitter taste is also drying, since it stimulates secretion (and so excretion) of fluids: it increments salivation, gastric, biliary and pancreatic secretions, and so on; this causes a transient moistening effect, but on the end all these fluids leave the body, for instance with defecation, producing a net drying effect. Some fluids are re-adsorbed in the bowels, but surely not completely.
Bitter herbs can be either stimulating or relaxing, according to the overall nature of the plant. For instance, vervain is decidedly bitter but it is also diaphoretic and relaxing, and, like all relaxing diaphoretic herbs, reduce the Tension excess and is indicated for chills, tremors and muscular tension.
Obviously, not all herbs are equally bitter: there are slightly bitter herbs that can be used as vegetables, and decidedly more bitter and medicinal herbs (e.g., gentian), that must be administered carefully to avoid producing an excessive dryness. If we need to treat people that are already dry and cold by constitution, we can use warmer and possibly mucilaginous bitters (e.g., cinnamon, if it is adapt for the specific condition) or rather we can use bitter herbs in a formula, together with other warm and/or mucilaginous herbs.
According to the Five Movements systematic correspondence of TCM, the bitter taste is associated to the Heart. What is called Heart in TCM corresponds, at least in part, to the nervous and circulatory systems of western medicine, and alkaloids, glycosides and flavonoids, typically bitter, strongly affect these systems. According to the taste/action association, bitter herbs have a purifying action (since they remove heat and toxins) and dry dampness [ITMOnline]. The latter action is connected to both the ability of bitter herbs of stimulating the evacuation (and so fluids discharge) and to resolve inflammation and infections that stimulate mucus production.
Secondary taste: acrid-bitter
Also called nauseating bitter by Matthew Wood [Wood], it’s the taste of bile, very bitter and with a more or less intense note of acridness; it’s a bitter so intense that it causes shivering. It is typical of plants like lobelia, vervain, hops, holy thistle, cinchona, some plants belonging to the genus Eupatorium.
This taste acts someway like bitter and acrid, so it dries, it is firming and hardening, purifying, it opens and removes obstructions, it’s evacuating; it resolves Tension excess and stagnation, periodicity (or fluctuating symptoms) and wandering symptoms. Acrid and acrid-bitter are the only two tastes able to act upon Tension excess.
So, the acrid-bitter can be useful, like acrid, in cases of excessive neuromuscular tension that manifests itself with spams, stiffness and/or chronic contractions, hypersympatheticotony or, on the contrary, syndromes due to parasympathetic deficiency. Moreover, acrid-bitter can act as expectorating and emetic (e.g., Lobelia) and can treat fevers, chills and syndromes characterized by wandering symptoms (that is, symptoms that shift from one place to another) and/or intermittent symptoms (or fluctuating or recurrent symptoms: that is, that appear and disappear suddenly and/or alternate), like periodic fevers (e.g., malaria) and conditions where, for instance, chills alternate with fevers or diarrhea alternates with constipation [Wood, Wood2].
Secondary taste: cyanogenetic glycosides bitter
It’s the typical taste, for instance, of the seeds of the plants belonging to the Rosaceae genus (bitter almond, apricot, blackthorn, hawthorn, …). It participates of the characteristics of the bitter taste, but it is far colder: the cyanogenetic glucosides interfere with the cell respiration cycle so diminishing cellular activity. It’s a toxic taste: small quantities decrease systemic inflammation and facilitate cellular respiration, but larger quantities inhibit such respiration and can result fatal.
It interferes with the thyroid activity. Indeed, cyanogenetic glycosides are hydrolyzed by the β-glycosidase enzymes contained in the plant or secreted by the gut flora forming cyanide, that is detoxified by rodanase: this enzyme transforms cyanide into thiocyanate, that is expelled with urine. But thiocyanate is a competitive inhibitor of the Sodium Iodide Symporter (NIS) even in small quantities, and so it is able to inhibit iodine accumulation into the thyroid, hindering the biosynthesis of thyroid hormones. It also inhibits iodine transportation into the milk.
 It corresponds to “heat clearing” in TCM and to “alterative” in western herbalism.
 Being purifying and drying, it “dissolves” the excess of moisture “thickened” by putrefactions and heat excess, clearing the blocked passages.
 A function that also depends on downward direction: e.g., anthraquinone laxatives.
 Or fluctuating symptoms: that is, symptoms that recur periodically.
 That is, it can be either directed downward/centripetally or upward/centrifugally according to the circumstances: mostly, this taste is able to resolve “alternating” conditions, so, for instance, constipation alternating with diarrhea.
 Cyanide prevents the cell from using oxygen during the oxidative phosphorylation, by binding to the ferric ion (Fe3+) of the mitochondrial cytochrome-oxidase complex. As a result, a shift toward anaerobic metabolism occurs, causing a diminished biosynthesis of ATP and a large increase of the lactic acid concentration, that causes metabolic acidosis. The final effect is the ensuing of a histotoxic hypoxia.
[Wood] Matthew Wood, “The Practice of Traditional Western Healing”, North Atlantic Books (2004)
[Wood2] Matthew Wood, “The Earthwise Herbal – A Complete Guide to New World Medicinal Plants”, North Atlantic Books (2009)