Alkaloids and the Bitter Taste

Angela Bassoli, Cigliola Borgonoi/o, Cilberto Busnelli 3.1 

A bitter taste has long been associated with alkaloids and in fact many of them have always been known as bitter. Actually, in most cases the bitter taste was initially associated with plant extracts containing alkaloids such as cinchona, belladonna, or aconitum, and subsequently with the pure compounds after isolation. This association is so strong that the bitter taste of food is stiH sometimes attributed to the presence of alkaloids, even when this has not been demonstrated by the isolation of any active principle and the sensory evaluation of single compounds. There are hundreds of bitter compounds from many other chemical classes, such as terpenes, aminoadds, salts, flavonoids, and a number of synthetic chemicals, and often the bitterness associated 

Modem Alkaloids Structure, Isolation, Synthesis and Biology. Edited by E. Eattorusso and O. Taglialatela-Scafati Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31521-5 

Despite this diversity, the alkaloids are still considered as reference standards for bitter taste in sensory evaluation methodologies caffeine and quinine solutions are used to train panelists to recognize the bitter taste and to give comparative evaluations of other bitter principles. This is obviously also due to the fact that these two alkaloids are among the most important for their use in food, not only as contaminants but as active principles selected for their taste profile and biological activity. 

Besides these, a number of other alkaloids are known to have a bitter taste, which was generally discovered accidentally, as many are toxins, poisons, and other biologically active compounds. Finally, the bitter taste of alkaloids also has some importance in the interaction between plants and animals, especially insects, which have a gustative system quite different from that of vertebrates these interactions have some important applications in ecology and agriculture. 

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Bassoli A, Borgonovo G, Busnelli G (2008) Alkaloids and the bitter taste. In Fattorusso E, Taglialatela-Scafati O (eds) Modem alkaloids. Wiley-VCH, Weinheim... 

Organic aromatic molecules are usually sweet, bitter, a combination of these, or tasteless, probably owing to lack of water solubiUty. Most characteristic taste substances, especially salty and sweet, are nonvolatile compounds. Many different types of molecules produce the bitter taste, eg, divalent cations, alkaloids, some amino acids, and denatoirium (14,15). 

Bitter taste, like sweet taste, is exhibited by a very diverse group of chemical compounds. Because the bitter taste is generally regarded as unpleasant, and because it is often associated with such compounds as those alkaloids and glycosides that are harmful to man, detailed studies have been minimal. There is thus very little information available from which to deduce the chemical grouping common to those compounds eliciting the bitter response. 

True alkaloids derive from amino acid and they share a heterocyclic ring with nitrogen. These alkaloids are highly reactive substances with biological activity even in low doses. All true alkaloids have a bitter taste and appear as a white solid, with the exception of nicotine which has a brown liquid. True alkaloids form water-soluble salts. Moreover, most of them are well-defined crystalline substances which unite with acids to form salts. True alkaloids may occur in plants (1) in the free state, (2) as salts and (3) as N-oxides. These alkaloids occur in a limited number of species and families, and are those compounds in which decarboxylated amino acids are condensed with a non-nitrogenous structural moiety. The primary precursors of true alkaloids are such amino acids as L-ornithine, L-lysine, L-phenylalanine/L-tyrosine, L-tryptophan and L-histidine . Examples of true alkaloids include such biologically active alkaloids as cocaine, quinine, dopamine, morphine and usambarensine (Figure 4). A fuller list of examples appears in Table 1. 

This is a non-chemical, and probably the first biological, method of determining the presence of alkaloids. It was first used particularly with quinolizidine alkaloids in lupine plants. The tasters were men or animals, even in ancient times. It is based on the fact that quinolizidine alkaloid has a bitter taste. This method is qualitative. Taste is a subjective and individual category, especially in the... 

PSILOCYBE A genus of mushroom which produces the bitter-tasting indole alkaloid psilocybin that causes hallucinations and other side effects. Sometimes Psilocybe mushrooms are referred to as psilocybin mushrooms. 

Despite the supposed universality of bitter taste rejection, many commonly consumed foods and beverages such as fruits, tea, coffee, chocolate, and alcohol have bitterness as a major sensory attribute which, in the overall taste profile of a food, is often appreciated by the consumers. Some alkaloids are certainly responsible for the bitter taste of known food the taste threshold is available only for atropine (0.1 mM), cocaine (0.5 mM), and morphine (0.5 mM) (Table 3.1). 

The Bitter Taste of Alkaloids in Other Drugs and Poisons... 

The bitter taste is therefore reported by occasional and accidental tasting or by the observation of rejection behavior by animals. The equation alkaloid atoxic = bitter is so strong that sometimes the presence of some bitter alkaloids was assumed in some bitter plants and never demonstrated by the isolation of any active principle of bitter taste. It is worth noting that many toxic alkaloids are reported as bitter not only in the popular tradition but in scientific literature. Table 3.2 lists some of these compounds, reporting the common name, CAS number and if the compound is listed on Toxnet. 

Since these compounds are mostly used as weapons or poisons, the bitter taste is generally not a problem in their use so, with few exceptions, it has seldom been studied systematically. One exception is strychnine, which is used as a potent rodenticide and for which the bitter taste is a limitation since it prevents ingestion by the animals. Some studies on the bitter taste of this alkaloid have been done with the aim of masking the taste of strychnine [48]. The threshold for the detection of strychnine in distilled water is 5.4 p,g, in tap water 6.5 p,g. Various substances were added to diluted solutions of strychnine to ascertain whether the bitter taste could be masked. It was masked to some extent by certain salts, sucrose, and extracts of yerba Santa. It was also noticed that the cation was the significant factor in the masking... 

The same conclusions were also recorded for vertebrate herbivores. For example rabbits (Cuniculus europaeus) and hares (Lepus europaeus) clearly prefer the sweet plants and leave the bitter plants almost untouched, at least as long as there is an alternative food source. In conclusion, although taste perception in mammals and insects differs in many aspects, there also some similarities both in anatomy and in the function of the bitter taste perception. A comparison of the effects of alkaloids, as well as of other bitter compounds, will be assisted by further advances in the knowledge of the structure of taste genes and receptors. 

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