## 2 Natural Toxic Plant Alkaloids with Relevant Occurrence in Food
### Origin, Toxicity and Occurrence of Tropane Alkaloids (TAs)  
Tropane alkaloids (TAs) are toxic secondary metabolites naturally synthesized by different plant families, such as Solanaceae, Brassicaceae, Convolvulaceae and Eryth myxplaceae. There are over 200 different compounds identified, all of them characterized by having a common chemical structure known as the tropane skeleton, which involves a two-ring structure with pyrrolidine and piperidine rings sharing a single nitrogen atom and two carbon atoms (Figure 2b) [7; 31]. The variety of compounds generally arise by the esterification of tropine with different acids, such as propanoic acid, isovaleric acid, acetic acid, isobutyric acid, (+)-\(\alpha\)-hydroxy-\(\beta\)-phenylpropionic acid, tropic acid, among others [31]. The most common TAs that can contaminate food are (\(-\))-hyoscyamine and (\(-\))-scopolamine (also known as hyoscine) (Figure 2b). The racemic mixture of (\(\pm\))-hyoscyamine is called atropine (Figure 2b), but of the two enantiomers only the (\(-\))-hyoscyamine shows toxicity. Nonetheless, the (\(-\))-hyoscyamine is unstable and undergoes racemization over time, so both enantiomers can appear (mainly in the oldest plant organs) in variable proportions, although they always favor the (\(-\))-enantiomer [32]. The presence of these compounds in food can have serious consequences on the health of consumers, due to their anticholinergic effects. TAs can produce mild or severe symptoms depending on the concentration ingested. Nonetheless, symptoms are generally acute since chronic toxicity caused by these alkaloids has not yet been demonstrated [33]. TAs act as muscarinic acetylcholine receptor antagonists; therefore, they produce several symptoms, such as changes in heart rate, excessive salivation, pupil dilation, a reduction in gastrointestinal tone, hallucinations and even death at higher concentrations [34; 35]. These symptoms are generally described for atropine and scopolamine; however, knowledge concerning the toxicity of other TAs is still limited. For instance, tropine increases intestinal contractions, but pseudotropine causes the opposite, an inhibition of intestinal contractions, while cusechoyrine suppresses the immune response in animals [36]. Nonetheless, for the rest of the TAs their toxicity is unknown. For this reason, to date they have been less studied than atropine and scopolamine. Moreover, they usually appear at lower concentrations. However, depending on the species and family, the compounds vary. In this sense, the Solanaceae family is the most problematic as it presents high concentrations of atropine and scopolamine. The Solanaceae family has 90 genera and over 4000 species [35]. _Datura, Brugmansia, Atropa, Hyoscyamus_ and _Mandragon_ are some of the most concerning genera in this family. Specifically, _Datura stranomium_ is one of the species that has generated most of the reported poisoning cases [7; 37]. On the other hand, the Convolvulaceae family includes the _Contovululus_ species (commonly known as bindweed), which are rich in TAs such as convolvine, convolidine and other low molecular weight TAs such as tropine, pseudotropine and tropinone [31; 36]. Although this family has been less studied than others, it should also be carefully studied as the plants belonging to this family often invade European fields. Therefore, their presence may pose a problem of contamination with TAs other than atropine and scopolamine. The most common route of TA contamination is related to crops that are harvested at the same time as these toxic plants, as they present similar maturation cycles to some food crops [33]. Moreover, the use of machinery during harvesting, the reluctance to use phytosanitary products and the trend towards organic products increase the contamination of foodstuffs with this type of natural toxin. Likewise, as in the case of PAs, other ways of contamination have recently been proposed, such as the natural horizontal transfer of TAs through the soil.