Czech J. Food Sci. Vol. 22, Special Issue

Natural in Food Crops and their Changes during Processing

J. HAJŠLOVÁ*, V. SCHULZOVÁ, P. BOTEK and J. LOJZA

Department of Food Chemistry and Analysis, Institute of Chemical Technology, Prague, Czech Republic, *E-mail: [email protected]

Abstract: Many food plant contain specific secondary metabolites which are classified as toxins or antinutrients for humans. In this short review main groups of these bioactive compounds are introduced, potential hazard posed for consumers and related regulatory aspects are mentioned. Substances which are currently of greatest concern are discussed in a greater detail. Phytoestrogens and glucosinolates are shown as compounds that may under certain conditions, exhibit health protecting effects, toxic glycoalkaloids, have been selected as an example of toxins occurring in staple crop, lectins and pyrrolizidine alkaloids represent as toxins responsible for recently documented outbreaks of food poisoning.

Keywords: natural toxins; plant secondary metabolites; food poisoning; phytoestrogens; glucosinolates; glycoalka- loids; lectins; pyrrolizidine alkaloids

NATURAL TOXINS contaminating food crop and/or animal products, their removal by common food processing prac- Occurrence of natural toxins in food and feed tices is almost impossible (thermolabile patulin represents undoubtedly an important food safety is an exception). issue which is of growing concern of both scientists (ii) Also some bacteria may produce toxins (e.g. and regulators [1]. botulin, and Bacil- Considering the current terminology, several ap- lus cereus enterotoxin). Many of them are proaches are applied for classification of natural which are not, contrary to most of , toxins. The narrow definition specifies natural tox- heat-stable and can be therefore in most cases ins as food constituents endogenously produced by reduced/eliminated by thermal treatment of con- food organisms and capable to give rise to adverse taminated matrix. effects when the food organism or product thereof (iii) Some unicellar microalgae that are occasion- is consumed. The broader definition of natural ally occurring in marine and freshwater bodies toxins includes also toxic compounds originating throughout the world produce toxins, so called from a contamination of the food. In addition to phytotoxins. Some of them like domoic acid, saxi- plant toxins which are discussed in a greater detail (paralytic shellfish poisoning or PSP toxin), below, several other sources of natural toxins in and brevetoxin are of significant human health food supply can be recognized: concern since they may bioaccumulate or are mag- (i) Microorganisms producing toxic co-metabo- nified in the food chain by fish and shellfish. The lites may contaminate food (or raw material used presence of cyanobacterial toxins (including ana- for its production). Mycotoxins such as aflatox- toxins, microcystins, and nodularins) in drinking ins, ochratoxin A, , , water represents another hazard to be considered fumonisins and patulin produced by toxicogenic in this context. species of Aspergillus, Penicillium, Fusarium and (iv) Non-toxic raw materials of plant origin may other genera of fungi are of greatest concern both in by accident or unlucky circumstances be contami- terms of toxicity and worldwide occurrence. Once nated or being mixed with a toxic non-nutritive

Proc. Chemical Reaction in Food V, Prague, 29. 9.–1. 10. 2004 29 Vol. 22, Special Issue Czech J. Food Sci. plant species, for example unripe berries of black the presence of toxic substances; nevertheless, nightshade (Solanum nigrum), containing sever- over the millennia the domestification process has al toxic glycoalkaloids may contaminate edible gradually reduced their levels in cultural species. crops). In food plants we eat today there is fairly less compounds hazardous for consumers as com- PLANT TOXINS pared to their wild relatives. On the other hand, the decrease/elimination of constituents that have Plants contain a large number of chemical com- evolved as a way to fight off predators unavoidably pounds which are distinct from the intermediates results in lowered resistance of modern cultivated and products of primary metabolism. These second- plants to diseases. ary metabolites that are produced from primary In spite of enormous progress in breeding/grow- metabolites and their precursor may contribute to ing agricultural practices and advances in food various specific features of plants such as odours, processing technologies aiming at production of tastes and/or colours; some of them are viewed as high quality and safe foods, number of antinutrients the key components of active and potent defence (compounds with negative effects on availabil- mechanisms against bacteria, fungi, insects and/or ity of nutrients) and toxic compounds of natural herbivores. However, the real function of these origin still can occur in our diet [2–4]. In Table 1 constituents for plant health and proliferation has the list of most important groups of plant toxins not been fully recognized yet. (some compounds constituting respective group are Worth to notice, most of plant species in the precursors of physiologically active metabolites) world are inedible for humans, many because of together with main plant sources are shown [5].

Table 1. Main groups of food plant toxicants

Toxicants Plant family Alkenyl benzenes Myristicaceae, Labiatae, Lauraceae, Piperaceae Anthraquinones Polygonaceae Capsaicinoids Solanaceae Coumarins Leguminosae, Rubiaceae, Umbelliferae Coumestans Leguminosae Cucurbitacins Cucurbitaceae Cyanogenic glycosides Leguminosae, Gramineneae, Rosaceae Furocoumarins Umbelliferae, Rutaceae, Glukosinolates Cruciferae Glykoalkaloids Solanaceae Glycyrrhizinic acid Leguminosae, Sapindaceae Hydrazons Morchellaceae Proteinase inhibitors Leguminosae Isoflavonoids Leguminosae, Rosaceae, Vitaceae Lectins Leguminosae Nitriles Leguminosae Oligosaccharides Leguminosae Oxalates Chenopodiceae Phenylhydrazines Agaricaceae Pyrrolizidin alkaloids Asteracea, Boraginaceae, Leguminosae, Sapindaceae Quinolizidine alkaloids Berberidaceae, Chenopodiaceae, Leguminosae, Solanaceae Saponins Leguminosae Sesquiterpen lactones Asteraceae, Convolvulaceae, Rutaceae, Umbelliferae Toxic amino acids Leguminosae Toxic fa�y acid Cruciferae Toxic pyrimidines Leguminosae Xanthin alkaloids Bu�neriaceae, Rubiaceae, Theacea

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Besides of chemical structure, the mode of action Brassicaceae family which comprises a large number is a common criterion used for classification of of common vegetables such as different types of plant toxins. cabbage, cauliflower, broccoli, and Brussels sprouts. More than 100 different glucosinolates are known BIOLOGICAL ACTIVITY OF PLANT at present [12]. Glucosinolates themselves exhibit SECONDARY METABOLITES minimal biological activity, nevertheless, upon cell damage, they undergo hydrolysis by endogenous As documented in several studies [6], bioactive myrosinase to yield, amongst others, the plant constituents might exhibit both health pro- biological active groups of isothiocyanates and tecting and adverse health effects, depending on indoles. Some of these breakdown products were in specific conditions. In the following paragraphs earlier research conducted on experimental animals selected groups of bioactive plant constituents found to be goitrogenic and/or mutagenic (for that which are currently subject of greatest concern reason, breeders developed and commercialised are presented. low glucosinolate rape varieties). However, the adverse effects in humans have been questioned COMPOUNDS WITH HEALTH and new research focusing on the health benefits of PROTECTING POTENTIAL glucosinolates and their degradation under various processing conditions has been initiated [13–15]. Phytoestrogens Based on available data, it has been concluded that glucosinolate breakdown products modulate The biological activity of phytoestrogens con- two distinct anticarcinogenic mechanisms that tained in soybean products (mainly isoflavons), can account for at least part of the large body of whole grain cereal food, seeds, and probably berries epidemiological evidence showing a protective and nuts (mainly lignans) are a good example of effect of Brassicaceae vegetables against cancers such controversy [7, 8]. Available epidemiologic of the lung and alimentary tract. Both raw and investigations [9] strongly support hypothesis on cooked vegetables are capable of exerting potent their cancer-protective role because the highest biological activity in vivo. levels of these compounds in the diet are found in countries or regions with low cancer incidence. COMPOUNDS EXHIBITING ADVERSE Phytoestrogens are assumed to prevent mainly EFFECTS hormone-dependent cancers like breast, ovarian and/or endometrial cancer. Since recently, various Glycoalkaloids food supplements containing soy estrogens daid- zein, genistein, glycitein are becoming popular for One of the widely discussed groups of plant toxins reduction of menopausal syndromes in women [10]. is represented by glycoalkaloids that are contained On the other hand, concern exists about the pos- in all Solanaceous plants including potatoes, an sible ill effects of phytoestrogens during prenatal important staple food [16].Various toxic effects and neonatal development. For instance, loss of have been shown for main glycoalkaloids, fertility and a number of reproductive lesions in α- and α-, and their hydrolysis farm animals were reported to result from grazing products [17]. Although historical reports on acute clover reach in coumestrol, a potent plant estrogen. poisoning of humans by ´solanine´ (its levels are In experimental rats, change of uterus weight, largely elevated in sprouted and/or greened po- and decline of neuroendocrine development was tatoes) are available, it is hardly a relevant issue observed when administered diet rich in ´plant at present; the main concern is currently focused hormons´ [11]. on chronic toxicity data. No adequate no-ob- served-adverse-effect level (NOAEL) is available Glucosinolates for determination of Tolerable Daily Intake (TDI) at present. Recommendation for maximum levels Glucosinolates are another group of compounds of 200 mg α-chaconine and α-solanine (sum) per that have been of great scientific interest for many kg potatoes is based on estimation of 300 g daily years because of both health concerns and expected intake [18], nevertheless, doubts exists whether health benefits. They are primarily found in the this value is sufficient for consumers protection,

Proc. Chemical Reaction in Food V, Prague, 29. 9.–1. 10. 2004 31 Vol. 22, Special Issue Czech J. Food Sci. since the safety margin in this particular case is monly edible plant foods such as leguminous rather narrow. ´Solanine´ content considerably species and many others hence they are ingested varies among individual varieties and is largely daily in appreciable amounts by both humans affected by environmental factors such as light, and farm animals. Due to their high stability, irradiation, mechanical injury, and storage [19]. lectins are able to survive digestion by the gas- On the other hand glycoalkaloids appear to be trointestinal tract of consumers and may cause largely unaffected by common food processing severe gastric upset. Number of outbreaks of practices such as baking, cooking, and frying. The food poisoning were reported, typically due to most efficient way of lowering dietary exposure the consumption of uncooked or partially cooked is peeling of tubers since their outer layers are kidney beans (Phaseolus vulgaris). Among many rich in these toxins. Depending on tuber size and adverse effects described, interferences with di- variety, up to 60% of total content can be removed gestion and absorption of nutrients have been by manual peeling. widely discussed [24]. Tomatoes are another important food crop of Solanaceous family contributing to the glycoal- Pyrrolizidine alkaloids kaloids intake. However, this crop seems in this respect to be much safer for humans than are pota- Pyrrolizidine alkaloids are a large group of natu- toes, since (contains mixture of dehydro ral toxins involving almost 350 chemicals [25], tomatine and α-tomatine) is largely degraded as many of them have been shown carcinogenic in the fruit ripens [20]. The development of animals and are therefore classified as potential transgenic tomatoes has also stimulated interest human carcinogens. Intoxication of people can oc- whether their glycoalkaloids content differs sig- cur in several ways, often due to contamination of nificantly from that occurring in standard varieties food supply by seeds containing these alkaloids. during different stages of fruit maturity. Another exposure route is occurrence of plants containing pyrrolizidines in traditional remedies Cyanogenic glycosides or certain dietary supplements.

Several other groups of food toxins have been PLANT TOXINS AND SAFETY placed on the priority list by expert groups, some of them are present only in fresh crops and can To evaluate/regulate compounds that might be be substantially removed by using appropriate responsible for food poisoning, complex informa- processing [21]. Glycosides of α-hydroxynitriles tion on all the related aspects (toxicity, exposure) (cyanogenic glycosides) releasing hydrogen cyanide has to be available. However, contrary to myco- via enzymic breakdown catalysed by endogenic toxins and some phycotoxins, only very limited β-glycosidases are contained in several economi- data are available. In the comprehensive review cally important plants, the most known of them concerned with this issue [26] it was concluded that being cassava; the main cyanogen contained in this the concept applied for estimation of ADI (Accept- crop is linamarin. Chopping, grinding, soaking in able Daily Intake) for pesticide residues and food running water as well as fermentation and cooking, additives, is not suitable for risk management and are traditional processing practices used by cass- regulation of food plant toxins. Typically, narrow sava African and South American consumers for margin between their actual intake and potentially detoxification. Amgydalin is another cyanogenic toxic levels would prohibit the use of particular glysoside that might be responsible for exposure to food as far as default uncertainty factors used to hydrogen cyanide when heat processed products establish ADI are employed. As emphasized by prepared from stone fruits from which seeds were expert group [26], the presence of plant toxins, not removed are consumed [22]. their bioavailability from matrix and interaction with other inherent plant constituents has to be Lectins evaluated together with potential health beneficial effects of the whole food (antioxidants and other Lectins or haemagglutinins are natural protective agents can be present). binding (glyco)proteins which are ubiquitous in Considering severity, incidence and onset of bio- nature [23]. They are present in the most com- logical symptoms induced in the context of the risk

32 Proc. Chemical Reaction in Food V, Prague, 29. 9.–1. 10. 2004 Czech J. Food Sci. Vol. 22, Special Issue to health of various food hazards, then following not realistic since natural plant toxins pose a far six principal categories, ranging from greatest to greater health risk than is that of many man-made least, are typically ranked by scientists: chemicals occurring in our foods. To clarify all the 1. Microbial contaminants relationships scientific effort in this field should 2. Nutritional imbalance (excess and deficiency) be further intensified. 3. Environmental contaminants 4. Natural toxicants Acknowledgement: The authors wish to express their 5. Pesticide residues thanks to Dr. C������� A��������, National Food Admi- 6. Food additives nistration Sweden, for suporting their work in the field of It should be noted that the ranking of these haz- natural toxins. ards is not linear, that of environmental contami- nants and natural toxicants being about 1/1000th that References of nutrient imbalance and pesticide residues and food additives being about 1/100th that of natural [1] V�� E����� H.P. (2004): Anal. Bioanal. Chem., toxicants. 378: 1152. Regarding the regulatory aspects related to the [2] S���� B.R., T� A.T. (eds) (1996): Natural Toxins 2: inherent plant toxins, the main problem encoun- Structure, Mechanism of Action and Detection. Kluwer tered in this area is, as already mentioned, is the Academic/Plenum Publishers, Hardbound. lack of essential data on toxicological properties [3] T� A.T., G������ W. (eds.) (2000): Natural and and information on occurrence of these substances Selected Toxins: Biological Implications. ACS Sym- in human diet. Generally, establishing maximum posium Series, No. 745. limits has to be based both on magnitude of the [4] O’M���� J.P.F. (ed.) (2003): Food Safety Contami- potential public health/safety risk and the capacity nants and Toxins. Wallingford, Oxon. of legally enforceable standard to serve an effective [5] R��� E., R����� H., A������ J., G����� E., H����- risk management function [27]. In setting priori- ������ A., H����� S., H��� S., L������ F., L������ ties for research and further evaluation, various P., S����� A., S������� H., V����-V������� C. (1998): criteria are considered. More than acute risk, the NETTOX List of Food Plant Toxicants Report No. 1, data (both experimental and epidemiological) on Danish Veterinary and Food Administration. chronic toxicity are important for such purpose. [6] Z��� S., K�� H.-L., G�� Y., G��� Z.-Y., L�� E.J.D. Compounds assumed to be associated with high (2004): Life Sci., 74: 915. consumers’ exposure in particular region are of [7] R����� K., B���� G. (1996): Environ. Health Persp., concern. Specific risk groups should to be taken 26: 123. into account as well. [8] B������ J. (1996): Health Persp., 104: 478. [9] B������ S.A., A������� C., L������ J., B���� J., CONCLUSION C����� A. (2000): British J. Nutr., 79: 393. [10] F���������� A.L. (2003): Maturitas, 44, Suppl. 1: As outlined above, foods consumed at times S21–S29. often contain not only synthetic chemicals but [11] W������ P.L., L���� C., N������� F. (1993): Biol. also natural constituents that are intrinsically Reprod., 49: 1117. toxic. Also present in foods, however, may be [12] T������ P., F���� J.W. (2001): J. Nutr., 131: 3027. detoxifying compounds which, together with [13] V������ R., ��� ��� G��� M.S., D����� M., J����� detoxification processes in human bodies, limit W.M.F. (1997): Cancer Le�., 114: 193. the damage inflicted on our metabolism. Con- [14] M����� R.F., D����� M., V������ R., R���� S., sumers are usually aware of the more acute re- J������ I.T. (2000): J. Sci. Food Agric., 80: 967. sponses which arise from allergens or microbial [15] J������ I.T. (2000): Acta Hortic., 539: 39. toxins, but are less aware of compounds which [16] M��� J.A. (1994): Food Rev. Int., 10: 385. cause chronic diseases over time. It also should [17] H������ J. (1995): Food Chem. Toxicol., 33: 323. be noted that scientific perception of thise facts [18] Nordic Working group on Food Toxicology and is almost the reverse of the public’s perception Risk Assessment (1991): Food and New Biotechno- of risk where pesticides and food additives are logy, Novelty, Safety, and Control Aspects of Foods seen as the greatest dangers. The popular notion Made by New Biotechnology. Nordic Council of equating ´natural´ and ´healthy´ is obviously Ministers, Nord.

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