Bioactive Compounds of Allium Species 14 Rajeev Bhat Contents 1 Introduction .................................................................................. 278 2 Botanical Distribution and Bioactive Compounds ......................................... 278 3 Bioactivity ................................................................................... 281 3.1 Antioxidant Activity ................................................................... 281 3.2 Anticancer Activity .................................................................... 283 3.3 Antimicrobial Activity ................................................................ 284 3.4 Antihyperlipidemic/Anti-hypercholesterolemic Activity ............................. 285 3.5 Other Bioactivities .................................................................... 285 4 Toxicity...................................................................................... 286 5 Conclusions .................................................................................. 287 References ....................................................................................... 287 Abstract Evaluation of different plant species of Allium has resulted in identification of several bioactive constituents/phytochemicals. Some of the bioactive phytochemical constituents include organosulfur compounds, thiosulfinates, polysulfanes, polyphenols, tannins, flavonoids, alkaloids, saponins, fructans, fructo-oligosaccharides, essential oils, amino acids, vitamins, pigments, and much more. Traditionally, majority of the plants belonging to Allium sp. have been proved to be effective in treating flu, cold, cough, asthma, headache, stomachache, arthritis, and other common ailments. Besides, bioactive compounds identified in some of the commonly used Allium sp., they are scientifically proven to contribute towards a wide range of bioactivities such as antioxidant, antimicrobial, anti-inflammatory, antidiabetic, anticancer, anti- R. Bhat (*) ERA-Chair for Food (By-) Products Valorisation Technologies (VALORTECH), Estonian University of Life Sciences, Tartu, Estonia e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2021 277 H. N. Murthy, K. Y. Paek (eds.), Bioactive Compounds in Underutilized Vegetables and Legumes, Reference Series in Phytochemistry, https://doi.org/10.1007/978-3-030-57415-4_17 278 R. Bhat hypercholesterolemic activities and much more. In the present chapter, attempts have been made to identify and report on some of the popular, widely consumed, and scientifically proven bioactivities of plants belonging to Allium species. Keywords Allium species · Bioactivity · Bioactive compounds · Bulbs · Phytochemicals · Toxicity 1 Introduction Nutraceutically valued plant-based food offers wide prospective to provide health benefits with a natural approach. Effective utilization of traditionally recognized agri-food produce (e.g. herbs and spices) not only supports reduction in the costs incurred for health care but can also be a potential source of revenue for farming communities. Among a wide array of herbs traditionally used for culinary and therapeutic uses, plants belonging to Allium sp. command high value. Allium species comprises of plants which serve the purpose either as an ornamental, as a culinary ingredient, or as a traditional therapeutic agent, routinely used in traditional herbal drug preparations. Nevertheless, since time immemorial, edible plant parts of Allium sp. have been used in cooking as a flavoring agent/spice or are consumed raw (as salads) or extracted with solvents (mainly water) for medicinal purposes. Being natural, majority of the edible plants of Allium sp. are safe for consumption without any undue toxicity exhibited in humans. Plants belonging to Allium sp. are com- monly referred to as “bulb crops” or as “bulb vegetables.” In traditional medicines, herbal formulations prepared from majority of the Allium sp. have been used to treat or manage common flu, cold, cough, asthma, headache, stomachache, and arthritis. Besides being abundant in nutrition, this group of plants is also high in bioactive compounds and exhibits rich bioactivity (e.g. antioxidant, antimicrobial, anticancer, anti-cholesterolemic activity). Some of the common and popular plants belonging to Allium sp. include onion, shallots, Welsh onion, Chinese onion, garlic (wild garlic, white garlic, and rosy garlic), leeks, wild leeks, chives, and others. In the present chapter, attempt has been made to identify and report on some of the popular, widely consumed, and scientifically proven bioactivities of plants belonging to Allium sp. 2 Botanical Distribution and Bioactive Compounds The distribution of plants of Allium sp. is covered under family Amaryllidaceae and sub-family Allioideae/Alliaceae. Previously, this group of plants was placed under family Liliaceae. The plants belonging to this species extensively grow in a varied range of temperatures (temperate, tropical, and subtropical conditions). It is broadly considered that there might be nearly 800–900 species of plants belonging to Allium. However, onion (Allium cepa), Welsh onion (Allium fistulosum), Chinese onion 14 Bioactive Compounds of Allium Species 279 (Allium chinense), tree onion or the Egyptian walking onion (A. cepa var. viviparum/ proliferum), garlic (Allium sativum var. ophioscorodon and var. sativum), leeks (Allium tuberosum), chives (Allium schoenoprasum), and shallots (Allium hirtifolium) remain as the most widely consumed species. In Fig. 1, a pictorial representation of some of the popular plants of Allium sp. is shown. Besides, there are wildly growing species without much commercial values such as Allium altaicum, Allium roylei, Allium galanthum, and Allium pskemense. Some of the other species are reported to have presence of high amounts of bioactive compounds exhibiting good bioactivity, and these include Allium ascalonicum, Allium autumnale, Allium ampeloprasum (var. porrum), Allium fistulosum, Allium hirtifolium (Allium stipitatum), Allium jesdianum, A. melanantherum, A. flavum, Allium macrostemon, Allium tripedale, Allium schoenoprasum, and Allium thunbergii. Majority of the plants belonging to Allium sp. are perennial. Further, depending on the species, the edible portion remains either single or clustered bulbs, flowers, leaves, stem, and/or stalks. The size of the bulbs and leaves varies depending on the plant types. The plants generally have a taproot system and grow up to a height of 120–150 cm [1]. In some species, the height recorded is as low as 5–10 cm. Further, the flowers are produced on the upper part of leafless stalk and represent the form of an umbel. In general, majority of the Allium sp. plants contain bioactive phytochem- icals such as sulfur (organosulfur compounds), alliin, alliinase, allicin along with polyphenols, quercetin, tannins, flavonoids, gallic acid, ferulic acids, cinnamic acid, N-caffeoyltyramine, carotenoids, anthocyanins, polysulfanes, alkaloids, saponins, vitamins (B1, B2, C, and E), selenium, organoselenium, fructo-oligosaccharides, and chlorophyll [2–6]. Fig. 1 Pictorial representation of some popular plants of Allium sp. (a) Shallot onion; (b) green onion; (c) tree onion; (d) altai onion; (e) black garlic; (f) wild leek 280 R. Bhat Besides, soluble dietary fibers such as fructans and fructosyl are present in ample amounts too. Nevertheless, most of the edible bulbs have a unique and a character- istic intense sulfurous aroma (odor). Henceforth, relationship of sulfur compounds in Allium sp. imparting unique flavor quality has been well established [7–9]. On the other hand, there are black onions and black garlic bulbs that possess a distinctive odor and are less pungent compared to fresh ones. This is owed to decreased levels of organosulfur compounds and nitrogen oxides during processing stages [9–11]. Recently, hairy garlic (Allium subhirsutum) which is widely consumed as a spice in the Mediterranean regions was identified to contain rich amounts of flavonoids, sulfur compounds, and phenylpropanoid derivatives and is recommended to be a good alternative for garlic (A. sativum)[12]. Bulbs of wild onions (genus Allium sect. Codonoprasum) have a mild odor and distinctive flavor and contains dimethyl disulfide and methiin (S-methyl-L-cysteine sulfoxide) as the dominant volatiles [13, 14]. In shallots, volatile compounds involved in imparting flavor were identified to be sulfides, dimethyl trisulfide, furfural, 2,5-dimethylpyrazine, and thiophenes [15]. In black garlic, presence of amino acids like leucine, isoleucine, and phenylalanine is reported [10]. Same authors observed substantial increase of amino acids during production of black garlic. Additionally, significant increase in fructose during black garlic production is also stated [11]. Further, wild leeks (Allium ampeloprasum) are reported to contain copious amounts of flavor precursor compounds as methiin and propiin [13]. The precursors of distinctive flavor and health-promoting potential of Allium sp. vegetables are linked with S-alk(en)ylcysteine sulfoxides [16, 17]. It was observed that bioactive sulfur compounds are released after their enzymatic breakdown in combination with other polyphenolic compounds imparting various bioactivities [18, 19]. In fresh garlic, S-allyl-L-cysteine sulfoxide (or the alliin) alone forms majority of the cysteine sulfoxides. Chive flowers have been reported to contain sulfur compounds like 3,5-dihydroxy-6-methyl-2,3-dihydro-4H-pyran-4-one, pro- ved to prevent colon cancer in humans