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Onion ( Allium cepa L.): Chemistry and Human Health, 2nd Edition

Chapter · October 2017

DOI: 10.1002/9781119158042.ch58

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1145

Onion (Allium cepa L.)Sunil Pareek,∗ Narashans Alok Sagar, Sunil Sharma, and Vinay Kumar

Department of Agriculture and Environmental Sciences, National Institute of Food Technology Entrepreneurship and Management (NIFTEM), Kundli, Sonepat,Haryana, India

58.1 Introduction

Onion (Allium cepa L.) has been valued as a food and amedicinal plant since ancient times. It is widely cultivated,second only to tomato, and is a vegetable bulb crop knownto most cultures and consumed worldwide (FAO, 2012).It is a short duration horticultural crop (Brewster, 1990)grown at low latitudes. It is commonly known as “Queenof the kitchen,” due to its highly valued flavor, aroma, andunique taste, and the medicinal properties of its flavorcompounds (Selvaraj, 1976; Griffiths et al., 2002). Onion isused throughout the year, for example in curries, in theform of spices, in salads, as a condiment, or cooked withother vegetables, such as boiled or baked. It is also used indifferent forms of processed food, e.g. pickles, powder,paste, and flakes, and it is known for its medicinal values.

58.1.1 Historical Aspects

Onions have been used by humans since theNeolithic age,and they are still being used all over the world. Over thislong period, there have always been people who haveappreciated the use of onions and used them in consider­able quantities, but there have also been those who haverejected and detested them (Koch and Lawson, 1996).Onions have been cultivated globally, in at least 175countries, for around 5000 years. Ancient Egyptiansregarded the spherical bulb as a symbol of the universe.The name is probably is taken from the Latin unusmeaning “one,” and the Romans introduced the onionto Britain, from where it may have been carried to theAmericas (Burnie et al., 1999). The first known writtenreport about the onion comes from the Sumerians anddates back to 2600–2100 BC. In the Papyrus Ebers, which isbased on ancient Egyptian writings and knowledge,

∗Corresponding author.

we discover that leek played an important role in thekingdom of old Egypt. The great physician Hippocratessuggested onion as a diuretic, laxative, and emmena­gogue. He also used onion for the treatment of pneumo­nia, and, externally, for healing putrid wounds (Koch andLawson, 1996).

58.1.2 Production Statistics

Onion is an extensively grown biennial bulb crop, withworld production of 74,250,809 tonnes from an area of4,364,000 hectares (Table 58.1). China and India are theprimary onion growing countries, followed by the USA,Egypt, Iran, Turkey, Pakistan, Brazil, the Russian Federa­tion, and the Republic of Korea (FAO, 2012). Onionproductivity is highest in the Republic of Korea(66.16 t/ha), followed by the USA (56.26 t/ha), Spain(53.31 t/ha), and the Netherlands (51.64 t/ha); the averageproductivity across the world is 19.79 t/ha. From aneconomic point of view, the international trade in onionexports is 6.77 million tonnes. The Netherlands is thehighest onion exporter (1.33 million tonnes) followed byIndia, China, Egypt, Mexico, USA, Spain, and Argentina.Bangladesh, Malaysia, the Russian Federation, the UK,Japan, and Saudi Arabia are the major onion importingcountries in the world.

58.1.3 Botany

The genus Allium is very large and consists of many wildedible species (only a small fraction is cultivated com­mercially), and is widely distributed over temperate zonesin the northern hemisphere (Hanelt, 1990; Gautam et al.,1997). The place of origin is purported to be in centralAsia, and the Mediterranean regions are considered to bethe secondary centre of origin (CSIR, 2003). The genus

Fruit and Vegetable Phytochemicals: Chemistry and Human Health, Volume II, Second Edition. Edited by Elhadi M. Yahia.© 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd.

1146 Fruit and Vegetable Phytochemicals

Table 58.1 Top 10 onion producers in the world.

Rank Country Total production (tonnes)

1 China, PR 20,507,759

2 India 13,372,100

3 USA 3,320,870

4 Egypt 2,208,080

5 Iran 1,922,970

6 Turkey 1,900,000

7 Pakistan 1,701,100

8 Brazil 1,556,000

9 Russia 1,536,300

10 Republic of Korea 1,411,650

Source: FAO, 2012.

Allium contains more than 780 species (Burnie et al.,1999) with large diversities in morphological characters.The chromosome number of onion is 16 (2n). It has beenclassified in hierarchical level as follows:

� Kingdom: Plantae� Subkingdom: Tracheobionta� Super division: Spermatophyta� Division: Liliopodia� Subclass: Liliales� Order: Liliaceae� Genus: Allium� Species: Allium cepa L.

Onion belongs to the family Amaryllidaceae; the plant iseither biannual or perennial (depending on the cultivar),and smells when crushed (WHO, 1999). The plant hasshallow adventitious fibrous roots (Ranjitkar, 2003), bulb,and tubular leaves. The stem grows 100–200 cm tallduring the second year of the plant’s life. The green leavesof the plant are an extension of the outer food storageleaves. The inflorescence is umbel-like and develops froma ring-like apical meristem. The umbel is the aggregationof flowers at various stages of development, and it con­tains 200–600 small individual flowers, although thisnumber can range from 50 to 1000 (Ross, 2001). It iscomposed of white or greenish-white small flowers whichgrow at the tip of the stem in the second year of the plant.The onion bulb ranges in shape from flat to globular to

oblong, and the onions are usually of three colors: red,white, and yellow (Fritsch, 2005). The fruits are capsuleand contain black seeds. The bulb is composed of fleshyand enlarged leaf bases. The edible onion bulb can growup to 10 cm in diameter, and it is composed of severaloverlapping layers on a central core. The outer leaf basesof the bulb lose moisture and become scaly by the time of

harvesting, and the inner leaves thicken as the bulbdevelops. The majority of the species of onion grow inopen, sunny, and dry land, mainly in humid climates.However, the Allium species have been adopted in otherecological niches of the world (Fritsch and Friesen, 2002).

58.1.4 Cultivars in Major Countries of Production

Many onion cultivars (Table 58.2) have been developedworldwide. Themajority of the cultivars are grown for drybulbs, salad purposes, and pickling. These cultivars havediversities in photoperiod, storage life, skin color, andbioactive compounds (Brewster, 1994). The major culti­vars of onions grown in the top 10 producing countriesare summarized in Table 58.2.

58.2 Nutritional Composition and MajorPhytochemicals

Proximate values and nutritional composition of onionsare given in Table 58.3. The whole bulb of onions are agood source of (+)-S-alk(en)yl-L-cysteine sulfoxide andγ-glutamyl peptide, which together account for over 70%of the total sulfur in onions (Lawson, 1996). There arethree main non-volatile and odorless alk(en)yl cysteinesulfoxides present in onions. These are S-trans-prop-1­enyl cysteine sulfoxide (isoalliin), S-methyl cysteine sulf­oxide (methiin), and S-propyl cysteine sulfoxide (propiin).The predominant flavor precursor is isoalliin, whichaccounts for more than 80% of the total amount of alk(en)yl cysteine sulfoxides. Methiin appears in lower con­centrations, while alliin and propiin are present in traceamounts (Jones et al., 2004).During tissue destruction, the alk(en)yl-L-cysteine sulf­

oxides are degraded by alliinase enzymes. This leads to theformation of new compounds, such as alkyl alkane-thi­osulfinates, which have an influence on the characteristicodor and flavor of alliums. These ingredients may bedecomposed to other organosulfur compounds, includingpropyl-1-propanyl thiosulfinate, 1-propanethial-S-oxide,allicin, dipropyl disulfide, diallyl sulfide, methyl propanyldisulfide, and ajoene. Simultaneously, γ-glutamyl cysteineis also converted to different organosulfur compounds,including S-allyl cysteine and S-allyl mercaptocysteine(Block, 1985).Onions are a very common and rich source of dietary

flavonoids, and contain three diverse and highly valuablephytochemicals in perfect proportion: flavonoids, fruc­tans, and organosulfur compounds. These compoundsare believed to provide beneficial effects for humanhealth.Onions are major contributors of flavonoids and organo­sulfur compounds, which are powerful antioxidants.

114758 Onion (Allium cepa L.)

Table 58.2 Onion cultivars grown in top 10 producing countries.

Country Cultivar

China, PR Fuzhou Red Skin, Texas Early Grano, Red Skin, White Skin, Shanghai Red, Nanjing Yellow, Yangzhou Red,Yunnan Red, Kunming Purple, Minhou Purple

India N-53, Nasik Red, Pusa Red, Bellery Red, Punjab Selection, Pusa Ratnar, Patna Red, Poona Red, White Patna, RedGlobe, Large Red, Agrifound Light Red, Agrifound Dark Red, Agrifound White, Early Grano, Arka Niketan, PusaMadhavi, Hisar-2, Arka Kalyan, Pusa White Round, Pusa Flat White, CO-3, CO-4, Agrifound Red, Punjab RedRound, Punjab Naroya, Punjab Selection, Arka Bindu, Pusa White Flat, Pusa White Round

USA Olé, Riverside, Ringmaker, Big Mac, Valiant, Gringo, Daytona, Orogrande, Robin, Celebrity, Sweet Sandwich,Colorado No. 6, Sweet Spanish, Buffalo, Walla Walla, Southport White Globe, Red Wethersfield, Norstar 210B,Rocket

Egypt Giza 6, Beheri, Giza 20

Iran Red Azarshahr, White Kashan, Texas Early Grano, Yellow Sweet Spanish, (Red) Tarom, Ray, Isfahan, (White) Qum

Turkey Çorum, Akgün 12, Kantartopu

Pakistan Red Tunic, Texas Early Grano, Local White, Phulkara, Faisalabad Early, Desi Early Red, Swat, Chaltan, Sariab Red,Hazara

Brazil Baia Periforme Precoce, Granex, Piro Ouro, Texas Grano, Pera Norte, Crioula, Texas Grano 502, Norte-14,Jubileu, IPA varieties, Granex 33, Granex 429, Alfa Tropical, Beta Cristal, Conquista, Roxa de Barreiro

Russia Krasnodar, Ispanskij, Karatalskij, Chernomorskij, Andizhanj, Masallinskiij, Vertiuzhanskij, Dnestrovskij, Kaba,Skwirskij, Arzamasskij, Bessonovskij, Danilovskij, Kilichinskij, Mstersky, Rannij Zheltyi, Rostovskij repchatyimestnyi, Strigunovskij, Pogarskij, Sibirskii, Skoropelyi, Tschernuschka

Republic of Korea Cheongdangnang, Paechong-Joseng, Chenju-Whang, Kinkyu, Bonganghwang, Yongangwhang, Samda

Table 58.3 Proximate values and nutritional composition of Table 58.3 (Continued )onion bulbs (per 100 g).

Component Unit Mean valueComponent Unit Mean value

ProximatesPantothenic acid mg 0.123

Water g 89.11Vitamin B6 mg 0.12

Protein g 1.1Folate μg 19

Lipids g 0.1Choline mg 6.1

Ash g 0.35Betaine mg 0.1

Carbohydrates g 0.34β-Carotene μg 1

Fiber g 1.7Vitamin A IU 2

Sugar total g 4.24Lutein+ zeaxanthin μg 4

Sucrose g 0.99Vitamin E mg 0.02

Dextrose g 1.97Vitamin K μg 0.4

Fructose g 1.29

Energy kcal 40 Minerals

VitaminsCalcium mg 23

Vitamin C mg 7.4Iron mg 0.21

Thiamin mg 0.046Magnesium mg 10

Riboflavin mg 0.027Phosphorus mg 29

Niacin mg 0.116Potassium mg 146

(continued )(continued )

1148 Fruit and Vegetable Phytochemicals

Table 58.3 (Continued )

Component Unit Mean value

Sodium

Zinc

Copper

Manganese

Selenium

Lipids

Myristic acid

Palmitic acid

Stearic acid

Total saturated acids

Oleic acid

Linoleic acid

Linolenic acid

Total monounsaturated acids

Total polyunsaturated acids

Phytosterols

Amino acids

Tryptophan

Threonine

Isoleucine

Leucine

Lysine

Methionine

Cystine

Phenylalanine

Tyrosine

Valine

Arginine

Histidine

Alanine

Aspartic acid

Glutamic acid

Glycine

Proline

Serine

mg 4

mg 0.17

mg 0.039

mg 0.129

μg 0.5

g 0.004

g 0.034

g 0.004

g 0.042

g 0.013

g 0.013

g 0.004

g 0.013

g 0.017

mg 15

g 0.014

g 0.021

g 0.014

g 0.025

g 0.039

g 0.002

g 0.004

g 0.025

g 0.014

g 0.021

g 0.104

g 0.014

g 0.021

g 0.091

g 0.258

g 0.025

g 0.012

g 0.021

are part of the polyphenols family. Flavonol is a subclass offlavonoids, which includes quercetin, a chief and promi­nent dietary flavonoid of onions. Along with quercetin,other flavonols, such as kaempferol and isorhamnetin,have also been measured in onions (Dorsch and Wagner,1991; Dorant et al., 1994; Goldman et al., 1996; Lanzotti,2006). Apart from these, onions contain various sulfox­ides, such as (+)-S-(1-propenyl)-L-cysteine sulfoxide(PRENCSO), (+)-S-methyl-L-cysteine sulfoxide(MCSO), S-propyl-L-cysteine sulfoxide, S-methyl-L-cys­teine sulfoxide, and S-propenyl-L-cysteine sulfoxide(Mateljan, 2015). The other phytochemicals found inonion extract are recognized as allicin (S-oxydiallyldisul­fide), alliin (S-allyl-L-cysteine S-oxide), diallyldisulfide(allyldisulfide), S-methyl-L-cysteine S-oxide (3-methylsulfinyl alanine), propanethial S-oxide (thiopropanal S-oxide), and 3-mercapto-2-methypentan-1-ol (Rose et al.,2003). Ascorbic acid is the most abundant vitamin foundin the onion bulb, with a concentration of 1mg/g dryweight (Breu, 1996). Onion contains steroidal saponins(Carotenuto et al., 1999), which prevent absorption ofcholesterol in the intestine. Fructans (polysaccharides)are the principal storage carbohydrates in onions. In astudy of 60 vegetables, onions were reported to have thehighest quantity of fructans, which have the potential todecrease the population of bacteria (Roberford, 2007).The main content of fructans is fructooligosaccharides.Experiments for the separation and identification offructooligosaccharides have been performed usingadvanced techniques, like HPAEC-PAD (high perform­ance anion exchange chromatography with pulsedamperometric detection) and MALDI-MS (matrix­assisted laser desorption/ionization mass spectroscopy)(Stahl et al., 1997), which revealed that the vacuoles of thecells possess fructooligosaccharides. The fructooligosac­charides class is made up of several fructofuranosylsucrose subunits that are collectively known as 1-kestose(3a), neokestose (3b), nystose (4a), and so on (Shiomiet al., 2005). Onion contains two types of flavonoids, i.e.flavonols and anthocyanins. The major flavonols arequercetin, kaempferol, and isorhamnetin (Slimestadet al., 2007). The secondary metabolites found in onions,phenolics, have antioxidant activity (Nuutila et al., 2003)and are composed of hydroxylated aromatic rings.The concentration and distribution of the flavonol

quercetin may differ in the different cultivars of onions(Table 58.4).

A wide range of organosulfur compounds is found inonions, mainly including four types of diallyl sulfides:diallylmonosulfide (DMS), diallyldisulfide (DDS), diallyl­trisulfide (DTS), and diallyltetrasulfide (DTTS). Onionsare considered an outstanding source of flavonoids, which

58.2.1 Chemical Structure of Prominent Phytochemicals

The phytochemicals present in onion are mainly saponin,quercetin, and anthocyanin. The structures of the differ­ent phytochemicals are described in the following.

1149

Table 58.4 Quercetin concentration in the different cultivars ofonion.

Cultivars Quercetin(mg/kg)

Reference

Yellowonion

300 Makris and Rossiter, 2001

Red onion 739–1000 Tsushida and Suzuki, 1996; Arabbiet al., 2004

Pinkonion

719–927 Price and Rhodes, 1997

Whiteonion

67–121.5 Lugast and Hovari, 2000

Figure 58.1 Chemical structure of saponin.

58.2.1.1 SaponinSaponins are amphipathic glycosides with foaming char­acteristics. Structurally they have one ormore hydrophilicglycoside moieties combined with a lipophilic triterpenederivative (Hostettmann and Marsden, 1995). They con­sist of polycyclic (C27) aglycones (C30) attached to one ormore sugar side chains, and the aglycone is either steroi­dal or triterpene (Figure 58.1).

58.2.1.2 QuercetinQuercetin contains five hydroxyl groups (Figure 58.2),which determine the biological activity of the compoundand the possible number of derivatives. The major groupsof quercetin derivatives that are found in onions areglycosides and ethers. Sulfate and prenyl substituents

Figure 58.2 Chemical structure of quercetin.

58 Onion (Allium cepa L.)

Figure 58.3 Chemical structure of anthocyanin.

are found in only small amounts (Harborne, 1994; Wil­liams and Grayer, 2004).

58.2.1.3 AnthocyaninAnthocyanins are organic compounds that are usuallyfound in the epidermal layer of plant cells. They have acomplex structure which consists of an aromatic three-ring molecular region, with one or more attached sugarmolecules. The basic chemical structure of anthocyanin isa flavylium cation (2-phenylbenzopyrilium) which linkshydroxyl (–OH) and/or methoxyl (–OCH3) and one ormore sugars (Figure 58.3). The anthocyanins aremostly 3­glucosides of the anthocyanidins, and are subdivided intothe sugar-free anthocyanidin aglycones and the anthocy­anin glycosides (Williams and Grayer, 2004).

58.3 Properties and Biological Role ofOnion Phytochemicals

58.3.1 Properties

Evidence from several investigations suggests that thebiological and medical functions of alliums are mainlydue to their high organosulfur compounds content(Augusti and Mathew, 1974). The primary sulfur-con­taining constituents in whole onions and garlic are the S­alk(en)yl-L-cysteine sulfoxides (ACSOs), such as alliin,and γ-glutamylcysteines, which, besides serving as impor­tant storage peptides, are biosynthetic intermediates forcorresponding ACSOs; from these, and by differentmeta­bolic pathways in each vegetable, volatiles such as allicin,and lipid-soluble sulfur compounds such as diallyl sulfide(DAS) and diallyl disulfide (DADS), are originated (Lan­caster and Shaw, 1989). These compounds provide onionswith their characteristic odor and flavor, as well as most oftheir biological properties (Lanzotti, 2006) (Figure 58.4).Apart from these main sulfur and flavonoid constitu­

ents, several other constituents of intact onion, such aslectins (the most abundant proteins in onion), prosta­glandins, fructan, pectin, adenosine, vitamins B1, B2, B6, C,and E, biotin, nicotinic acid, fatty acids, glycolipids, phos­pholipids, and essential amino acids, have been studied for

1150 Fruit and Vegetable Phytochemicals

Figure 58.4 Formation of organosulfur compounds during metabolic pathways in processed onion. (Source: Corzo-Martinez 2007.Reproduced with permission of Elsevier)

their biological effects over several decades (Fenwick and treatment of different diseases, e.g. asthma (Bayer et al.,Hanley, 1985). In addition to these, certain steroidal 1989), diabetes (Sharma et al., 1977; Jain and Vyas, 1979;saponins and sapogenins, such as β-chlorogenin, have Dey et al., 2002), viral diseases (Rastogi and Dhawan,been shown to have a role in biological and pharmaco- 1990), hypercholesterolemia (Kumari and Augusti, 2007),logical activities, such as having antifungal, antibacterial, cancer (Winston, 1999), and osteoporosis (ACS, 2005).antitumor, anti-inflammatory, antithrombotic, and hypo- Curative powers have been attributed to onions through­cholesterolemic properties (Lanzotti, 2006). out the centuries, and they have been recommended forSeveral clinical trials and animal research studies have various ailments, such as laryngitis, animal bites, warts,

suggested that there are various uses of onions for the and colds.

115158 Onion (Allium cepa L.)

Although each onion phytochemical has its ownunique property and course of action, they are allultimately beneficial to humans. Organosulfur com­pounds have the properties of working as antiallergen­ics, anti-inflammatories, antimicrobials etc. because ofthe presence of sulfur–oxygen or sulfur–sulfur linkages(Augusti, 1996; Block et al., 1997). Physically, the quer­cetin molecule contains five hydroxyl groups, but it islipophilic in nature. The derivatives of quercetin canhave both lipophilic and hydrophilic characteristics,depending on the type of substituent in the molecule.These flavonols can be extracted easily from hydrophiliccompounds by reaction of plant tissue acetone (Wil­liams and Grayer, 2004). The hydrophilicity of quercetinis increased when glycosylation occurs at least on onehydroxyl group. This change in quercetin character,from lipophilic to hydrophilic, is very important forplants, as it makes quercetin more cytosol soluble, whichis necessary for transportation to vacuoles and variousparts of the plants (Malgarzata, 2008).Antioxidant activity is the primary, and most investi­

gated, chemical property of phenolic compounds. Quer­cetin and kaempferol, both considered as naturalantioxidants, are found in onions (Calderon-Montaño,2011). Antioxidants neutralize the free radicals of foodproducts and also neutralize the reactive oxygen species(ROS) of human cells (Bartosz, 1995). The antioxidantactivity of flavonols is often accelerated by antibacterialand antiviral activities (Rotelli et al., 2003; Chun et al.,2005).Over the years, many researchers have investigated the

relationship between the structure and the antioxidantactivity of flavonoids. The results reveal that the antiox­idant activity of a compound is determined by the pres­ence of free hydroxyl groups and their mutual locations(Wang et al., 2006).

58.3.2 Biological Roles

Onions not only provide flavor, but also provide healthpromoting phytochemicals. Phytochemicals are naturalcompounds found in onions which have the potential topromote health benefits in humans and offer protectionfrom a variety of diseases, including cancer. The organo­sulfur compounds have antimicrobial, antiallergenic,anti-inflammatory, and antithrombotic activity (Blocket al., 1997). As well as this, flavonols in onions, suchas quercetin and kaempferol, also possess different crucialbiological roles for health maintenance, like antiviral,antimicrobial, anti-inflammatory, and anticancer activity,along with protection of the heart and brain (Alexander,2006; Harwood et al., 2007; Utesch et al., 2008; Ansariet al., 2009).

58.3.2.1 Antioxidant ActivityIt is a well established fact that flavonoids have antiox­idant properties. It is the best studied and describedactivity of onion flavonoids, which protect cells andtissues against reactive oxygen species (ROS). ROS pro­duce free radicals, which damage the cells of differentorgans exogenously (De Groot, 1994; Harwood et al.,2007; Kumar and Pandey, 2013). It has also been provedin vitro that flavonoids, such as quercetin and kaempferol,stabilize the free electrons that originate from ROS(Hanasaki et al., 1994; Pietta, 2000). The hydroxyl con­figuration of the B ring of flavonoids has a significant rolein the scavenging of ROS, by donating hydrogen and anelectron to hydroxyl and peroxyl, to stabilize them (Caoet al., 1997). The flavonoid heterocycle initiates conjuga­tion between a free 3-OH and the aromatic rings, whichleads to antioxidant activity (Bors et al., 1990). Moreover,reports reveal that position, number, occurrence andnumber of sugar residues play a crucial role in antioxidantactivity (Ratty and Das, 1988).The major flavonoid found in onion is quercetin, pres­

ent in conjugated form as quercetin 4´-O-β-glycopyrano­side, quercetin 3,4´-O-β-diglycopyranoside, and quercetin3,7,4´-O-β-triglycopyranoside (Sellappan and Akoh,2002). The dry outer layers of onion, which are discardedbefore food processing such as cooking, contain largeamounts of quercetin, quercetin glycoside, and theiroxidative products (Gulsen et al., 2007), which are effec­tive antioxidants against non-enzymatic lipid peroxida­tion and oxidation of low-density lipoproteins (LDL).Quercetin, and its dimerized compound, show the highestantioxidative activity, which is comparable to that ofα-tocopherol. Therefore, the outer layer extract of onionis expected to be a resource for food ingredients (Ly et al.,2005).Flavonoids also possess metal chelating properties,

which stops the generation of free radicals (Mishraet al., 2013). Quercetin itself has iron-stabilizing andiron-chelating attributes (Van Acker et al., 1996). Kaemp­ferol is considered as a strong antioxidant because itshigher concentration accelerates the formation of anti­oxidant enzymes, like catalase, superoxide dismutase, etc.It also provides protection in atherosclerosis by prevent­ing the oxidation of LDLP (low-density lipid protein)(Calderon-Montaño et al., 2011).

58.3.2.2 Antibacterial ActivitySeveral studies (Table 58.5) have revealed that flavonoidshave strong antimicrobial properties (Mishra et al., 2011;Pandey et al., 2010; Kumar and Pandey, 2013). Theirantibacterial inhibitory action involves impeding the for­mation of microbial enzymes, adhesins, transport pro­teins, and so on (Cowan, 1999; Mishra et al., 2009).

1152 Fruit and Vegetable Phytochemicals

Table 58.5 An overview of antimicrobial activities of onion phytochemicals.

Activity Model Reference

Antifungal Dermatophytic fungi Zohri et al., 1995

Plant antifungal Plant pathogenic fungi Cammue et al., 1995

Antifungal Aspergillus sp. Yin and Tsao, 1999

Antifungal Fusarium oxysporum Phay et al., 1999

Antifungal Scopulariopsis sp. Mahmoudabadi and Nasery, 2009

Antibacterial, antifungal Bacteria and yeast species Dankert et al., 1979

Antibacterial Several bacterial sp. Elnima et al., 1983

Antibacterial Bacterial sp. Zohri et al., 1995

Antibacterial Oral pathogenic bacteria Kim, 1997

Antibacterial Streptococcus mutans, S. sobrinus, Porphyromonas Bakri and Douglas, 2005gingivalis, Prevotella intermedia

Antibacterial Helicobacter pylori, Staphylococcus aureus Ramos et al., 2006

Antibacterial Mycobacterium tuberculosis Abubakar and Ado, 2009

Antiviral Human immune virus Van Damme et al., 1993

Antiviral Enhance bioavailability of antiviral drug Wu et al., 2005

Antiparasitic Various parasites Reuter et al., 1996

Antiparasitic Leishmania sp., Trichomonas vaginalis Saleheen et al., 2004

Antiparasitic Cryptosporidium parvum Abu El Ezz et al., 2011

Quercetin is one of the most studied flavonoids thatinhibits bacterial growth (Wu et al., 2008). It has showngreat potential to inhibit the growth of Staphylococcusaureus completely (Havsteen, 1983). Experiments haveshown that kaempferol works as an inhibitor againstHelicobacter pylori (Calderon-Montaño et al., 2011).It has been reported that onion and garlic extracts exert

bactericidal effects towards Streptococcus mutans, Strep­tococcus sobrinus, Porphyromonas gingivalis, and Prevo­tella intermedia (Gram-positive bacteria), considered asthe main bacteria responsible for dental caries and adultperiodontitis, respectively (Bakri and Douglas, 2005).However, onion is not effective against Gram-negativebacteria (Griffiths et al., 2002). In addition to organosulfurcompounds, it has been reported that certain quercetinoxidation products found in onion also present antibac­terial activity against H. pylori and MRSA (multidrug­resistant S. aureus) (Ramos et al., 2006).

58.3.3 Antiviral Activity

Studies suggest that flavonoids have been known to bevery effective against viruses for a long time. Manyscientists have proved that flavonoids have antiviral

activity, and can inhibit or kill viruses (Bakay et al.,1968; Tripathi and Rastogi, 1981; Tsuchiya et al., 1985;Hayashi et al., 1993). The mechanism of inhibiting viralgrowth lies in blocking and destroying the synthesis ofviral protein and nucleic acids (Castrillo and Carrasco,1987; Vrijsen et al., 1987; Zandi et al., 2011).In particular, phytochemicals found in onions, such as

quercetin and kaempferol, play a big role in reducing thegrowth of various viruses (Kumar and Pandey, 2013).Moreover, quercetin and kaempferol have shown theirvirucidal activity (Table 58.5) against the herpes simplextype I virus, rabies virus, polio virus, mengo virus, pseu­dorabies virus, sindbis virus, and parainfluenza type 3virus (Kaul et al., 1985; Kell et al., 1987; Vrijsen et al., 1988;Ghazal et al., 1992; Cushnie and Lamb, 2005; Calderon-Montaño et al., 2011). As well as this, cell culture datahave shown that the quercetin flavonol may inhibit thereplication of different respiratory viruses, reducing theirviral count (Chiang et al., 2003; Chen et al., 2006).Quercetin also enhances the bioavailability of some anti­viral drugs (Wu et al., 2005). Lectins are a very heteroge­neous group of glycoproteins, with the ability to recognizeand bind specifically to carbohydrate ligands. Onionlectins have a pronounced anti-HIV activity (Van Dammeet al., 1993).

58 Onion (Allium cepa L.) 1153

Table 58.6 Overview of cancer prevention by Allium cepa.

Effect Model Reference

Antimutagenic effects

Antimutagenic effects

Protective effects

Stomach carcinoma protection

Breast cancer protection

Induction of phase II enzymes

Chemopreventive activity

Chemopreventive activity

Lung cancer protection

Brain tumor protection

Gastric cancer protection

Colorectal cancer protection

Prostate cancer protection

Tumor protection

Ames test

Salmonella strain

Human case-referent study

Human case-control study

Human case-control study

Wistar rats

Rats

Ito rat liver test

Human case-control study

Human case-control study

Human case-control study

Human case-control study

Human case-control study

Rats

Ikken et al., 1999

Kato et al., 1998

Gao et al., 1999

Dorant et al., 1996

Challier et al., 1998

Guyonnet et al., 1999

Siess et al., 1997

Takada et al., 1997

Sankaranarayanan et al., 1994

Hu et al., 1999

González et al., 2006

Millen et al., 2007

Hsing et al., 2002

Shrivastava and Ganesh, 2010

58.3.4 Anticancer Activity

Onion phytochemicals play a crucial role in preventingvarious carcinogenic activities (Table 58.6). Many studieshave revealed the positive role of flavonoids in the reduc­tion of cancer risk (Appelt and Reicks, 1999; Kamarajet al., 2009). Flavonoids adopt or follow different mecha­nisms to prevent cancer, including cell cycle arrest, inhi­bition of tyrosine kinase, regulation of p53 protein, heatshock protein inhibition, and blocking the expression ofRas protein (a class of cellular protein) (Duthie et al.,2000). Kumar and Pandey (2013) have suggested thatonion consumption reduces the cancer risk for differentorgans, such as prostate, stomach, breast, and lungs.Quercetin has been tested as a tyrosine kinase inhibi­

tory compound in a human phase I trial (Ferry et al.,1996). Kaempferol has also shown its positive potentialagainst ovarian cancer, lung cancer, leukemia, bladdercancer, prostate cancer, breast cancer, and pancreaticcancer (Nöthlings et al., 2007; Jaganathan and Mandal,2009; Calderon-Montaño et al., 2011; Chen et al., 2012;Kim and Choi, 2013).Various studies have reported that garlic and onion

intake diminishes the risk of sarcoma and carcinoma invarious tissues and organs, such as stomach, colon,esophagus, prostate, bladder, liver, lungs, breast, skin,and brain (You et al., 1998; Lau et al., 1990; Hu et al.,1999; Le Marchand et al., 2000).These effects appear to be mediated by various mecha­

nisms, which are not fully understood. It is possible to

state that garlic and onion exert their anticarcinogenicaction in an indirect way by different mechanisms:

� Alteration of the carcinogenmetabolism, increasing theactivity of the detoxification enzymatic system, whichincreases the carcinogen polarity, facilitating its excre­tion from the body (Guyonnet et al., 1999);� Inhibition of oxidative damage due to antioxidantaction (Perchellet et al., 1990);� Inhibition of cellular proliferation by induction ofapoptosis and inhibition of cell division (Perchelletet al., 1990);� Prevention of chromosomal damage (anticlastogeniceffect) (Lau et al., 1990; Khanum et al., 2004);� Inhibition of the lipoxygenase and cyclooxygenaseactivities (anti-inflammatory effect) (Perchellet et al.,1990; Rose et al., 2005).

Several investigations have shown that both water- andlipid-soluble sulfur compounds from onion provide anti-carcinogen benefits. Dipropyl sulfide (DPS) and dipropyldisulfide (DPDS) from onions (Guyonnet et al., 1999) caninhibit both early and late stages of carcinogenesis. Othersulfur compounds, such as SAMC (Sigounas et al., 1997),ajoene (Dirsch et al., 1998), andmethiin, alongwithDADSand DATS (Sakamoto et al., 1997), can inhibit the cellularproliferation by inducing apoptosis in human cell cul­tures, for example in human leukemic cells.In addition to organosulfur compounds, organosele­

nium compounds are largely responsible for the anti-carcinogenic activity of onions. Selenium-enriched

1154 Fruit and Vegetable Phytochemicals

Table 58.7 Antidiabetic activity of onion phytochemicals.

Effect Model Reference

Renal lesions, diabetes Wistar rats Babu and Srinivasan, 1999

Antihyperglycemic activity Rabbits Roman-Ramos et al., 1995

Antidiabetic activity Alloxan diabetic rats Sheela et al., 1995

Antidiabetic activity Alloxan diabetic rats Kumari et al., 1995

Antidiabetic activity Alloxan diabetic rats Kumari and Augusti, 1995

Hypoglycemic effects Alloxan diabetic rabbits Mathew and Augusti, 1975

Hypoglycemic effects Alloxan diabetic rabbits Augusti et al., 1974

onions have higher anticarcinogenic activity than thecommon plants (Matsuura, 1997; El-Bayoumy et al.,2006). This increased effect of cancer prevention isachieved at least partly by sulfur substitution with sele­nium. The pure selenium compounds have proved to besuperior anticancer agents than their corresponding sul­fur analogs. For example, diallyl selenide is at least 300times more active than DAS in the reduction of tumors ofmammary cancer (El-Bayoumy et al., 1996). The twomajor selenium compounds possessing anticancer activ­ity in onions are γ-glutamyl-Se-methyl selenocysteine andSe-methyl selenocysteine, with Se-methyl selenocysteineand Se-allyl selenocysteine being the most chemopreven­tive selenium compounds (Block et al., 2001). Identifica­tion and quantification of selenium compounds inselenium-enriched Allium are particularly important inorder to study the anticancer mechanisms in detail.

58.3.5 Anti-inflammatory Activity

Inflammation is a process of complex biological response.It is generally initiated by factors such as pathogeninfection, chemical irritation, and injury in cells andtissues. Many animal models have been studied to dem­onstrate the role of flavonoids against inflammation, andit has been reported that quercetin and kaempferol havean active role as anti-inflammatory agents (Kim et al.,1998; Jachak, 2001). Similarly, it has been observed in vitrothat quercetin has a role in the inhibition of differentisotypes of immunoglobulins such as IgM, IgG, and IgA;all are mitogen stimulated (Cumella et al., 1987).

58.3.6 Hepatoprotective Activity

The hepatoprotective property simply means the protec­tion of the liver from damage. The property of flavonoids

in this regard has been well analyzed. Quercetin supple­ment has shown a protective effect against iron overloadhepatic injury in the liver cells of mice (Zhang et al., 2006).Anthocyanin has also been observed as a hepatoprotectivecompound. Zhu et al. (2012) found that anthocyanincyanidin-3-O-β-glucoside (C3G) promotes hepatic Gclcexpression for the activation of protein kinase A byincreasing the cAMP level, which helps in phosphoryl­ation of the element binding protein for better transcrip­tion of Gclc.

58.3.7 Antidiabetic Activity

Flavonoids also possess an antidiabetic property (Table58.7) that is directly related to the increase of insulinproduction by control of pancreatic cells (Vessal et al.,2003). Another study shows that quercetin helps insuppressing diabetic symptoms (Kobori et al., 2009).Moreover, Kobori et al. (2009) found that the consump­tion of quercetin may help to control type 2 diabetes(Knekt et al., 2002). It was observed that the flavonolquercetin has also accelerated the secretion of insulin andincreased uptake of Ca2+ from separated pancreatic cells(Hii and Howell, 1984, 1985).Babu and Srinivasan (1997) observed that dietary onion

intake for 8 weeks produced a significant hypolipidemiceffect besides hypoglycemic influence in diabetic rats. Ithas been reported that long-term absorption of naturalflavonoids, such as quercetin, could be useful in prevent­ing advanced glycation of collagens, which contributes tothe development of cardiovascular complications in dia­betic patients (Urios et al., 2007). The bioactive constitu­ents from alliums, such as methiin and S-allyl cysteinesulfoxide (SACS), exert their antidiabetic action by stim­ulating insulin production and secretion by the pancreas,interfering with dietary glucose absorption, and favoringinsulin saving (Srinivasan, 2005).

58 Onion (Allium cepa L.) 1155

Table 58.8 Antiplatelet activities of onion phytochemicals.

Effect Model Reference

Antithrombotic activity

Antiplatelet activity

Antiplatelet activity

Antiplatelet activity

Antiplatelet activity

Antiplatelet activity

Antihyperlipidemic activity

Rats

Human and rabbit plasma

Human platelet-rich plasma

Human platelet-rich plasma

Human platelet-rich plasma

Rabbits, human platelets

Rats

Bordia et al., 1996

Ali et al., 1999

Debaene et al., 1999

Goldman et al., 1996

Goldman, 1996

Makheja and Bailey, 1990

Kumari et al., 1995

58.3.8 Antihypertensive Effect

A study of hypertension, using several rat models, hasindicated that quercetin and its methylated metaboliteisorhamnetin, found in onion, can reduce blood pressureand prevent angiotensin-II-induced endothelial dys­function by inhibiting the overexpression of p47phox, aregulatory subunit of themembraneNADPHoxidase. Thesubsequent increased superoxide production resulted in ahigh nitric oxide bioavailability (Sanchez et al., 2007). Incontrast, other investigations carried out with ethanolicextracts of onion and garlic in hypertensive rats reportedthat oral administration of extracts during a normal saltdiet or during a high salt diet do not influence bloodpressure (Kiviranta et al., 1989).

58.3.9 Antiplatelet or Antithrombotic Effect

Onion inhibits platelet aggregation in vitro (Ali et al.,1999; Lawson et al., 1992) and several platelet inhibitorshave been isolated and characterized from onions(Table 58.8). Studies on the antithrombotic action ofonion have reported that its aqueous extracts inhibitformation of thromboxane, a potent inducer of plateletaggregation (Moon et al., 2000). Several epidemiologicstudies have reported that the antiplatelet activity ofonions is considered to be a property of organosulfurcompounds. In particular, a class of α-sulfinyl-disulfides(cepaenes), found in onion extracts, has demonstratedantithrombotic activity (Block et al., 1997).

58.4 Conclusions and FuturePerspectives

Onion is one of the most important vegetables usedworldwide for enhancing the flavor and taste of a diverse

variety of foods. Besides this, onion, when eaten raw insalad, plays an important role due to the health benefitsaccrued by the direct intake of phytochemicals in the rawform. The importance of phytochemicals in curingdiverse human ailments marks out the onion as beingof great value to the food and pharmaceutical industry.The future of onions in the food industry lies in thedevelopment of the elite varieties with a high contentof phytochemicals. More research is required for everystage of production, beginning from the farmer’s field tothe processing center, and suitable technologies must bedeveloped to preserve the valuable phytochemicals for thebenefit of human health.The phytochemicals present in onions are biologically

active and possess immense health benefits. Onions dem­onstrate a remarkable variation in the development ofphytochemicals of high biological importance, accordingto the type of variety and the geographical area where theywere grown. A suitable breeding strategy for the devel­opment of onion varieties possessing ample amounts ofphytochemicals should be developed. The impact ofpreharvest practices adopted on the farmmust be studied,and biotic and abiotic stresses, which reduce the endog­enous synthesis of phytochemicals, must also be studiedthoroughly. Development of processable varieties ofonions, along with standardized procedures for process­ing and engineering technology, is the need of the hour;this, in turn, will boost the development of onion-basedprocessed products by simultaneously keeping the phy­tochemicals in a stable form. The phytochemicals inonions need to be preserved by developing a quality basedmethodology for processing raw onions. There is a scar­city of scientific information regarding the impact ofpostharvest processing methods on the phytochemicalspresent in onion. Further research is also needed on theimpact of preharvest practices on the development ofphytochemicals, which has so far not been exploredthoroughly.

1156 Fruit and Vegetable Phytochemicals

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