“In exchange for a few tears, you can take a lot of health benefits from onion”

Onion worshiped by the Egyptians as far back as 3500 B.C., the onion symbolized eternity because of its concentric-circular construction, in today's world with scientific advances we understood whyonion symbolized eternity.  In the world food preparations Onion (Allium cepa) is the most commonly used ingredients for food preparations, especially in tropical countries. Although it is classified as a vegetable, it has special qualities as a condiment, which add to taste and flavour to food and hence it is mainly used in India for cuisine and culinary preparations. Besides adding a delicious taste and flavour, onion serves as a good medicinal compound for cataract, cardiovascular disease, and cancer due to its hypocholesterolemic, thrombolitic and antioxidant effects as stated by many researchers. Onions are rich in powerful sulfuric compounds, responsible for their pungent odor and for irritating our eyes. Onions are consuming in different ways like uncooked form to value added onion produtcs such as onion flakes and other dehydrated onion products.

Nutritional Composition and Health Benefits of Onion

Onion bulb contains water (89.1% w/w), carbohydrate (9.3% w/w), protein (1.1% w/w), fat (0.1% w/w) and vitamin, minerals, and bioactive compounds. The potential health benefits of onion are mainly associated with its bioactive compounds. Bioactive compounds of onion are classified into three groups-

1. Organo sulfurs compounds
2. Flavonoids and phenolics compounds
3. Sapogenins and saponins.

Flavour related compounds come under the category of organo sulfur compounds whereas antioxidants properties of onion mainly associated with flavonoids and phenolic compounds.  Ewald et al. reported that an intake of 20g onion delivers approximately 8mg quercetin, which is tantamount to the recommended daily intake of vitamin E for adults (8mg/day). The quantity of quercetin is higher in red onion than a white/ yellow onion.Red onion has more radicle scavenging activity than white or yellow onion because red onion has a higher amount of phenolic and flavonoid compounds. Majority of the bioactive compounds in onion are heat sensitive in nature so it is suggested that onion should be consumed raw or only moderately cooked to deliver maximum health benefits. Potential health benefits of onion bioactive compounds are enlisted in table 1, with their respective bioactive compounds group. With the mercury soaring high, our skin seems to be sucking the heat deeper into the body. It is suggested that we should include onion in our diets mainly as salad because onion helps to beat the excessive heat and avoid sun stroke. Onion also has aphrodisiac property even it beat the hair loss also. Onion helps to keep the body cool because quercetin believed to have anti-histamine effect, histamine is irritant that causes heat rashes.

Table 1. Major bioactive compounds in onion and their potential health benefits. (Bahram‐Parvar et al., 2018)

Chemistry of Onion flavor compounds-

Onion contains S-alk(en)yl cysteinesulfoxide (ACSO) compounds in cytosol of the cell. ACSO is the precursor molecule for other organo sulfur compounds. Table 1 represents the major ACSO compounds in onion. The amount of these sulfur-containing compounds depends upon the environmental factors (such as temperature, humidity, and sulfur compounds availability in the growing area) and genetic factors. Isoallilin compound found more in proportion than methiin and propiin.  Isoallilin also known as PRESCO (Propenyl- S-cysteine sulfoxide), foremostly come to contact with Allinase enzyme after chopping.

At the time of chopping of onionAllinaseenzymes come out from the cell vacuole and react on ACSO in the cytosol (Fig.2). Allinase enzyme converts ACSO into thiosulfinates which is unstable in air, further rearrangements of the groups in these compounds lead to a wide variety of sulfur derived compounds. Some of these compounds are volatile organic compounds responsible for flavor of fresh onion; some are responsible for flavor of fried onion and few compounds responsible for tears in eyes .

Fig. 1 Action of Allinase enzyme on ACSO.

Bis-sulfines, thiosulfonates, thiosulfinates, propanal, methylpentanols and capaenes are the responsible compounds for fresh cut onion flavor. Mono/ Di sulfides, thiophenes di/tri sulfides and zwiebelanses are the responsible compounds for flavor of fried onions. ThiopropanalS-oxide is the volatile compound release after chopping of onion and irritates the cornea of the eyes. For the protection purpose of the cornea, eyes secret the eye water (tear) for the dilution of the thiopropanal-S-oxide.In other words, tears become armor to protect the cornea from injury.

Fig.2 Schematic diagram of flavor compounds formation after chopping of the onion bulb. (Griffiths et al., 2002)

Conclusion

In summer, with the mercury soaring high, our skin seems to be sucking the heat deeper into the body; onion can be helpful to avoid heat stroke. Onion has numerous bioactive compounds which have high potential health benefits. Incorporation of onion as a salad in diets makes you healthier by reducing the free radicle in the body.

References

Bahram‐Parvar, M., & Lim, L. T. (2018). Fresh‐Cut Onion: A Review on Processing, Health Benefits, and Shelf‐Life. Comprehensive Reviews in Food Science and Food Safety, 17(2), 290-308.

Ewald C, Fjelkner-Modig S, Johansson K, Sjoholm I, Akesson B. 1999. Effect of processing on major flavonoids in processed onions, green beans, and peas. Food Chem 64:231–5.

Griffiths G, Trueman L, Crowther T, Thomas B, Smith B. 2002. Onions – A global benefit to health. Phytother Res 16:603–15.

Lanzotti, V. (2006).The analysis of onion and garlic. Journal of Chromatography A, 1112(1-2), 3-22.

Rose P, Whiteman M, Moore PK, Zhu YZ. 2005. Bioactive S-alk(en)yl cysteine sulfoxide metabolites in the genus Allium: the chemistry of potential therapeutic agents. Nat Prod Rep 22(3):351–68.