Onion (Allium cepa L.) bioactives: Chemistry, pharmacotherapeutic functions, and industrial applications

Abstract

Bioactive compounds possess different health benefits. Onion contains various bioactive compounds, such as organosulfur compounds, flavonols, ascorbic acids, and carbohydrate prebiotics, and its by‐products have more content of flavonoids than the bulb. Diallyl monosulfide, diallyl disulfide, diallyl trisulfide, and diallyl tetrasulfide are the major organosulfur compounds, whereas quercetin, kaempferol, anthocyanin, and luteolin are considered as main flavonoids. Ascorbic acid and fructooligosaccharides are also regarded as bioactive compounds. Onion bioactive compounds have the strong antioxidant potential for neutralizing oxidative stress of the cells. These bioactive components are beneficial as anticarcinogenic, antibiotic, anti‐inflammatory, antiplatelet, antidiabetic, and cardioprotective agents along with other nutritional benefits. However, various postharvest practices have an impact on these bioactive compounds, for example, curing mostly enhances the bioactive level and processing temperature generally decreases the concentration of many of them, whereas storage studies suggest an increase of others under optimized conditions. Additionally, conventional extraction techniques showed a negative impact on bioactive compounds of onion, whereas innovative methods yielded a higher amount of bioactive components. There is a need for innovative and integrated procedures in the postharvest sector to maintain or enhance the level of bioactive compounds without compromising the quality of onions. The present review comprehensively describes different bioactive compounds of onion, their chemistry, and their pharmacotherapeutic roles. Moreover, it also explores the effects of various postharvest factors, such as temperature, storage duration, and extraction conditions on the level of the bioactive components. It also suggests industrial applications of onion waste and its bioactive compounds in the food sector.

Differences in Antioxidant Potential of Allium cepa Husk of Red, Yellow, and White Varieties

“…Red onion husks had the highest contents of polyphenols, followed by the yellow onion husks, with the white onion containing the lowest amounts [ 36 , 47 , 48 ], but in our study no compounds were detected in the white onion husk extracts. It was reported that white onions contained trace amounts of total quercetin [ 37 ], but it was detected in husks, albeit in much lower concentrations than in red and yellow ones [ 11 , 14 ]. Quercetin and myricetin, found in large quantities in red and yellow onion husk extracts, are the main representatives of flavonols, which are considered to be the most abundant type of flavonoids in foods.…”

“…More than 550,000 tons of bio-waste are generated during onion processing [ 5 , 6 ], which consists of peeling, slicing, and dicing, and these operations generate a large amount of waste [ 11 ]. Interest in onion waste has grown significantly in recent years, as evidenced by the increase in the number of related scientific studies in the past 2 years.…”

“…This is due to the cheapness and availability of this waste. The waste biomass mainly consists of skin/peel/husk (the outermost layers), roots, tops of the bulbs, and deteriorated bulbs [ 6 , 12 ]; however, onion skin represents the main waste component in onion processing (up to 60%) [ 7 , 11 ]. Onion waste is characterized by an unpleasant taste and smell; therefore, it is not suitable for animal feeding or as organic fertilizer, and is usually sent to landfill, causing an environmental problem [ 5 , 6 , 7 ].…”

“…Concerning onion varieties, quercetin and its derivatives, along with glucosides, are the predominant flavonols in all types of onions, regardless of the white, yellow, or red color, while anthocyanins are mainly present in red onions, wherein they make up approximately 10% of the total flavonoid content in fresh weight [ 7 , 9 ]. Contents of prominent flavonoids such as quercetin and its glucosides are the highest in pearls, followed by red, yellow, and white onion husks [ 11 , 14 ], and strongly depend on extraction conditions [ 15 ].…”

Integrating Genetic and Chromosome Maps of Allium Cepa: From Markers Visualization to Genome Assembly Verification

“…Onion (Allium cepa L.) is ubiquitously used as flavorful and healthy food, not to mention its remarkable medicinal properties, both preventive and curative. Onions also possess anti-cancer properties due to the accumulation of a number of bioactive compounds, including organosulfur compounds (review [22]), as well as antibiotic, anti-inflammatory, antiplatelet, antidiabetic, and cardio protective properties (review [23]). In terms of global production value, onion is ranked second after tomato (www.fao.org/faostat).…”