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Scientific Value Of Solid Onion Waste

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Solid wastes from the onion are a natural source of phenolic substances with a prospect of being exploited in the food industry. The use of cyclodextrins in aqueous solutions as an extractant makes it possible to encapsulate their phenolics in inclusion complexes. The extract produced by “green extraction” of onion waste is of interest as a natural food dye, as well as an improvement in its nutritional value. Its production process is aimed at extracting the phenolic components of onion waste with antioxidant and chromophore action.

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The onion (Allium cepa), also known as Crummyon or Alion the Commonwealth, is one of the most important species cultivated and consumed throughout the world. Onion is a biennial or perennial plant, but is usually grown as an annual crop and harvested during its first growing season. The plant has been cultivated by humans for 7,000 years.

Every year, 55 million tons of onion is produced worldwide, with production showing growth trends to meet growing demand. On the onion have been attributed from the antiquity tonic and healing properties. These properties have gained scientific evidence by identifying specific phytochemicals in recent decades. Onions contain a range of phytochemicals and their consumption promotes health as it has been linked to the reduction of incidence of carcinogenicity in various tissues, cardiovascular diseases, neurodegenerative disorders and the formation of cataracts and inflammations (Kwak et al., 2017, Lee et al. ., 2012).

These phytochemicals with beneficial effects include phenolics with antioxidant activity, with the main representatives of this group being quercetin and its glucosides. Their high content makes onions a major dietary source of quercetin.

Solid onion waste

The edible part of the dry onion consists of the underground bulbs which are cleaned by removing the outer leaves and edges. The process of sorting and processing onion produces a large volume of solid waste consisting of inedible dry dry and semi-dry leaves and honeycomb cuttings. In 2000 more than 450,000 tonnes of solid wastes of onions (SWR) were produced in Europe. These quantities burden the environment and remain unused (Kiassos et al., 2009).

The outer dry layers of the onion bulb, which are the main waste of the onion, are a source of valuable polyphenols such as flavonoids, anthocyanins and dihydroflavonols (Slimestad et al., 2007).

Flavonols Flavanols are the main flavonoids of the onion. The principal flavonols are based on quercetin (3,5,7,3 ‘, 4′-pentahydroxyflavone) and in the CAC at higher concentrations are the gluco-quercetin, 4-glucoside of quercetin and its 3,4’-diglycoside quercetin. Minor flavones of onions are highly structural in nature and include derivatives of amphotericin, equinamenetine, and myricketin. A total of twenty-five different onions have been identified.

Onion anthocyanins include a large group of flavonoids with different structures, making onion a particularly rich source. Most of the detected anthocyanins are cyanidine derivatives, but there are also small amounts of pentidine, dolphinine and peptunidine derivatives. The main anthocyanins of CAGs are cyanidine 3-glucoside, cyanidine 3- (3 “-glycosylglucoside), cyanidine 3- (3” -malonylglucoside), cyanidine 3- (6′-malonylglucoside) and 3- 3 “-glycosyl-6” -malonylglucoside) of cyanidine. A total of twenty-five anthocyanins have been identified on the onion.

The dihydroflavonols detected on the onions are based on taxipholine (3,5,7,3 ‘, 4 pentahydroxyflavanone), which exhibits structural affinity for quercetin and cyanidine. Dihydroflavonols do not exhibit (ROS) are produced in the body as products or in the form of a mixture of flavonoids and anthocyanins, and include four compounds of which taxoglobulin 3 -glucoside and taxigoline 4’-glucoside.

In addition to endogenous ROS production, there are also extrinsic sources such as tobacco, radiation, exhaust gases, pollutants and pesticides. The resulting active oxygen species interact with cells and their structures and can oxidize cellular biomolecules, such as nucleic acids, proteins, lipids and carbohydrates. The damage they cause is a cause of aging, carcinogenesis, cardiovascular, inflammatory and neurodegenerative diseases.

The ability of flavonoids to bind free radicals derived from oxygen by electron donation is due to their particular structure. The structure of their flavonites imparts an antioxidant effect due to the presence of a double bond between C2 and C3 and the carbonyl at the 4-position of the ring C, the hydroxyls at the 3-position of the C-ring and the 3-position and 4-rings of the B ring. The flavonoids from various extracts (Singh et al., 2009) and protect the organism from oxidative stress.

Anthocyanins, in addition to their bioactive properties associated with certain health benefits, are also natural colorants that can to be used for food staining, satisfying the increased demand for ‘clean’ labels (Chung et al., 2016).

Due to their properties, these wastes are of great interest as sources of natural dyes and antioxidants with potential for use to enhance the nutritional value of food as well as product staining under the “green label”. SCR extracts have already been incorporated into food matrices by exploiting their properties as antioxidants (phenolics), pigments (anthocyanins) and melanoma agents (thiol-containing compounds) (Roldan et al., 2008)


Cyclodextrins are oligosaccharides, which have biological properties similar to those of linear oligosaccharides. But they show differences in their physicochemical properties. This differentiation lies in the formation of water-soluble inclusion complexes with low solubility lipophilic components.

The cyclodextrin molecule consists of six to eight glucopyranose units linked together by α-1,4-glycosidic bonds and forming an annular tatra structure. The most common cyclodextrins consist of 6, 7 and 8 glycosyl units and are named α-, β- and γ-cyclodextrin, respectively. Β-cyclodextrin has limited water solubility, and for this reason derivatives such as 2-hydroxypropylated β-CD or hp-β-cyclodextrin have been synthesized with improved water solubility (Kurkov et al., 2013).

The inside of the cyclodextrin ring is hydrophilic whereas the inner walls of the ring are made up of the hydrophobic carbon backbones of the glucopyranose monomers, rendering the inner hydrophobic. This particular arrangement also relies on their ability to form inclusion complexes. Cyclodextrins are an ideal tool for enhancing the water solubility of insoluble molecules and improving their bioavailability. These oligosaccharides result from the enzymatic hydrolysis of starch and are recognized as non-toxic and inert excipients, making it possible to use them in food systems (Singh et al., 2009).

Molecular encapsulation with cyclodextrins (CDs) involves incorporating a hosted molecule or group within the cyclodextrin moiety. The hydrophobic inner portion of the structure of the cyclodextrin molecules can vary in size by varying the number of anhydroglucose units in the cyclodextrin molecules. Polyphenols from plant tissues such as onion solid waste can be encapsulated in cyclodextrins, as the benzene ring of polyphenols can be introduced into the cyclodextrin cavity. The formation of complexing complex with polyphenols increases their antioxidant activity, which can be attributed to increasing their solubility. Aqueous solutions of cyclodextrins could be considered as alternative, green solvents, as the formation of complexes between the cyclodextrin hydrophobic cavities and the nonpolar compounds can reduce the free energy of the system resulting in their increased solubility.

In addition, the nontoxic nature of cyclodextrins allows them to be used in food applications. The extraction of phenolic components and their utilization in food production is of technological and economic interest to the food industry and is gaining ground due to their reduced costs and the added value they bring to the product. The industry’s interest in these ingredients has led to exploration of how they can be extracted and picked up from the solid wastes of the onion. There are numerous methods for extracting polyphenols from plant waste.

Conventional methods include: solid liquid extraction wherein the plant tissues are lyophilized and extracted into liquid solvents by soaking or in a Soxhlet extraction device and liquid-liquid extraction. Basic disadvantages of traditional extraction techniques are the use of petroleum solvents, high energy consumption, thermal degradation of heat-sensitive polyphenols and long extraction times (Conidi et al., 2014).

Recently, the principles of green engineering and green chemistry have been introduced into various systems for the reception of these substances, aiming at the promotion of renewable extraction processes, using non-oil and plant-based solvents as raw materials. The insoluble nature of polyphenols makes it difficult to use water as an extractant. To improve solubility of the polyphenols in the solvent, microencapsulation may be applied to carrier molecules. The cyclodextrin structure allows the molecular encapsulation of biophenols by virtue of their ring-shaped structure, which makes it possible to incorporate the host molecule into the internal cavity of the carrier molecule.

Microencapsulation increases the solubility of polyphenols and improves their functional properties, and the use of aqueous cyclodextrin extracts is a green extraction method. ” The polyphenolic composition of the extracts is vital to stimulate the use of natural extracts in food applications. Generally, cyclodextrins appear to alter the solubility of the bioactive components in aqueous media resulting in increased biophenol extraction potential. In addition, less soluble or even nonpolar components can be solubilized in aqueous solutions. Vegetable extracts rich in anthocyanins have been used as a substitute for synthetic food colorants such as FD & C Red 40 and carmine in foods such as jams, dairy products, beverages or confectionery products (Helal et al., 2018).

Several foods are considered healthy but lack phenolic compounds that are known for their health benefit. Therefore, a possible incorporation of plant extracts from anthocyanin-rich onion solids into such products could provide a desirable red color while enhancing their nutritional value due to the presence of phenolic compounds. The process of extracting and microencapsulation of the phenolic components from the onion solid wastes influences the functional properties of the ingredients of interest to the industry by increasing the antioxidant action of these extracts to optimize their use as food enhancers.

The effect, however, of the process of producing these extracts in their flavor profile has not been thoroughly explored. Encapsulation in B-cyclodextrin has been found to reduce the bitter taste of certain substances (Konno et al., 2014), but this property has not been investigated in solid onion waste extracts. The main bio-phenols in the onion solids extract are Kerketin and Sweetcorn, which are characterized by a bitter taste, with a secondary flavor characteristic of certain peppery. The bitterness of these substances could restrict the use of the extract as a pigment in food. Improving the flavor profile of these extracts will facilitate their use in food as colorants and antioxidants.

Exploring the effect of extraction with β-cyclodextrin on the flavor characteristics of the final extract, particularly bitterness, which is an undesirable feature, requires organoleptic testing by trained test panels to identify the threshold of perception. Identifying the perception threshold would allow better use of these additives and ensure their future use.

03 December 2019

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