A comprehensive review of the nutritional, pharmacological, and industrial value of polyphenolic antioxidants found in onions

Onions are an excellent source of a variety of essential antioxidants, polyphenols, and other phytonutrients. These onions were used to enhance the meal's flavour and aroma. Additionally, onions have long been recognised for their ability to change food texture, making it more pleasant for eating. Among the most significant phenolic compounds identified in onions was Quercetin, which has been proven to be very helpful for maintaining good health by reducing the risk of heart disease, cancer, and a number of other severe conditions. Polyphenol extraction is essential for the effective usage and research of these phytochemicals. Previously, the extraction process was complicated by a variety of traditional methods, but with the aid of contemporary technology, the procedure was simplified. Additionally, it was found that red onions had greater health benefits than yellow onions, which were the most commonly used kind at the time. Apart from their health benefits, polyphenols were well recognised for their industrial use in dyeing, polymer manufacturing, and resin synthesis.


INTRODUCTION
Allium cepa L. ,Onion is one of the most valuable vegetable crops globally owing to its excellent taste and several health-beneficial properties. Onion bulbs are a rich source of antioxidants and sulfur-containing compounds, well-known for their potent anti-inflammatory effects, thereby helps in minimizing the chronic diseases, including cancer and diabetes [1].
According to Bolton, the onion name is given as a reasonable explanation where the onion mocks the campus bulletin called The Union. Onions usually produce chemical irritation called syn-prudential-s-oxide that stimulate the lachrymal gland in the eyes that cause tears. Onions are tolerant to cold and harsh conditions also, they can grow well in high temperatures and different growing environments. As a most cultivated crop, onion faces several breeding programme challenges. Onions are generally cross-pollinated, through insects and may sometimes result in genetic contamination [2]. There are several health benefits and medicinal properties of onions where they treat mouth sores, colds , headaches etc. Onions are packed with several vitamins and minerals; also, they contain low calories having reasonable amounts of Vitamin C and Vitamin B 9 and B 6 and are a good source of potassium promoting cellular function and proper muscle contraction. Onions maintain heart health due to Quercetin and anthocyanins, an antioxidant highly concentrated in onions. Onions fight against cancers due to the presence of sulphur compounds and flavonoid antioxidants. Also, onions aid in proper digestion due to inulin and fructo-oligosaccharides [3].
Along with the above essential factors and benefits of onions, polyphenols also are of primary importance due to their antioxidant properties and protection against certain diseases.
These polyphenols contain flavonoids, phenolic acids, phenolic amides and lignans, which are also present in fruits, vegetables and grains. Out of these, the significant polyphenols observed in onions are flavonoids which include three diverse classes like flavonoids, fructans and organosulphur compounds [4].
As discussed, these polyphenols are associated with several health benefits by maintaining the blood sugar levels, thereby reducing the risk of type 2 diabetes in people consuming a high amount of polyphenols comparing to people who consume fewer polyphenols. Among polyphenols, anthocyanins provide an antidiabetic effect found in onions.
Also, the continuous supplement of polyphenols proved to reduce the blood pressure and also LDL bad cholesterol levels increasing HDL good cholesterol [5]. Lower risk of heart disease is observed with high enterolactone levels obtained by lignan intake, a critical polyphenol.
Proper intake of polyphenols is also known to help in reducing the risk of breast and prostate cancers. The flavonoids present in onions help improve blood flow to the brain and improve working conditions and memory of the brain. Polyphenols are also known to reduce blood clots, thereby promoting proper brain function, improved digestion, and protection against several cancers [6]. Along with onions, several other vegetables and fruits like carrots, potatoes, asparagus, apples, lemons, and pomegranates contain a good quantity of polyphenols.
Sometimes higher polyphenols would result in kidney damage in rare cases [7] It was observed that the total polyphenol content is more in yellow onions than red onions, making them very beneficial for consumption. In this review, we will concentrate mainly on the total polyphenols in onion, their presence in the peel and layers of onion and their composition, their extraction methodology, also concentrating on the health benefits and industrials uses of polyphenols[8].

Extraction, Analysis, and Polyphenolic Composition of Onion Bulb and Skin
Several publications were available on the extraction and analysis of the polyphenols in onions over the las few years. But the traditional methods were replaced by newly emerging advanced techniques. The conventional methods include sample preparation, separation, detection and identification, which are altered by modern techniques. Firstly, the extraction procedure should be understood properly along with the chemical nature of the substance, sample size, and the presence of interfering substances. Before starting the experiment, the time, temperature, and the number of extractions should be known because of their effect on the extraction yield. Additional steps were also performed to eliminate compounds like waxes, terpenes, fats etc., that were not required. Polyphenols can also possibly be extracted from fresh, dried or frozen plant samples [9]. The water bath method is the primary method for conventional extraction. This method of water bath extraction is done at temperatures ranging between 20-so degrees; the more the time of water bath, high will be quality of the final product, i.e., polyphenol content. However, the use of higher temperatures above 70 degrees will decrease the polyphenol content because of their degradation. Also, the stability and quality of the extract are influenced by the pH of the medium. Therefore, pH between 4-5 is considered appropriate for polyphenol extraction in onions [11].

Modern Extraction techniques
Due to changes in temperature conditions and reduced quality of polyphenols by the influence of temperatures, specific new methods are adopted to reduce the quality issues. These modern techniques are pretty accurate and are used as alternative techniques for phenolic extractions [10].

Microwave-Assisted Extraction
This is the most effective and popular method, which reduces the quality losses and is very efficient in polyphenol extraction. Due to its ease and availability, this method was designed to work in two ways, pressurized (PMAE) microwave-assisted extraction and (SFMAE)solvent-free microwave-assisted extraction. This method eases the polyphenol extraction using less solvent. Most plants contain higher amounts of water, so this is an effective extraction tool. Low water content plants need longer to heat the microwave. The temperature of the heated plant in the microwave cannot exceed the boiling point of water (100 ° C). Microwave radiation consists of oscillation and magnetic fields. When the microwave electric field passes through water molecules, it forces water molecules to oscillate, resulting in friction between molecules and warming up. This method is the best so far and has less extraction time with energy-saving mode [12].

Ultrasound-Assisted Extraction (UAE)
UAE is beneficial to enhance bulk transfer, cell disorders, more enhanced penetration and capillary effects. Very high temperature in the UAE increases solubility, diffusivity, and pressure, which helps the waves penetrate the contents of the network and transportation in various organic and inorganic solvents. Ultrasonic probes and baths are the

Analysis of polyphenols
In addition to organic solvent extraction under this technique, it can be applied to

Major polyphenolic compounds
There are several polyphenolic compounds available in nature primarily found in fruits and vegetables. These compounds were highly beneficial due to their antioxidant properties and medicinal values. Several phenolic compounds are available, but phenolic acids, flavonoids, stilbenes, tannins and lignans are the significant classes for these phenolic compounds 4.1. Polyphenolic compounds in red and yellow onions Several reports indicated that onions are very healthy and beneficial for health. Therefore, more attention was drawn to the phenolic compounds of onions, which were essential for good health [20]. Generally, both red and yellow onions are instrumental in preparing dishes; in some parts, red onions are used for cooking purposes and white onions are used for garnishing. In India and some Asian countries, red onions play a significant role in cooking due to their flavour and color. But in the USA and few other countries, white onion was preferred over red onion due to its spice and high Sulphur content. Therefore, the availability of polyphenolic compounds was discussed by comparing the red and yellow onions [21].
It was observed that the total phenol content, flavonoid content and proanthocyanin content of yellow onions was slightly higher than that of the red onions. When compared within all the layers of the onions, the total phenol content was higher in yellow onions than red onions. Also, the flavonoid content from inner to outer layers of onions was higher in yellow onions and less in red onions. It was also observed that the highest flavonoid content was observed in the first layer of the onion and gradually reduced in the inner layers; the content is 7-fold in case of yellow onions and just 5-fold in case of red onions [22].
Proanthocyanin content in contrast to flavonoids and total polyphenolic content was higher in red onion and less in yellow onions, may be due to the red color of the onion. The anthocyanin pigment not only imparts color to the red onions but provide high antioxidant capacity. It was observed that the anthocyanin content in all the layers of red onions was steady without any maximum difference [23]. Thus it was concluded that the total phenolic content, flavonoid content and promising antioxidant activity was highest in case of yellow onions compared to red onions which is having high antioxidant properties. The reduction of the contents from outer to inner layers was observed in case of both the yellow and red onions.
From our understanding it was advisable to prefer onions which are having maximum phenol, flavonoid content with antioxidant activity is very beneficial compared to antioxidant activity alone [22].

Anti-cancer and anti-diabetic properties
Apart from providing deep flavor, color and consistency to the food, onions are well known to protect from cancers. It was investigated that the regular use of onions would reduce bowel cancer. Along with several skin cancers, breast cancers colorectal cancer, aka bowel cancers, occupied third position as the most common cancer. It was proposed that the regular use of onions would reduce cancer risk [24]. A lot of attention was drawn regarding the clinical hypoglycaemic effects of onion due to its anti-diabetic properties. Diabetes was a heterogeneous group of disorders caused by several abnormalities especially in carbohydrates, proteins and lipids. It was proven from Environment reveals insights that consumption of fresh onions on daily basis would help to reduce the risk of type 1 and 2 diabetes showing positive impact on diabetes. It was observed that onions have a protective effect for diabetes by enhancing insulin resistance by encouraging multiplication of beta cells thereby increasing insulin secretion. These onion was proven to have several essential health benefits and can protect humans from health issues like cholesterol, cancers and diabetes [25]. As stated in Table   1, many studies on onions were performed, and different literatures were reviewed to get a better knowledge of the uses and importance of onions in the treatment of several diseases.
Onions have been shown to help mitigate the negative effects of cholesterol, diabetes, cancer, and post-cancer sickness, which explains why we should eat onions on a daily basis (Table 1).  [47,48]. The outer layer of onions which is generally unutilized and wasted, contains higher proportions of anthocyanins and antioxidants. This outer papery layer can be an excellent source of beneficial antioxidants. This antioxidant property was adequately measured with the help of Trolox equivalent antioxidant capacity (TEAC) , which then is used for calculating the amount of antioxidants present [49]. A report assessed and compared the antioxidant potentials in different onion cultivars belonging to Nigeria using an in-vitro assay technique. It was proven that the red onions have antioxidant potential over other cultivars, which is greater than 69%, making red onions essential add-ons in our diet. These red onions contain the highest health-promoting potential factors when compared to others [50,51].

Bioaccessibility and Bioavailability of Onion Polyphenolic antioxidants
Polyphenolic antioxidants gained much importance in recent years due to their beautiful effects on human health and well-being. These compounds were known to treat several important diseases and health issues in our day-to-day life. A little consumption of onions regularly would help us improve several ailments due to their antioxidant activity. The wide range of antioxidants present in polyphenols is known to positively affect non-communicable diseases like cardiovascular diseases and diabetes [52]. Also, positive effects were observed on treating neurodegenerative diseases like Parkinson's and hunting diseases, but in early stages.
The polyphenolic compounds present in onions reduce Obesity by promoting lipolysis so that bad cholesterol can be eliminated [53]. Interestingly, polyphenols interact with gut microbiota, thereby promoting healthy and good bacteria while preventing harmful bacteria. Also, these polyphenols increase the activity of probiotics by promoting their beneficial actions [54]. These polyphenols can be measured and isolated by volumetric titration. Permanganate, an oxidizing agent, was used to oxidize standard solutions, thereby creating a standard curve. Calorimetric measurements were also used to quantify polyphenolic contents.
New methods like biosensors were used to measure the polyphenolic contents. These measurements were adequately assessed with the help of HPLC and concluded. In general, the bioaccessibility of polyphenols is determined by their amount detected in the fruit or vegetable and released into the human body for easy access [55]. Polyphenols were wonderful natural phenols available from the phenylpropanoid pathway for the phenolic acids and shikimic way for tannins and analogues. Polyphenol Oxidase (PPO) is an enzyme that catalyzes o-diphenol oxidation to produce o-quinone. This is a quick polymerization of O-Quinon to produce black, brown or red polyphenolic pigments, which causes colouration of fruits and vegetables.
Polyphenols from fresh samples were highly water-soluble and are obtained in large quantities [56]. Consuming standard servings of onion, coffee, wine, chocolate, legumes or tea may also contribute to about one gram of intake per day. Herbs, spices and beans also have the potential to be significant to supply certain polyphenols. These polyphenols were obtained from several fruits and vegetables [57]. Different cooking methods can change the polyphenol content. These polyphenols were better available in steamed vegetables than deep-fried vegetables because they can be destroyed by heat [58].

Industrial applications
In the Indian continent, pomegranate peel contains high tannins and other polyphenols, whereas juice, used in non-synthetic cloth colouring [47,59]. After being processed, the aim is to utilize plant residues from wine, olives (called pomacing) or pecan shells [48,60]. Especially in onion quercetin is one of the major polyphenolic compounds with several advantages and abundantly available in the skin and layers of onions. The availability of these compounds in different layers of onions are stable except for flavonoids [61,62]. The immense availability of polyphenols and its production would increase the preservation of the food by reducing its oxidation which is also an important property of polyphenols [63,64].

Conclusion and prospects:
Not just a culinary plant, Onions are a common accompaniment in every Indian meal.
These onions have gained much importance not only because of their flavour but also their colour, pungency, taste, and several health benefits. Different onion varieties were available throughout the world depending upon the people's usage like red, white, yellow. Out of these, red onions are a good source for curry making and white onions are usually used as salads.
Several anti-bacterial, anti-diabetics, anti-cancerous and nutritious properties of onions made them a topic of interest to our study. These onions can grow in any temperature conditions except high chilling conditions, which results in stress and ultimately leads to death [65]. In our manuscript we have focussed mainly on the total polyphenols present in the onion and their importance. There are several health benefits associated with polyphenols which act as reducing agents such as vitamin C, E and carotenoids protecting the body tissues against several ailments. These include anti-cancer activity, anti-ageing activity, anti-inflammation activity, lung-protective effect, neuroprotective effect, kidney protective effect and cardio protection.
Among all the polyphenols in onion, Quercetin is one of the significant and dominatingpolyphenolsl presents. It is also quite helpful in preventing allergies, ageing, mood swings, cancers, asthma etc [66][67][68][69][70]. When compared red onion which is the primary