Onion, a very commonly used vegetable, ranks third in the world production of major vegetables. Apart from imparting a delicious taste and flavor due to its pungency in many culinary preparations, it serves several medicinal purposes also.
Onion is the most commonly used vegetable in the world food preparations especially in the tropical countries. Although, it is classified as vegetable, it has special qualities, which add to taste and flavor to food and hence it is mainly used in India for cuisine and culinary preparations. Besides adding a delicious taste and flavor, onion serves as a good medicinal compound for cataract, cardiovascular disease and cancer due to its hypocholesterolemic, thrombolitic and antioxidant effects.Several antioxidant compounds, mainly polyphenols such as flavonoids and sulfur-containing compounds, have been described in onion and garlic by the researchers namely Kourounakis and Rekka.
At least 175 countries grow onions. According to the latest information available from United Nations Food and Agriculture Organization. India produces all three varieties of onion viz. red, yellow and white. The production as well as market value of this potential vegetable is increasing day by day.
Onions are low in calories (50 kcal/100 g) yet add abundant flavor to a wide variety of foods. Onion is known for its nutritional value and for the utility as herbal medicine in our country. It has moderate amounts of protein, fat, fibre and good amounts of calcium, phosphorous and potassium, vitamin C and B6.
Apart from onion as such even the stalk is edible. The stalk contains good amount of carotene and iron. Onion has both glucose (reducing sugar) and sucrose (non-reducing sugar). The pungent taste of onion is due to volatile oil Allyl-propyl-disulphide present in it. The proximate composition and energy values of raw and dehydrated onion are shown in Tab
Proximate composition and energy values of raw and dehydrated onion (per 100 g of onion)
| Particulars | Big Onion | Small Onion | Onion Stalks | Dehydrated Onion |
| Moisture, g | 86.6 | 84.3 | 87.6 | 4.6 |
| Protein, g | 1.2 | 1.8 | 0.9 | 10.6 |
| Fat, g | 0.1 | 0.1 | 0.2 | 0.8 |
| Minerals, g | 0.4 | 0.6 | 0.8 | 3.5 |
| Fibre, g | 0.6 | 0.6 | 1.6 | 6.4 |
| Carbohydrate, g | 11.1 | 12.6 | 8.9 | 74.1 |
| Energy, K Cal | 50.0 | 59.0 | 41.0 | - |
| Calcium, mg | 46.9 | 40.0 | 50.0 | 300.0 |
| Phosphorus, mg | 50.0 | 60.0 | 50.0 | 290.0 |
| Iron, mg | 0.6 | 1.2 | 7.43 | 2.0 |
| Carotene, μg | - | 15.0 | 595.0 | - |
| Thiamin, mg | 0.08 | 0.08 | - | 0.42 |
| Riboflavin, mg | 0.01 | 0.02 | 0.03 | 0.06 |
| Niacin, mg | 0.4 | 0.5 | 0.3 | - |
| Folic Acid, mg | 6.0 | - | - | - |
| Vitamin C, mg | 11.0 | 2.0 | 17.0 | 147.0 |
| Magnesium, mg | 16.0 | - | 104.0 | - |
| Sodium, mg | 4.0 | - | 2.2 | 40.0 |
| Potassium, mg | 127.0 | - | 109.0 | 1000.0 |
| Copper, mg | 0.18 | - | 0.45 | - |
| Manganese, mg | 0.18 | - | 0.74 | - |
| Molybdenum, mg | 0.03 | - | 2.29 | - |
| Zinc, mg | 0.41 | - | - | - |
Onions contain significant amount of a flavonoid called quercetin. Although quercetin is available in tea and apples, earlier research proved that absorption of quercetin from onions is twice that from tea and more than three times that from apples . Onions are stimulant and mild counter irritant. Crushed raw onion can be applied on the forehead to get relief from headaches.
Red small onions can be used as an expectorant. Eating raw onions help to reduce cholesterol levels because they increase levels of high-density lipoproteins. It is advisable to include raw onions in the salads daily. It helps in controlling coronary heart disease, thrombosis, and blood pressure.
Onions are also used in the treatment of anaemia, urinary disorders, bleeding piles and teeth disorders. Anti tumor and anti cancer effect, platelet-anti-aggregating agent, anti-hypercholesterolemia, anti-ulcer and anti-gastric cancer agent activity of onion are also found by several researchers.
Processed and value added products are gaining importance in the worldwide markets.onion has 6% share in the overall production of vegetables in India and about 93% of the total export of fresh vegetables from India. Onion is mainly exported in the form of dehydrated onion, canned onion and onion pickle.
Free water is removed from the vegetables during the drying process so that microorganisms do not survive and reproduce. Simultaneously, the solids such as sugar and organic acids are concentrated thereby exerting osmotic pressure to further inhibit the microorganisms. Drying process involves the application of heat to vaporize water and removal of moist air from the dryer.
Dehydrated onions are considered as a potential product in world trade and India is the second largest producer of dehydrated onions in the world. There is a large demand of dehydrated onion in the European countries found a positive and significant growth rate in onion export which is of 6.27% per annum. Onions are generally dried from an initial moisture content of about 86% (wb) to 7% (wb) or less for efficient storage and processing.
Dehydration of food is aimed at producing a concentrated product, which when adequately packaged has a long shelf life, after which the food can be simply reconstituted without substantial loss of flavour, taste, colour and aroma. Several types of dryers and drying methods, each better suited for a particular situation are commercially used to remove moisture from a wide variety of food products including fruits and vegetables.
Moyis described the construction and performance of a natural convection solar dryer for drying the fruits and onion and found that from 342 m2 area, 500 kg of material can be dried in 3–5 days with a peak air temperature of 63°C and an air flow rate of 79 m3/min.
At a commercial level the convective drying of onion is used mainly now-a-days. Researchers have found that three stage drying of onion flakes had better effect on organoleptic characteristics of onion compared to one constant drying temperature of 50°C. However this convective drying method has some adverse effect on the finished product. Conventionally air dried products tend to be difficult to rehydrate satisfactorily due to structural changes in the product and of indifferent quality due to excessive thermal damage.
They recommended that the onion flakes could be dehydrated in two stage; in the first stage by employing 90°C temperature removing moisture upto 50% and in the second stage employing 50°C temperature removing the remaining moisture.
Microwave drying of onion with a hot air pretreatment and compared the products with traditionally dried ones. The colour of microwave and freeze dried onions was similar to that of air dried onions at 80 and 60°C, respectively. However, rehydration was better in case of freeze drying. Higher level of shrinkage was observed in case of microwave dried samples.
Onion slices were dried in a single layer of thickness varying from 1 to 5 mm in the temperature range of 50–70°C in a laboratory scale vacuum dryer. The effect of pretreatment, drying temperature and slice thickness on the drying kinetics of onion slices was studied using different thin layer models. The moisture diffusivity values were found ranging from 1.32E−10 to 1.09E−09 m2/s for untreated and 1.32E−10 to 1.09E−01 m2/s for treated onion slices. Effective moisture diffusivity showed increasing trend with increase in temperature and thickness.
Onion slices of 4 to 6 mm thickness were rapidly frozen to −30°C for 7–8 h at 6.1 mm Hg pressure. The colour, flavour, nutrient content and rehydration were found better than traditionally dried product. In this process best quality onions were subjected to preliminary freezing to −20°C while the amount of water evaporated at the sub zero temperature amounted to 85%.
Hot air drying of onion slices to 50% of their initial weight at 100°C followed by freeze drying occupied only about half the volume of conventionally freeze dried products but possessed similar rehydration properties except having a deeper colour. Freeze drying is recognized as to be the most superior method among all the dehydration procedures for many fruits and vegetables. Only drawback which hinders its large commercial use is its high operating cost compared to other dehydration methods.
Osmotic dehydration of onion slices is gaining importance now-a-days as an emerging drying technique. Osmotic drying is carried out in order to remove the moisture prior to mechanical drying. Three salt concentration levels of 5, 12.5, and 20% and three temperature levels of 28, 43 and 58°C.
The sample to solution ratio of 1:5, agitation of 100 shakes per min, sample thickness of 4 mm and 0.2% potassium metabisulphite mixed with osmotic solution were used for the study. It was derived that equilibrium moisture loss and solid gain are related to solution concentration and solution temperature logarithmically. Osmotic dehydration at 20% salt concentration at 28°C solution temperature for 1 h was found optimum for further drying of onion slices.
Review of different dehydration techniques of onion reveals that several analytical and numerical methods are available for analyzing the drying behavior as well as quality parameters. However, there are some other methods of drying such as vacuum drying, dehumidified air drying etc. which can be explored in order to assess the effect of different operating parameters on quality of onion as it contains several essential nutrients and has enormous medicinal value as well. Combination of two or more drying methods or multimode drying techniques can also be adopted for drying of onion. Most of the modeling of drying kinetics has been done for hot air drying method. These models can be tested for other drying methods also. Moreover, there is a scope for establishing proper correlation between drying conditions and energy consumption.
