Abstract: Onions are native to Central Asia and Mediterranean coastal areas. Featured by cold resistance, preferring a warm climate, high yield, good storage, long supply period; fleshy bulbs with fine texture, soft fiber, and delicate flavor, they are regarded as Vegetable Queen in Europe. At the same time, onions are able to improve appetite and help digestion, as they are of relatively high nutritional value and medicinal value. With the popularity of healthcare around the world, the food scientists, medical experts, and biologists have been trying to do a wide and deep study of onions. The use of advanced technology in developing onion functional products and researching onion liveness is gradually expanded, so developing new onion products has great prospects.

1. Overview of Onions

1.1 Species and Characters of Onions

According to the color of onion skin, the onion can be categorized into white onions, yellow onions, and purple onions. In terms of the nutrition, the purple kind is better than the other two kinds.

1.11 White onions

White onions have tender flesh, high moisture, and sweetness but low yield, and they are of gold color and rich sweet after long-time cooking; they are good to be fresh consumed, roasted or stewed.

1.12 Purple onions

Reddish flesh, strong acridity but poor storage; suitable for a fried dish or lettuce salad.

1.13 Yellow onions

Most are exported. Bright yellow flesh with fine sweet and mid-spicy taste; suitable for fresh eating or eating with sauce; good storage; commonly used as the dehydrated vegetable.

1.2 Value and Efficacy of Onions

1.21 Nutritive value

Onions are rich in nutrition, and per 100g onion consists following elements:

In the meantime, onions also have 18 types of amino acid and selenium, zinc, copper, iron, etc. Micro-elements, so onions are kind of treasured health food welcomed by many people, and exported increasingly.

1.22 Medical value

In traditional Chinese medicine, onions are considered to have mild nature and spicy taste; have the functions of warming lung & reducing phlegm, helping digestion, detoxifying & killing insects, treating wound and swelling; decreasing blood pressure and fat; and be able to relieve the crymodynia in abdomen, ascarid in children, rheumatic arthritis, dermatophytosis, skin ulcer, dysentery, pertussis, insomnia, etc..
Moreover, onions have some supportive therapeutic efficacy for diabetes, CHD, cerebral thrombosis, atrophic gastritis, atherosclerosis, etc.. Modern medical research has shown that frequent eating of onions has a certain curative effect on cancer, heart disease, hypertension, lithiasis, bronchitis, etc..

1.3 Consumption Trend of Onions and Its Deep-processed Products

Except for fresh eating, onions are also raw materials for food industry, and the major product is onion paste that has a wide market and can be used as a seasoning to lessen the bad smell of meats and fishes, or as added food into the soup, dessert or vegetable salad.

2. Deep-processed Products and Processing Technology of Onions

Other than the onion paste, there are many other deep-processed onion products, such as dehydrated onion slices, onion powder, onion essential oil, onion juice and multi-flavor onions. Among these products, the onion essential oil and onion juice are common in western developed countries, as one of the essential flavorings in their daily lives.

2.1 Onion Paste

2.11 Onion paste processing technology

Fresh onions-onion peeling-onion root cutting-onion rinsing-onion slicing/shredding-onion crushing-onion milling-onion acidifying and heating-onion enzymolysis-onion beating-secondary milling-secondary heating-onion concentrating-onion paste canning-sealing-sterilizing-cooling-finished product
---Choose the onion with strong spicy and with no mildew.
---Employ the onion peeling and root cutting machine to complete removing outer skin and roots; the cut them into slices or shreds with a thickness of 3-5mm.
---The pH value of citric acid should be 4.4-4.6; the soluble solid content should be 6-7% after enzymolysis
---The additive amount of enzyme should be 0.15-0.2%; the enzymolysis temperature should be 40-45℃, the pH value should be 4, time should be 15-20 minutes.
---The quality onion paste should be fine and smooth, light yellow in color, fragrant in flavor, no water separation, fiber, and impurities.

2.2 Dehydrated Onion Slices and Onion Powder

The dehydrated onion slices and onion powder is the result after onion dehydration, which has a significant proportion in further-processed onion products. These two kinds of products are resistant to long-distance transport, good for storage and easy to use. And both of them need to be dried, having no much different processing technology, but the onion powder needs crushing and screening process.

2.21 Processing technology

Fresh onions-cleaning/rinsing-onion slicing-original flavor preservation-onion dehydrating-picking-onion crushing-packing
---The ripe onion with dry outer skin, big bulb, small stem, tight structure, strong spicy, and internal consistency should be chosen.
---The thickness of slices should be 3-5 mm and they need a full washing, and color-protecting by dipping them in the baking soda solution.
---After cleaning and color-protecting, put the slices into the dewatering machine to remove the surface water. The centrifugal dewatering machine is widely used in this process.
---Preheat the drying machine to a temperature of 60 ℃, and spread the onion slices evenly in the drying machine for dehydration. The drying temperature should be 55-60 ℃, drying time should be 6-7 hours. When the moisture is reduced to ≦5%, the drying process is finished.
---The dextrin and lactose should be used for a rapid drop of water to keep the original flavor of onions so that the product can have a long-time storage.
---Pick out dried slices with inferior-quality and pack; or grind them into onion powder.

2.3 Onion Essential Oil

The commonly-used processing methods are hydrodistillation method, solvent extraction method and supercritical extraction method by which better-quality oil with a flavor that is close to nature onions.

---Hydro-distillation Method

Hydro-distillation method has three types of process, and they are a direct process, indirect process and the combination of the direct and indirect process. Hydro-distillation method is large in energy consumption. During the extraction, the onion may lose some constituents; some other constituents will be resolved into propyl mercaptan that has 50-70 times sweetness of sugar. So, the ripe onion has no spicy taste, and the dehydrated onion tastes crisp and sweet. The effective constituents of onion’s cellulolytic activity are stable to heat, insoluble in water, not easy to volatilize, which makes it possible that use the hydro-distillation to get onion oil.

---Solution Extraction Method

By this method, the low-polar solvent, such as ether, dichloromethane, etc., is used to extract onion oil from the fresh onions. The solvent extraction method has advantages of mild extraction conditions, less destroy of flavor components and high extraction rate. However, there is a problem of residual solvent, as a result, the application scope of this method is being narrowed down.

---Supercritical Extraction Method

This kind of extraction is conducted under not really high temperature with less consumption of heat-sensitive compounds. High-quality onion oil with no residual solvent can be obtained.

2.31 Onion oil technological process

Fresh onions-cleaning/rinsing-onion slicing-onion freezing-oil extraction-finished oil product

2.32 Tips

---When using the supercritical extraction, the onion should be freeze-dried into onion slices or powder.
---In the extraction process, 15% absolute ethyl alcohol should be added as entrainer.

2.33 Processing parameter

Taking the freeze-dried red onion powder with a particle size of 0.45-0.90 mm as the raw materials, the extraction pressure is 27.98 MPa, the temperature is 42.18 ℃, time is 216.6 minutes, extraction rate is 0.483%. The finished onion oil is of a strong flavor of fresh onions.

2.4 Multi-flavor Onions Production

---Peel the fresh onions and cut their roots without making the bulb scattering; then make rip cuttings every 1cm along the onion’s waist part with a half depth to the center.
---Steep the onions into the saturated clarification water for about 10 hours and then rinse them by clean water.
---Add 280g dry ginger powder, 200g Curcuma powder, 200g table salt, 80g saccharin, 80 white pepper powder, 80g red pepper powder, 80g garlic juice into 20kg sugar solution with 50% sugar content; after boiled, place 30kg prepared onions in the mixture for 5 minutes’ boiling, then stop heating for 2 days’ steep; 2 times of stirring is needed during steep.
---When the steep is finished, a second boiling is done until the sugar solution mixture half remains.
---Dry the outer layers of steeped onions under a temperature of 55-60℃; then split them to dry their center part to a moisture content less than 20%.
---Settle the finished onions, then cool and pack.

3. Onion Products with Wide Development Prospects

Onions have wide-range cultivation areas around the world, and their production comes right after tomatoes, accounting for about 10% of the world’s total vegetable production.
The onion is a kind of seasonable crop for both medical and edible uses as garlic, so its use is easy to be affected by various factors. Nowadays, to meet the market demands, the highly-processed onion products that can be produced are the supercritically-extracted onion oil, onion powder, onion paste, etc.
With the rapid development of the economy and the improvement of people’s living standard, people have a growing awareness of health care. So, the deep-processed onion products will become a necessity of people’s diet.
In addition, with the popularity of healthcare around the world, the food scientists, medical experts, and biologists have been trying to do a wide and deep study of onions. The use of advanced technology in developing onion functional products and researching onion liveness is gradually expanded, so developing new onion products has great prospects.
In view of the current situation and the nutrition and health-care value of onions, exploiting more onion products with high quality is not only a great chance but a great challenge for the onion processing industry.