ONION VALUE ADDITION

0 Introduction

Onion (Allium cepa L.) is an important vegetable that is highly used in food preparation worldwide. It is the third-largest produced vegetable worldwide and is among the most commercial vegetables. Despite being low in calories (50 kcal/100 g), onion is loved in preparation of almost all ethnic cuisines due to its delicious taste and flavour from its pungency (Kaack, 2017). It is also much adored due to its health benefits and medicinal applications for conditions such as cardiovascular disease, cataracts, gastric ulcers and cancer due to its hypocholesterolemic, thrombolitic and antioxidant effects. Onions are also rich in nutrients that include vitamin B, Vitamin C traces and calcium (Mitra et al., 2011).

The onion consists of about 9-10% Carbohydrates, 1.1% proteins and 0.1% fat. Onion bulb (Allium cepa) is one of the crops with highest fructan content, with a content of 8.60% of the fresh weight of the bulb. This fructan component is referred to as prebiotic fiber because it feeds the beneficial microorganisms in the gut, improving the colon health and reducing the risk of colon cancer attack. Fructan has also been attributed to increased calcium and magnesium absorption by the colon and secretion of satiety hormones. Onion has also been found to contain several vitamins which are of great importance to the human health. These include Vitamin B6 and Folic acid (B9). In addition, onion also contains a number of minerals including calcium, magnesium, Phosphorus and potassium that are important in nutrition and health of populations consuming onions.

Harvested onions should be stored in a well-ventilated, cool, dry and dark location to avoid fast quality deterioration. Dark location is necessary to prevent onion sprouting. Harvested onions should not be stored at temperatures of 20-25 degree Celsius as these temperature ranges support the activity of bacteria and fungi that are onion pathogens.

Storage condition is one of the most important factors to consider in postharvest handling of onion. The relative humidity and temperature should be tightly controlled for effective preservation as it controls physiological activity, biochemical activity and microbial invasion of the onion bulb. Chemical treatment of the onion bulbs has been shown to inhibit the growth of the black mould and other fungal microbes in onion bulbs. Among chemicals that could be used in this treatment include salicylic acid, bronopol and maleic hydrazide.

Therefore, onion value addition plays an important role in minimizing huge post-harvest losses, employment creation, stabilization of market prices and promoting nutritional benefits. Some of the techniques used in value addition on onions include solar drying, convective air drying, infrared drying and pickling. Onions are classified depending on the skin colour (red, white, yellow or brown coloured varieties), taste (sweet and pungent) and shape of the bulb (round, flat, or globe).

3.1 Cleaning

Cleaning of onions is meant to get rid of dirt, pesticide residues and microorganisms that can pose health risks, onion decay and quality loss. Fresh-cut industry commonly uses chemical-based washing of onions using chlorine dioxide solution. There are alternative techniques that are being applied instead of chlorine dioxide. The use of mild‐temperature water is also effective in controlling insect infestation and surface microorganisms in onions. Onions that are meant for storage should be dried after cleaning. Additionally, the roots should be kept clean, free from moisture and soil to avoid rotting.

3.2 Removal of inedible parts

Removing inedible parts of the onion involves simple steps as follows:

Cut away the stem end. The peduncle residues should be twisted and not cut off using a knife to avoid onion neck rot.
Peel off the papery skin to remain with the edible thicker, moist and tender onion.
Cut off the root end of the onion to remain with the edible portion of the onion.

3.3 Disintegration or size-reduction

Onion can be cut manually to desirable sizes using a sharp knife. Both ends of the onion are trimmed, and the onion cut into two halves. Onion peels are then removed from both halves, and edible portion held firmly in a chopping board and cut into the desired pieces.

4.1Principles and Methods of Value Addition

Value addition involves any process that transforms a product to a more valuable form. Onion processing technology enables development of numerous value-added products that are more desirable by consumers. Vale addition yields multiple benefits to the onion industry. Advances in the field of processing makes it possible to produce different value-added products from onion i.e. minimally processed ready to use or ready to cook fresh onion, onion paste, dehydrated onion flakes, onion powder, onion oil, onion vinegar, onion sauce, pickled onion, onion wine and beverages.

4.1Importance of onion value addition

Reduction in high post-harvest losses that usually occur to raw onions to their perishable nature.
Allows the availability of onions in the off-season and controls the market price of the products.
Value-added products are produced from locally-sourced raw materials that are cheap and not capital intensive.
Value addition process takes advantage of the local labour markets and provides a source of income to those who would otherwise be lacking job opportunities.
Businesses involved in the production of value-added onion products contribute to the economic development of the country.

4.0 Value Adding Technologies

Value adding technologies involve the processes that allow production of products that are of more value than the raw products.

4.1 Vegetable Processing Technologies

The main aims for vegetable processing technologies are to prevent microbial spoilage and natural physiological deterioration of plant cells to prolong the shelf life of perishable vegetables. Processed vegetables allow ease in transportation and distribution of a wide variety of vegetables.

Onion Processing majorly involves onion dehydration or pickling. Flavonoid-free varieties are desirable for pickling because flavonoids are responsible for causing yellow spots that reduce the bulb quality.

The following are the requirements for onion varieties for dehydration ("Onion Harvesting and Storage", 2020)

High pungency because dehydrated onions are mainly used as flavouring agents, and some npungency is lost in the dehydration process.

Flesh that does not discolour or develop bitterness on dehydration. Yellow bulbs contain pigment quercetin that causes a bitter taste and inferior flavour; thus, not a suitable variety for processing.
High solid content, i.e. total soluble solids of 15-20 % or more.
High insoluble solid content (1 % or more)
Low reducing to non-reducing sugar ratio.
Full globe to tall globe shape of the bulb with a thin neck.
Bulbs which are resistant to diseases, pests and should hold up for a considerable time of 2-3 months in storage.

4.1.1 Choice of vegetable processing technology in developing countries

The choice of appropriate vegetable processing technology in developing countries should be based on the following factors:

Promoting farmer income through the utilization of local resources, i.e. indigenous raw materials and part or all of the processing equipment.
Reduction of production costs through the utilization of local natural resources and reduced transport costs.
Generating and distributing income by decentralization of processing activities and engaging various beneficiaries in processing activities.
Maximizing national output by reducing capital expenditure and royalty payments.
Maximizing availability of consumer goods by maximization of high-quality, standard processed produce for internal and export markets, reducing post-harvest losses, giving added value to indigenous crops and increasing the volume and quality of agricultural output.

5.1.2Techniques for vegetable processing

Blanching

Blanching is a heat treatment process in which vegetables are washed and dipped in hot water at 88 ° C for two to five minutes. Alternatively, they can be heated using steam in a conveyor for thirty seconds to a minute. After heating, vegetables are cooled rapidly in cold water or cold air better retention of quality.

Objectives of blanching

To inactivate undesirable enzymes that result in colour discolouration and off-flavours.
Reduces microbes in the vegetables.
Stabilizes the color of vegetables for consumer appeal
Allow ease of vegetable packaging.
Extends the shelf stability of vegetable products

Dehydration

Dehydration of vegetables is an ancient vegetable processing technology. It serves to reduce water in vegetables leading to weight and volume reduction that aids in easy storage and transportation. Drying also prolongs the shelf life of vegetables by inhibiting growth of spoilage microorganisms through decrease in water activity. Dehydration can be done in several forms such as vacuum drying, spray drying, solar drying and freeze-drying.

The objectives of drying onions are:

To remove all the surface water or moisture from the onion.
To dry the layers of the onion skin.
To dry the stem and seal it after being cut off.

Dried onions are best stored at temperatures of 25 and 30° Celsius.

Canning

Canning process involves cutting vegetables into small pieces, packing in containers, and passing through severe heat treatment to destroy bacterial spores. The containers are then sealed while hot to create a vacuum during cooling at room temperature.

Canned vegetables can be stored for years without getting spoiled. Heat processing should be adequate to kill all the bacteria spores, including the heat-resistant Clostridium botulinum that produces deadly toxins causing botulism. Bulging of containers is one indication of the presence of spores, and the contents should not be consumed for safety reasons.

Freezing

Freezing is a nice way of vegetable processing that maintains flavour and quality of vegetables. Individually quick-frozen (IQF) technology applied do not allow the formation of large crystals on vegetable cells and thus, maintaining the quality of frozen products. The freezing technologies utilized include blast freezing, plate freezing, belt-tunnel freezing, fluidized-bed freezing, cryogenic freezing, and dehydrofreezing.

Fermenting and Pickling

Fermentation and pickling use acid in vegetable processing and thus, require less heat treatment for preservation. Some of the pickled vegetables include onions, cabbages, cucumbers and green tomatoes. The variety of vegetables used for fermentation or pickling may not be the same as fresh market vegetables. Vegetables are easily perishable commodities due to their high water activity and nutritive values. These conditions are more critical in tropical and subtropical countries which favour the growth of spoilage causing microorganisms. Lactic acid fermentation increases shelf life of vegetables and also enhances several beneficial properties, including nutritive value and flavours, and reduces toxicity. Fermented vegetables can be used as a potential source of probiotics as they harbour several lactic acid bacteria such as Lactobacillus plantarum, L. pentosus, L. brevis, L. acidophilus, L. fermentum, Leuconostoc fallax, and L. mesenteroides.

Irradiation

Irradiation using gamma rays is being applied in vegetable preservation in various countries. Irradiation technology has been showed to be effective in reducing post-harvest losses, and controlling the stored product insects and the microorganisms. Irradiation is extremely beneficial in prolonging the vegetable shelf life by 3–5 times. In order not to expose vegetables to high irradiation doses another approach is to use the “hurdle technology,” that is to apply more than one technology toward better quality and longer shelf life. However, low-dose ionizing radiation in foods has been shown to be safe. Ionizing radiation is mainly applied in potato preservation to prevent sprouting.

5.0Value-Added Onion Products

Onion processing is gaining importance worldwide. Processed onion is mainly in the form of dehydrated onion, canned onion and onion pickle (Gorrepati & Khade, 2020). Examples of value-added onion products include minimally processed onions like frozen onion rings, onion paste, dehydrated onion flakes, onion powder, onion oil, onion vinegar, onion salt, onion sauce, pickled onion, vacuum-packed onion, onion wine and beverages. Dehydration is widely used in onions to reduce the onion’s moisture content of about 86% to 7% or less for efficient storage and processing (Grewal et al., 2013).

Minimally processed onions: Comprise the freshly cut onions aimed to retain freshness. These onions are available in a ready to cook form that enables consumers to save on the preparation time (Bahram-Parvar & Lim, 2018). Frozen onion rings can be made in the form of raw onions sliced into rings, freeze-dried and stored in a suitable packaging material under low temperatures. The availability of minimally processed onion for ready to cook or ready to use purpose reduces the inconvenience of peeling and cutting before food preparation there by reducing overall food preparation time. Minimally processed onion is processed by peeling and cutting/size reduction which retains its freshness. The peeling and cutting can be done either manually or mechanically. Although peeled onion offers convenience, due to the removal of protective layer by peeling, the storage life can be compromised. The storage life depends on type of variety, length of storage and storage conditions i.e. temperature and relative humidity (RH) of bulbs before peeling. The most visible quality changes that can be seen in peeled garlic include sprouting, rooting and discoloration apart from the invisible nutritional quality changes. Due to high moisture content microbial growth is a major concern in case important to reduce weight loss by controlling respiration rate and to augment the storage life.

Fresh Onion rings

Onion paste: Onion paste comprises onions that have been grounded to form a paste. Paste can be processed by peeling and grinding, as it contains high moisture, proper packaging and storage conditions are required to store for longer duration without any microbial spoilage. Addition of preservatives or thermal treatments can be followed to store it for longer duration. The final paste can be packaged in glass jars or metal cans.

Onion paste

3. Onion oil: Onion oil results from distillation of minced onions. It has a flavour strength 500 times that of the dehydrated product. Onion contains exceedingly small quantity of oil. This can be extracted by solvent extraction method or super critical fluid extraction technique. Oil is used as a flavoring substance, preservative and as a medicine. As the oil contains specific pungent aroma, encapsulation of oil is also done to reduce the pungent flavor while consuming.

Onion Oil

Fermented Onion Products: The fermented onion products are used to flavour food. They are made from ground onion leaves and tops. These products are popular in parts of West Africa. Lactic acid fermentation can beused to processed sweet, white, and yellow storage onions to produce sour onion product. The onions are sliced to 0.3 cm thick, salt is added at 1.5, 2.0, and 2.5 g/100 g without or with sugar at 1.0 and 2.0 g/100 g, and the fermentation temperature is 18°C. Since onions do not have the necessary lactic acid bacteria for the anaerobic fermentation, onions are inoculated using either brine from sauerkraut or slices of cabbage. The fermentation produced sour onion with pH between 3.25–3.35 and 1.2–1.5 g lactic acid/100 ml, which is in the range as that of sauerkraut. The sour onion has a tart acidic taste, characteristic of sauerkraut, with onion flavor but without the pungency of raw onions.

Fermented onion product

Dehydrated onion flakes are made from the dried onions and can be grounded to powder that dissolves more than the flakes. Dehydration in to flakes reduces bulk to store and transport besides increasing shelf life. Flakes are prepared by peeling, cutting, pretreating and drying using different drying techniques such as sun/solar, hot air/vacuum oven, microwave, fluidized bed, freeze drying and infra-red. Pretreatment before drying improves quality of dried product, prevents browning, accelerates drying rate and retains volatile compounds. Selection of proper drying technique is important to maintain quality of the product and make profit.

Dehydrated Onion Flakes

Onion dehydration involves the following basic steps:

Harvesting
Transporting to the processing plant
Curing
Washing
Slicing,
Dehydration in a solar/mechanical/oven dryer
Milling
Packaging

6. Dehydrated onion powder is prepared by grinding the dried onions, i.e. dehydrated onion flakes. Powder dissolves very easily and reconstitute quickly compared to flakes. It is a common ingredient in seasoned salt and spice mixes, such as royco seasoning. Some varieties are prepared using toasted onion. The dehydrated flakes and powder are used in the preparation of baked products like bread and pizza, seasoning, soups, sauces, salad sprinkles, among other uses. Dehydration decreases water activity of the material, reduces microbiological activity, and minimizes physical and chemical changes during its storage and extends the shelf-life. Due to this, dehydration is one of the most common processes used to improve food stability of semi-perishable crops like the onion.

Dehydrated onion powder

Importance of dehydrated onions

Preserves onion from contamination of microorganisms and maintains onion’s quality attributes.
Promotes export earnings.
Avails onions throughout the year including off-seasons.
Promotes comparatively cheaper and easier handling, packaging and transportation than the raw fresh onion.
Provides a wider market of horticultural produce.

5.1 Preservation and methods that can be utilized in onion processing

Vegetable preservation poses a challenge in keeping the vegetables for a long duration while maintaining the nutritional, physical, and sensory properties. Hurdle technology is utilized by combining safer additives that act as hurdles that inhibit microbiological growth and prevent undesirable biochemical reactions. The hurdles used in the process depend on the type of product, but the main hurdles in vegetable preservation are temperature, redox potential, acidity, water activity and preservatives (Value Addition of Vegetable Crops, 2020). Onions can be preserved using the methods below.

1. Drying

Drying is the main mode of preserving onions. Onions can be dried whole or chopped before drying. After harvesting, bulbs are air-dried before storage. Good storage conditions for whole onions should be cool, dry, dark, well-ventilated areas to offer good preservation of onions up to an average of 5 months.

The chopped onions should be washed and the inedible portion removed before initiating the drying process. Chopping is done to make slices of 1/8 to 1/4 inch thick. The slices are dehydrated for 3 to 9 hours hat 130°F to 140°F.

Advantages of dried onion

Dehydrated onions are a great food storage item.

They are cheap to pack compared with canned goods.
They do not require refrigerated storage as do frozen onions.
Dehydrated onions will make your meals easy and delicious.
Dried onions can be used just like fresh onions. Easily reconstituted either by adding them to dishes with lots of cooking liquid like soup, stew, chili, etc.

2. Canning

Onions can be canned under strict conditions because they are low acid foods and therefore susceptible to bacterial growth. Canning and preserving low-acid foods such as onions requires pressure canning to kill microorganisms that are harmful if not destroyed before ingesting the food. Pressure canning at 240 degrees kills the botulism bacteria. The onions for canning should be filled in hot jars under hygienic conditions. The jars should be airtight, and air bubbles should be removed before closing- leaving an inch headspace in the jars.

3. Freezing

Freezing can be done for onions even though it is not a recommended method for long storage because it causes a mushy texture to the onions. Mature onions need to be blanched for 3-7 minutes until the inside of the onion is heated. The onions are then quickly cooled and excess water drained before packaging. Proper packaging follows, and half an inch headspace should be left in the packaging container. Freezing is usually not recommended for onions. Garlic tends to get strong and bitter during freezing. The texture of both onions and garlic after freezing becomes mushy. To preserve under cold temperature storage, pick mature onions and clean thoroughly then blanch for 3 to 7 minutes or until center is heated. Cool promptly, drain and package, leaving ½-inch headspace. Seal and freeze. These onions are suitable for cooking only. Young green onions may be chopped for salads and sandwiches and frozen without blanching, but they will not be crisp. They will be highly flavored but may be slightly tough.

4. Steeping Preservation with hurdle concept

Steeped onions are best used as salads after washing. The process of steep preservation of onions include steps as follows:

Choose a mature onion bulb
Wash the bulb
Step the onion bulb in 2% sodium chloride solution, acetic acid steeping (pH 2.5-2.75),300 ppm sulphur dioxide for 7 days followed by heat treatment at 100 degree Celsius for 3-5 minutes
Store the onion bulb in steeping solution for 60-75 days at ambient storage (25-30 degree Celsius).

Steeped Onions

Source: ("Preserving Onions and Garlic | College of Agriculture, Forestry and Life Sciences | Clemson University, South Carolina", 2020)

6.0Introduction to Product Quality Assurance

Product quality assurance is a quality management concept that involves processes that determines if the products manufactured meets the quality standards. Products have different requirements specified to meet the desired quality.

Minimum requirements for quality of onions

The bulb should meet some minimum requirements for quality ("Common Standards Of Quality For Onions Regulation 2213/83 Annex Iderogating Reg 3398/84 (as amended by Reg 1854/85)(as amended by) 2193/86,1654/87", 2020).

The bulb should:

Intact
Clean, practically free of any visible foreign matter,
Sufficiently dry for the intended use (in the case of pickling onions, at least the first two outer skins and the stem should be fully dried)
Free of all abnormal external moisture
Free of foreign smell and taste

6.1 Food safety and Standards applied

Food safety refers to the absence (or presence of acceptable safe levels) of contaminants, hazards or other substances that render the food unfit for human consumption. The objective of food safety is to protect foods from microbial, chemical and physical hazards or contamination at the entire food supply chain from production and processing, to storage and transportation to consumption.

Food safety measures need to be applied at all stages of the value chain right from production to consumption, i.e. from farm to fork. Both the policymakers, food handlers and consumers are responsible for ensuring food safety.

Responsibilities of Policymakers in ensuring food safety

Build and maintain adequate food systems and infrastructures to respond and manage food safety risks.

Enhance multi-sectorial collaboration for all parties who promote food safety.
Integrate food safety into broader food policies and programmes.
Ensure that domestic foods are safe and fit for exportation.

Responsibilities of Food handlers and consumers

Be knowledgeable of the food they use. Read food packages labels, be aware of food hazards and make informed choices.
Handle and prepare food safely.
Grow crops safely following the strategies to avoid microbial contamination.

The general food safety standards applied in an onion production firm involves the following steps:

Ensure that all personnel follow the recommended good hygiene and manufacturing practices such as hand washing, covering of hair and proper dressing code.
Inspect all production equipment for safety to ensure that no availability of active microorganisms. The parameters checked to include the presence of onion residue, damages in the equipment that can harbour microorganisms, and wetness in the operation area that poses a risk of bacterial contamination. All the safety issues are fixed before production begins.
Check the safety of all the knives and blades to be used in cutting to ensure no risk for metal contamination.
Check peroxyacetic acid (PAA) concentrations that are used in disinfecting conveyor lines to ensure that it meets the food and drug administration standards.
Verify the functioning of wash water systems (both the temperature and cleanliness) and chlorine dioxide levels as per the FDA regulations.

Codex Alimentarius Commission (CAC) is a global body for food standards that came up with Codex Alimentarius, a collection of harmonized international food standards, guidelines and codes of practice to protect the health of consumers and ensure fair practices in food trade. Some of the general food standards developed include:

Food Additives: The general standards for food additives regulate the food additives applied. The principles for the use of additives and the maximum levels for maximum use is clearly laid down.
Food Hygiene: This comprises avoiding microbial contamination of food by harmful microbial contamination.
Other food contaminants: These include chemicals, heavy metals and radioactive substances. There are set principles and levels for the number of contaminants and regulations on how to avoid them.

6.2 Basic Characteristics of a Standard

According to the Private Food Safety Standards: Their Role in Food Safety Regulation and their Impact, 2010, a standard is characterized by the following:

Define the required characteristics of products such as the maximum residue limits.

Conform to similar fundamental requirements
Displays guidelines for performing tasks
Available and applied in all stages of production.
Defines the requirements of a management system like documentation requirements.

6.3 Importance of Standards

Ensure the safety of products.
Safeguard health.
Promote international acceptance of products.
Helps in pointing out what is missing and builds on the existing strengths in the available systems.
Demonstrate conformance to international standards and applicable regulatory requirements.
Reduces the risk of product liability claims.

7.0Product Traceability

Product traceability is an important food safety measure. Traceability describes the movement of a product through specified stages of production, processing and distribution ("Food Traceability Guidance", 2017). It is key in tracing back the movement of foods through the value chain to point out the specific areas where the problem emanates. Food safety checks should be developed in every food industry to enhance food safety and guarantee the quality of the products.

7.1 Components of a traceability system

The three basic components of a traceability system (The World Bank Group, 2019) include:

Product identification
Information required to track the movement of the product/item through the supply chain
Establishing a link between the product/item and the supply chain participants.

8.0Equipment/processor Maintenance and Operation

All onion processing stages require the use of specified equipment like onion peelers and sorting machines that should be in proper operation to enhance high productivity and safety. These equipment/processors need to undergo preventive, predictive and corrective types of maintenance.

8.1 Preventive Maintenance

Preventive maintenance involves the equipment/processor care and servicing to maintain optimum operations through regular inspections and observations to detect and correct defaults before the occurrence of a total breakdown. It aims to extend the equipment/processor's life by predicting failures and is meant to promote safety and improve production. Preventive maintenance guarantees reliable and efficient performance of the equipment/processor and improves operation.

The seven basic preventive maintenance milestones include testing, servicing, calibration, inspection, adjustment, alignment and installation.

8.2 Predictive maintenance

Predictive maintenance involves determining the functioning of equipment to estimate when maintenance will be required. It is also known as condition-based maintenance and works by monitoring the equipment’s performance and condition during normal operation to reduce the chances of equipment failures. Condition-based maintenance predicts the likelihood of equipment failure at certain times, depending on some factors followed by preventing the failure through regularly scheduled and corrective maintenance. There must be continuous monitoring of equipment and production processes. The data is collected periodically on critical speeds and changing spindle positions. The importance of predictive maintenance is to allow accurate scheduling of corrective maintenance and prevent unexpected equipment failures.

8.3 Corrective Maintenance

Corrective maintenance involves repair or restoration of equipment that has failed in operation. This maintenance works by identifying the failure and rectifying the failure to bring back the equipment or to normal operation. It is important in rectifying the equipment to reinstate the equipment to normal operation and prioritize on tasks that may be safety-related or affecting production.

The steps involved in corrective maintenance include:

Confirmation of the detected failure.
A confirmed equipment failure is prepared for maintenance and failure report done.
Localization and isolation of a failed part in the assembly.
Removal of the failed part for repair or disposal. If disposed of, a new part is acquired for installation.
After repair, reassemble, realign and adjust the equipment and return to use.

9.0 Conclusion

Onion is one of the most commercial vegetables grown and consumed worldwide. It forms an important source of revenue to commercial farmers from local, regional and export earnings. High production of onions with huge post-harvest losses involved necessitate onion processing using the available technologies to produce several value-added products for better commercialization. Onion preservation is crucial in in the value addition process. Some of the preservation methods include dehydration, canning and freezing with dehydration being the main mode of onion preservation.

Food safety is critical in the entire onion value chain to enhance the production of quality and safe products. Onion quality and safety standards offer regulation and guidance in the value addition process. The ability to trace a product to its source forms part of the food safety management in onion industry to be able to trace back the products in case of emergencies and do necessary withdrawals from the food supply. Moreover, equipment/processor maintenance in onion processing is critical for safety and optimum productivity. Farmers should utilize value addition technologies for better commercialization of their produce.

10.0 References

AgriBiz. 2020. Onion Value Chain | Agribiz. [online] Available at: [Accessed 28 November 2020].
Bahram-Parvar, M., & Lim, L. (2018). Fresh-Cut Onion: A Review on Processing, Health Benefits, and Shelf-Life. Comprehensive Reviews In Food Science And Food Safety, 17(2), 290-308. https://doi.org/10.1111/1541-4337.12331
Clemson.edu. 2020. Preserving Onions And Garlic | College Of Agriculture, Forestry And Life Sciences | Clemson University, South Carolina. [online] Available at: [Accessed 28 November 2020].
Common Standards Of Quality For Onions Regulation 2213/83 Annex Iderogating Reg 3398/84 (as amended by Reg 1854/85)(as amended by) 2193/86,1654/87. Msu.edu. (2020). Retrieved 28 November 2020, from https://msu.edu/~sindijul/Fruits%20and%20Vegetables/Onions.htm.
FAO. (2010). Private Food Safety Standards: Their Role in Food Safety Regulation and their Impact [Ebook]. Retrieved 27 November 2020, from http://www.fao.org/3/ap236e/ap236e.pdf.
Food Traceability Guidance. Fao.org. (2017). Retrieved 29 November 2020, from http://www.fao.org/3/i7665en/I7665EN.pdf.
Gorrepati, K., & Khade, Y. (2020). Mouth-watering value-added products of onion and garlic [Ebook]. Retrieved 29 November 2020, from https://www.researchgate.net/publication/342521088_Mouth-watering_value-added_products_of_onion_and_garlic.
Grewal, M. K., Jha, S. N., Patil, R. T., Dhatt, A. S., Kaur, A., & Jaiswal, P. (2013). A less energy intensive process for dehydrating onion. Journal of Food Science and Technology, 52(2), 1131-1137. https://doi.org/10.1007/s13197-013-1092-x
Kaack, K. (2017). Industrial Processing of Pickled and Pasteurized Onion (Allium Cepa L.). International Journal Of Forestry And Horticulture, 3(2). https://doi.org/10.20431/2454-9487.0302005
Mitra, J., Shrivastava, S., & Rao, P. (2011). Onion dehydration: a review. Journal Of Food Science And Technology, 49(3), 267-277. https://doi.org/10.1007/s13197-011-0369-1
Onion Harvesting and Storage. Ikisan.com. (2020). Retrieved 27 November 2020, from http://www.ikisan.com/ka-onion-harvesting-storage.html.
Onion packing & processing storage and sales quality. Farmsoft.com. (2020). Retrieved 28 November 2020, from https://www.farmsoft.com/traceability/onion-packing-processing-storage-and-sales-quality.
Preserving Onions and Garlic | College of Agriculture, Forestry and Life Sciences | Clemson University, South Carolina. Clemson.edu. (2020). Retrieved 28 November 2020, from https://www.clemson.edu/extension/food/canning/canning-tips/31preserving-onions-garlic.html.
Sudhir Singh and B. Singh. (2020). Value Addition of Vegetable Crops [Ebook]. Retrieved 28 November 2020, from http://agrigoaexpert.res.in/icar/category/horitculture/value_added/Value%20Addition%20of%20Vegetable%20Crops.pdf.
Tekle, G. (2015). Growth, Yield, and Quality of Onion (Allium CepaL.) as Influenced by Intra-row Spacing and Nitrogen Fertilizer Levels in Central Zone of Tigray, Northern Ethiopia [Ebook] (p. 2). Retrieved 25 November 2020, from https://cgspace.cgiar.org/bitstream/handle/10568/90462/Tekle.pdf?sequence=1&isAllowed=y.