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Valorisation of onion waste for effective utilisation in food systems
Wednesday, 09 September, 2020, 14 : 00 PM [IST]
Bhosale Yuvraj Khasherao, Aruna Nair UK, VR Sinija
1.    Introduction
Onion (Allium cepaL.) is an important vegetable crop not only for internal consumption but also as highest foreign exchange earner among fruits and vegetables.

Being one of the most commonly used ingredients for Indian foods, pan-season availability and cost-effectiveness of this commodity is very important. The requirement of onion is almost constant throughout the year and availability of fresh onion is limited to 7 or 8 months and there is lean period when prices shoot up because of poor storage conditions available in the country.

Small onion is one such variety of onions in India, known for its characteristic flavour with higher pungency and its role in the south Indian cuisines. But this variety has a low shelf life especially in humid areas. This leads to sudden price hikes which have become the norm in India in case of onions. To maintain this balance, availability throughout the year is to be ensured through value addition and shelf life extension.
Processing of the onion for value addition and shelf life extension will lead to the generation of large quantity of bio-waste. This bio-waste ends in landfills causing environmental issues. However, this article provides glimpse of its potential onion waste valorisation.

2.    Composition of onion waste
Onion skin is most explored when it comes to studies on utilisation on onion waste. Onion skin is the major onion processing industry waste. Onion peel is good source of insoluble (698 mg/g) and soluble dietary fibre (52 mg/g). It contains the fibres that are composed of glucose, uronic acid. Small amount of xylose, mannose and rhamnose. These polysaccharides are bind together by kalson-lignin and intercellular proteins. Onion peel is very good source of minerals as it contains around 10.6% of ash in it. The major minerals that found in onion peel are Calcium (30.7 mg/g) and Potassium (4.2 mg/g). There is good amount of Iron also present in the onion peel (1.1 µg/g).

Onion peel is good source of Total phenols (52.7 mg/g) and total flavonoids (43.1 mg/g). The antioxidant activity of peel is 227.8 µmol/g which is far more than whole onion (83.5 µmol/g). One of the most highlighted phytochemical in onion peel is quercetin due to its antioxidant activity and enzyme protective action. The studies showed that there is varying amount of quercetin based on the variety and cultivation conditions. Onion peel contains 1.61 mg/g of quercetin in it. However, the amount of its glucosides is 5.76 mg/g.

3.    Valorisation of onion waste
3.1    Extraction of active compounds
Due to the presence of such high quality nutrients, many attempts have been made to utilise this waste. In an attempt to convert the onion peel waste into sugars, onion waste was treated with various enzymes such as cellulase, pectinase and xylanase at various concentrations.

The most effective observed concentration was of cellulase, pectinase and xylanase was 0.72 mg/g, 0.16 mg/g and 1.00 mg/g. At this enzymes concentration, 98.5 % of polysaccharides were converted into respective sugars. The breakdown of the polymeric materials resulted into the separation of quercetin from complex matrix increasing extraction efficiency by1.61-fold.

3.2    Modification of onion waste fibres and quercetin
The cell wall polymers are highly susceptible towards pressure cooking. The pressure cooking found to break polymeric structures reducing pectic polymers and molecular weight of soluble materials. Similar effect of extrusion cooking was observed on cell wall material from onion waste.
Temperature and shearing by screw rotation causes depolymerisation causing increase in polymeric solubility and swelling. However, when treated with divalent cation followed by cooking, ionic Ca2+ and Sr2+ get adsorbed to the wall polymers. This resists depolymerisation giving rise to increased molecular weight and thermal stability of the polymers and decreased thermal solubility. High uronic acid content and swelling capacity represents high cation exchange of the polymers in onion waste. This will lead towards higher retention time in the small intestine giving very less time for cholesterol and fat to be absorbed in small intestine resulting in the decrease in blood lipid level.

 Attempts have been made to modify quercetin glycosides to free quercetin making it bio-available. Quercetin glycosides from onion waste were treated with glycosidase enzyme to form free quercetin and respective sugars. The reactions converted 90% of the glycosides into free quercetin in mere 10 minutes at 90°C. The time of extraction can be reduced to 5 min at 120°C and 51 Bar pressure increasing free quercetin content to 50mg/g from 10mg/g.

3.3    Application of onion waste in water treatment
Onion peel has ability to absorb dissolved salts present in water and hence can be used in the water treatment. Along with the salts, onion peel is known to absorb many pigments and dyes that come out of printing industry. Apart from the dyes, it has potential to absorb many hazardous chemicals. The absorption capacity of onion waste can be increased by activation techniques such as microwave treatment.

3.4    Enzyme production from onion waste
Different onion wastes can be implemented for production of various enzymes. Peroxidase is the enzyme that exhibits the antioxidant activity and has the ability to change many flavonoids. The reaction between peroxidase and flavonoids compounds forms secondary flavonoids that also exhibit antioxidant activity. Moreover, the enzyme can be used in the treatment of effluent and reduce the oxygen demand before disposal. Pectinases produced from onion waste were found heat-stable with optimum temperature of 80°C and pH range from 3.0 to 6.0.

3.5    Alcohol and vinegar production from onion waste
Onion waste is rich source of polysaccharides that can be hydrolysed into sugars and utilised for the development of vinegar, alcohol and lactic acid. Two-stage process can be applied for the vinegar production. In initial stage, juice with 30 g/l sugar was converted to alcohol by S Cerevisiae followed by conversion of alcohol into vinegar. The maximum productivity of acetic acid was 37.0 g/l.

4.    Conclusion
Onion waste is good source of macro and micronutrients having great potential for utilisation directly as food ingredient or as source of some nutrients for extraction and bioconversion. This waste streams can be utilised effectively avoiding qualitative and quantitative losses and environmental impact.

(The authors are from Food Processing Business Incubation Centre, Indian Institute of Food Processing Technology, Pudukkottai Road, Thanjavur. They can be reached at sinija@iifpt.edu.in)
 
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