Evaluation of drying methods on some nutritional and volatile components of Bombay red onion (Allium cepal L.)

Onion (Allium cepa L.) is a strong-flavoured vegetable consumed in different ways and its distinctive flavor or simply pungency. Onion has also important natural compounds effective for medical functions. Its importance is directly related with high content of high organosulphur compounds. Shelf life of fresh onion bulb is short enough due to the presence of high moisture content where postharvest loss of onion bulb reaches up to 50% in the production season. Consequentially Onion bulb had extreme variable market price during production and off season which affect both growers and consumers. So in this study the nutritional and volatile components of different drying methods of onion were evaluated. Effect of different drying method on protein, carbohydrate, total sugar, fat, pyurvic acid, ascorbic acid, total phenol, total flovonol, rehydration ratio, colour and sensory properties of onion slice were evaluated and found insignificant at (P < 0.05) for microwave and modified direct solar dryer. But oven drying method had significant effect on onion bulb quality attributes at (P < 0.05) than other two drying methods. Key word: Onion, drying, bioactive, nutritional and organoleptic Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 14 March 2018 doi:10.20944/preprints201803.0101.v1 © 2018 by the author(s). Distributed under a Creative Commons CC BY license.


Introduction
Onion (Allium cepa L.), an incredibly used vegetable, ranking third from major vegetables produced in the world. Onion is a strong-flavored vegetable used in a wide variety of ways mainly for its distinctive flavor, aroma or simply pungency (Kumar,S., et.al, 2007). Some researcher also reported about its importance as a biological compound and medical functions.
Such use is also directly related with their high content of organo-sulphur compounds (Corzo-Martinez, M., et.al, 2007). Onion bulb is a perishable agricultural produce which limits economical growing of onion bulb (Endalew W, et.al, 2014). Evaluating and recommending long different preservation techniques is a solution to extend shelf life of onion bulbs so that it will be available in all seasons and localities to meet the demand during the whole year in all areas (Miedema, P., 1994). Processing and stabilizing of onion bulb have the advantage of both the growers and consumers (Ayalew D, et.al, 2017). In the production of processed commercial foods such as soups, sauces, sausage, and meat food products dried onion is basic ingredient used as flavouring agent. Dried onion products are sometimes preferred than fresh onion bulbs because of its simplicity of for use and greater shelf stability (Mazza G properties of the agricultural produces (Mazza G and Lemaguer M., 1980). The domestic demand for energy substantially exceeds supply and on the contrary solar energy is an ideal way for drying of agricultural produces in sub-Saharan countries like Ethiopia. The supply of solar energy is abundant in almost all locations of the country. This concern have focused our attention to the potential of harnessing the opportunity to develop modified direct solar dryer and evaluation its efficiency by comparing with commercial dryers like oven and microwave dryers. Therefore in this study the effect of three drying methods was evaluated on physical, biochemical, nutritional and sensory quality attributes of onion flake and recommended the technology to the end user.

Sample collection and preparation
Bombay red onion sample was collected from Adet agriculture research center and trimmed to remove contaminated, injured and other extraneous materials. Then inedible part of the onion bulb was removed and sliced uniformly at slice thickness of 5mm. Finally sliced onion was dried up to a moisture content of 12% using three different drying methods (drying temperature of oven and microwave was adjusted at 50 o C while the drying temperature of modified direct solar dryer was uncontrolled so it was simply measured using data logger). Selection criteria for drying temperature of oven and microwave dryer were literature based approach. Dried onion slice was packed in polyethylene bags and set further analysis of quality attributes.

Proximate composition analysis
Proximate composition: The proximate compositions such as moisture, crude fiber, total carbohydrate, ash and crude fat contents of onion were determined according to a method (AOAC, 1990).

Bioactive component analysis
Total phenol content: Onion sample total phenolic content was determined by Folin-Ciocalteu method as described by (Gao A. et.al. 2000). Distilled water of 0.44 ml and 0.02 ml of Folin reagent were added to 0.02 ml of extract/suspension of the sample (2 mg/ml). After 3 minutes resting, 0.4 ml of 20% Na2CO3 was added. The mixture was vortexed and incubated for 20 min at 40°C using a water bath; thereafter the absorbance was read against a blank at 760 nm using a UV-spectrophotometer. The total phenolic content was determined using the standard curve (y = 0.022 x; r2 = 0.9945) obtained with Gallic acid. The contents were expressed as mg of Gallic Acid Equivalent/100g of the samples.
Ascorbic acid content: Ascorbic acid content of the fresh and dehydrated onion was determined using titration method by (Ranganna S., 1986). The reduction of 2, 6-dichlorophenol indophenol dye by ascorbic acid and expressed in mg per 100 g. 4 Total Flavonol content: Aluminum tri-chloride method, as described by (Padmaja M. et al. 2011) was used to determine the total flavonoid content onion samples. One hundred (100)μL of extract/suspension was mixed with 1.49 ml of distilled water before introduction of 0.03 ml of 5% NaNO2. After 5 min resting, 0.03 ml of 10% AlCl3 was added and the mixture allowed to rest. After 6 min, 0.2 ml of 1 M NaOH and 0.24 ml distilled water were respectively added and the mixture was vortexed and the absorbance was measured at 510 nm using UVspectrophotometer.The flavonoid content was determined using the standard curve (y = 0.1972 x; r 2 = 0.9972) obtained with Catechin. The contents were expressed as mg CE/g of onion sample. tube. The reaction mixture was vortexed and kept for 10 min at 37°C temperature and after cooling 5mL of 0.6 mol/L NaOH was added, and the absorbance was measured at 420 nm with Shimadzu UV-1700 spectrophotometer. The calibration curve was made by preparing pyruvic acid solutions at concentrations 0.04-0.4 mmol/L in water and the pyruvic acid concentration were expressed in terms of (μmol/g fresh weight (FW)).
Rehydration ratio: Rehydration ratios of dehydrated onion slice were determined by the methods suggested by (Ranganna S.1986). The rehydration characteristics of the dehydrated onion slice were studied in terms of the rehydration ratio.

Sensory analysis:
A trained panel of 15 members evaluated the color, appearance, texture, flavour, taste and overall acceptability of the products on a nine-point hedonic scale. The panelists were unaware of the project objectives. Samples were coded with three-digit random numbers and then served. Panelists were provided with a glass of water, and were instructed to rinse and swallow water between samples. They were given written en instructions and asked to evaluate the overall acceptability of the products based on their appearance, texture, taste and color using a nine-point hedonic scale (1=dislike extremely to 9=like extremely) a method described by (Meilgaard M., et.al,1999).

Statistical analysis
Data collected from the experiment were analyzed by statistical analysis such as descriptive statistics and analysis of variance (ANOVA).

Result and discussion
Drying environment of direct solar dryer As it is observed in the figure-one below the maximum drying temperature of modified direct solar dryer was about 45 o C. Drying temperature and relative humidity of modified direct solar dryer was recorded by data logger.      two days prepared using modified direct solar dryer was found acceptable by the panelists. The taste of onion flake had higher values than the fresh onion scores and this increment may be due to high concentration of total sugars in onion flake than the fresh onion (Achanta S.et.al, 2000). Rehydration: The rehydration characteristics of a dried product are widely used as indicators of quality dried product. Rehydration is a complex process that is influenced by both physical and chemical changes associated with drying and the treatments preceding dehydration and reported rehydration ration of onion was 6.87 (Lewicki PP.et.al.1998). The result found in the experiment was nearly similar with the one reported previously. The rehydration quality parameter data of onion slice is given in table-4 below. The drying process causes changes in the permeability of the cell walls, loss of osmotic pressure and solute migration which affects the rehydration ratio. The rehydration ratio of onion slice ranges 7.87 to 5.65 along the drying methods. Each drying method had significance difference at (p<0.05) in rehydration ratio dried onion flake. The rehydration ratio of solar and microwave dried onion products show insignificance difference at (p<0.05). When rehydrating a dried product, it will never regain the same condition as before drying. The less elastic cell walls and the reduced water holding capacity of protein and starch, all decrease the rehydration ratio of the products but this phenomenon will be reduce significantly by optimizing drying process as it is in our finding. So, the negative factors regarding rehydration of the cells will be less than with a poor drying technique as stated (Kumar HSP.et.al, 2004).

Conclusion
It can be disclosed from this work that Bombay red onion in Ethiopia has better crude proteins, total carbohydrates, crude fat and bioactive components like ascorbic acid, pyruvic acid, falvonol and phenol content. The overall interpretation of this present investigation may offer a scientific basis for increased and versatile utilization of these carbohydrate-rich onions as a food component and carbohydrate supplement. In this study it appeared that modified direct solar drying method had more quality dried product than oven and microwave dried onion products.
The quality parameters of dried onion were also influenced by the drying temperature and time.
Drying environment of modified direct solar dryer was an appropriate technique of extending shelf life of onion with preserving its physical and chemical characteristics of fresh onion bulbs.