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Rehydration Characteristics of Dried Summer Onion

Md. Masud Alam
Senior scientific officer
Spices Research Center, Bangladesh Agricultural Research Institute, Shibganj, Bogura

Muhammad Zakaria Hossain
Scientific Officer
Spices Research Center, Bangladesh Agricultural Research Institute, Shibganj, Bogura

Md. Nazrul Islam
Professor
Department of Food Technology and Rural Industries, Bangladesh Agricultural University, Mymensingh

Abstract/ Executive Summary:

Different reconstitutional properties of summer onion were studied. During rehydration quality test for both samples (mechanical and solar dried) it was observed that the rehydration ratio (RR) was higher for mechanical dried onion than that of solar dried counterparts and RR was also higher for blanched sample rather than unblanched sample for both the drying method. The coefficient of reconstitution (CR) is found to be highest for mechanical dried blanched onion and was followed by mechanical unblanched, solar blanched and unblanched dried onion. It is also seen that both RR and CR of osmosed onion (55/15% sugar-salt, 60% sugar and 25% salt) showed better reconstitution properties than nonosmosed onion.

Keywords: Summer onion; Mechanical and solar drying; Rehydration ratio; Coefficient of reconstitution; Blanched and osmosed

Location: Department of Food Technology and Rural Industries, Bangladesh Agricultural University Mymensingh, Bangladesh

Start Date: 01-07-2010

End Date: 30-06-2011

Program Area: Postharvest and Agro-processing

Commodity: Onion

Objectives:

The aim of this study was to asses the rehydration quality of the dehydrated products.

Methodology:

Freshly harvested summer onions were procured from the Spices Research Center (SRC) farm of Bangladesh Agricultural Research Institute (BARI), Bogra. Onions were cleaned and washed with tap water and spread on a plastic filter basket to drain out excess water. Onions were transversely cut into 5 mm thick slices using slicer. Fifty percent cut samples were blanched for 3 min. All blanched and unblanched samples were soaked in 1500 ppm potassium metabisulphite (k2S2O5) solution for 20 min. The treated slices were divided into four samples for four treatments. The treatments were T₁(Blanched mechanical drying), T₂ (Unblanched mechanical drying), T₃ (Blanched solar drying) and T₄ (Unblanched solar drying). Samples were dried in the solar (45-50°C) and mechanical drier (60°C). For another experiment the 5 mm thick raw onion slices were osmosed for 24 hr in 25% salt solution (T₅), 55/15% sugar- salt solution (T₆) and 60% sugar solution (T₇) and dried in a cabinet dryer at 60⁰C. After cooling in the room temperature, all the dried samples were packed quickly in different foil packet (130μm) and heat sealed. The samples were stored at room and refrigeration temperature. After twelve months of storage, the dried samples were taken for rehydration studies. About 500 ml capacity beakers were taken and 150 ml of water and 2 g of dried sample were poured into each. After quick blotting with the filter paper weight of the reconstituted samples were taken each after 15 min. interval and selected the optimum textural condition of pre-soaked samples up to 60 min. The samples were subsequently boiled in the same pre-soaked water for 3, 7, 10, 13, 15, 18 and 21 minute as per Sarker and Setty (1976) and counting of time began after heating started. After expiry of the specific boiling time, the liquid portion was drained off while the solid content was transferred to a funnel covered with a coarsely porous filter paper. Gentle suction was applied and drained with careful stirring for one-half to one min. or until the drip from the funnel has almost stopped. The rehydrated samples were removed from the funnel and weighed individually. The dehydration ratio, rehydration ratio and co-efficient of reconstitution were calculated using following formulas.

Dehydration ratio (DR) = weight of prepared material before drying/weight of dried material-------------(i)

Rehydration ratio (RR) = weight of rehydrated material/weight of dehydrated material----------(ii)

Co-efficient of reconstitution (CR) = Rehydration ratio/Dehydration ratio---------------------(iii)

The percent water in rehydrated material was determined as per methods of Ranganna .

% Water in rehydrated material = (Drained wt. of rehydrated material)-(Dry matter content in sample taken for rehydration)/Drained wt. of rehydrated material?--------(iv)

Source of Information: International Journal of Research (IJR), Vol-1, Issue-7, August 2014, ISSN 2348-6848

Article Link (Online Journal):

Funding Source:

1. Government

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Temperature, blanching and their combined effect have a reasonable impact on the rehydration of dried onion. With increasing drying time and temperature, contractile stresses occur in the cell wall structure since the amount of heat given to food material increases. Thus, the porosity of the dried samples increases leading to an increase in shrinkage and rehydration value. Reconstitutional properties of dried onion depended on the method of drying. Blanched sample shows always higher rehydration ratio (RR). The longer time of rehydration was, the higher losses of dry substances were caused by dilution of soluble compounds.

Knowledge Management: Journal