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Study on Water Sorption Isotherm of Summer Onion

MD. MASUD ALAM
Senior Scientific Officer
Spices Research Center, Bangladesh Agricultural Research Institute (BARI), Bogra

MD. NAZRUL ISLAM
Professor,
Department of Food Technology and Rural Industries, Bangladesh Agricultural University (BAU), Mymensingh

Abstract/ Executive Summary:

The water sorption characteristics of dehydrated onion and onion solutes composite by vacuum drying (VD) and air drying (AD) were developed at room temperature using vacuum desiccators containing saturated salt solutions at various relative humidity levels (11-93%). From moisture sorption isotherm data, the monolayer moisture content was estimated by Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) equation using data up to a water activity of 0.52 and 0.93 respectively. Results showed that in case of non treated samples the monolayer moisture content values (W_o) of BET gave slightly higher values than GAB (9.7 vs 8.2) for VD, while GAB gave higher value than BET (11.0 vs 9.8) for AD. It is also seen that the treated and non treated onion slice and onion powder absorbed approximately the same amount of water at water activities below about 0.44 and above 0.44 the treated samples begin to absorb more water than the non treated samples. It was observed that 10-20% added of sugar gave no change in water sorption capacity while the amount of sorbed water increases with increasing amount added salt for mix onion product.

Keywords: : BET equation, GAB equation, Monolayer moisture content, Water activity

Location: Bangladesh Agricultural University (BAU), Mymensingh

Start Date:

End Date:

Program Area: Quality and Nutrition

Commodity: Onion

Objectives:

To determine relationship between aw and quality characteristics of different dried onion and onion solute composities.

Methodology:

The experiment was conducted in the Department of Food Technology and Rural Industries, Bangladesh Agricultural University (BAU), Mymensingh during the period of January, 2011 to July 2011. Freshly harvested summer onion variety of BARI piaz-2 was procured from Spices Research Center (SRC) of Bangladesh Agricultural Research Institute (BARI) and stored at room temperature. Sucrose and salt were bought from local market. All other chemicals were of reagent grade and collected from the scientific store. Onions free of mechanical injury and disease were cleaned and washed with tap water and spread on a perforated tray to drain out excess water and subsequently used for preparation of samples. After peeling onions were cut into slices of approximately 5mm thickness by an electric slicer or made into slurry by electric blender before being made into samples. When studying sorption behavior of osmosed samples, onion slices were first osmosed (24 hr, in appropriate solution) and slices were then either i) directly dried in vaccum dryer at 70⁰C or ii) ground to a slurry before vaccum drying. In some cases the ground slurry was blended with known amount of solutes prior to being vaccum dried. Procedures for determining equilibrium moisture content as influenced by water activity in food materials are described in detail by Gal (1983) and Shatadal and Jayas (2000). The principal methods are gravimetric and menometric/ hygrometric. The gravimetric method is divided into static and dynamic methods. As the static gravimetric method is simple and reliable, procedures noted by Islam (1980) with certain modifications were applied in determining sorption isotherm in this study. The dehydrated 2g onion samples were taken in each crucible. Nine desiccators with nine saturated salt solution were used to determine the equilibrium moisture content for vaccum dried onion at room temperature (about 25⁰C) over38 ALAM AND ISLAM a perforated plate to avoid mixing. The dessicators were evacuated to less than 50 torr. The crusibles were kept in vaccum desiccators over saturated salt solution with two replication. At various intervals, the vacuum was broken with air, the sample weighed and replaced in the desiccators, which was then re-evacuated. After attaining equilibrium the crucible were collected from each desiccators and the equilibrium moisture contents of the dried samples were determined by oven method (AOAC, 2000).

Source of Information: Bangladesh J. Agril. Res. 40(1): 35-51, March 2015, ISSN 0258-7122

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Results showed that in case of non treated samples the monolayer moisture content values (Wo) of BET gave slightly higher values than GAB (9.7 vs 8.2) for VD, while GAB gave higher value than BET (11.0 vs 9.8) for AD. It is also seen that the treated and non treated onion slice and onion powder absorbed approximately the same amount of water at water activities below about 0.44 and above 0.44 the treated samples begin to absorb more water than the non treated samples. It was observed that 10-20% added of sugar gave no change in water sorption capacity while the amount of sorbed water increases with increasing amount added salt for mix onion product.

Knowledge Management: Journal