Experimental procedure for osmotic dehydration
Selection of raw materials
The fresh summer onion was collected from the spices research center, Bogra, Bangladesh Agricultural Research Institute.
Preparation of sample and solution
The selected variety of summer onion was peeled and washed with water and unwanted material like dust, dirt, and surface adhering were removed. The onion bulbs were sliced with an electrical slicer of approximate 7 mm ± 1mm thickness. Each slice was weight and subsequently individually marked by using different clour threads, the ratio of solute to onion slices was 5:1 w/w.
Measurement of initial moisture content
The moisture content of fresh as well as osmotically dehydrated onion samples was determined by using air oven method and calculated by using following equation (Ranganna, 2000).
Percent moisture content (db) =
Measurement of total soluble solids
The total soluble solids of prepared solution were found out by using hand refracto meter of various ranges, which give the reading directly in Brix. (Ranganna, 2000).
Osmosis of onion slices
In osmotic dehydration the prepared samples (onion slices) were weighed approximately 100 gm for every experiment and immersed in sucrose and salt solution (40, 50, 60 sucrose 5, 10, 15, 20, 25 °Brix salt and of 40:15, 45:20 50:15 and 55:15°Brix sucrose-salt combine) contained in a 1000 ml glass beaker. The beakers were placed inside the constant temperature of refrigerator (5°C) ambient (25°C) and water bath (40°C). The solution in the beakers was manually stirred at regular intervals to maintain uniform temperature (5, 25 and 40°C). Every concerned sample was removed from each keeping condition at designed time (0.5, 1, 2, 4, 6, 8, 12, 16 and 24 hours), samples were taken out and placed on absorbent paper for 5 minute or were immediately rinsed in flowing water and placed on tissue paper to remove the surface moisture to eliminate excess solution from the surface before weighing. Finally the samples were weighted and their moisture contents were determined.
Osmotic dehydration characteristics
Water loss
Water loss is the quantity of water lost by food during osmotic dehydration. The water loss (WL) is defined as the net weight loss of the onion slice on initial weight basis and will be estimated as, (Hawkes and Flink, 1978)
Solid gain
The solids from the osmotic solution get added to the samples during osmotic dehydration. The loss of water from the sample takes place in osmotic dehydration consequently it increases the solid content. The solid gain is the net uptake of solids by the slices on initial weight basis and computed using following expression (Hawkes and Flink, 1978)
Normalized solid content (NSC)
The overall Normalized solid content of the sample do affect the final weight of the sample and was determined by Hawkes and Flink.