After attending to this experiment, we shall be able to learn moisture content determination in food products by Dean and Stark method
The loss in weight on heating is not entirely due to the moisture content but may also result due to the loss of volatile substances which are present in most of the foods.
Most of the spices contain notable quantities of volatile oil which pass off with the water.
The moisture content of spices and oils/fats containing 2% or more of water may be determined by Dean and Stark distillation method using xylene, toluene or heptane.
This method is based on the principle that during heating, water and any immiscible solvent (toluene or xylene) distil off together at a constant ratio (azeotropic property) at a temperature lower than the boiling point of either component.
As water is denser than toluene/xylene, the water is collected in the receiver measuring tube where it separates from the extracting solvent.
Apparatus- Dean and Stark apparatus
Heat source – an electric heater provided with a sliding rheostat or other means of heat
Copper wire – long enough to extend through the condenser, with one end twisted into
a spiral. The diameter of the spiral should be such that it fits within the graduated
portion of the receiver and yet may be moved up and down.
Clean the entire apparatus with potassium dichromate-sulphuric acid cleaning solution to minimize the adherence of water droplets to the sides of the condenser and the receiver.
Rinse thoroughly with water and dry completely before using. Place the specified quantity of material, accurately weighed, in the distillation flask, add an equal volume of xylene or toluene, as desired, or at least 100 ml, if less than 100 g of the material is used, and swirl to mix.
Assemble the apparatus and fill the receiver with the solvent by pouring it through the condenser until it begins to overflow into the distillation flask.
Insert a loose cotton plug in the top of the condenser to prevent condensation of atmospheric moisture within the tube.
In order that the refluxing may be under control, wrap the flask and the tube leading to the receiver with asbestos cloth.
Heat the flask so that the distillation rate is about 100 drops per minute. Purge the reflux condenser occasionally, during the distillation with 5 ml portions of xylene or toluene to wash down any moisture adhering to the walls of the condenser.
The water in the receiver may be made to separate from the xylene or toluene by moving the spiral copper wire up and down in the condenser and receiver occasionally, thus causing the water to settle at the bottom of the receiver.
Reflux until the waterlevel in the receiver remains unchanged for 30 minutes and then shut off the source of heat.
Flush the condenser with either xylene or toluene or cyclohexane, as required, making use of the spiral copper wire to discharge any moisture droplets.
Immerse the receiver in water at about 27°C for at least 15 minutes or until the xylene or toluene or cyclohexane layer is clear, and then read the volume of water.
Moisture, % (v/m) = (V × 100)/W
V = volume, in ml, of water collected, and
W = weight, in g, of the test sample taken.
RESULTS AND INFERENCE
Results of moisture are generally expressed as % v/m and taken as a mean of duplicate values. The repeatability should be within ± 0.1 per cent. The amount of water collected in the receiver tube gives the measure of the moisture content in the given food products after applying the suitable calculation formula as given in the text.
• Sample should be properly ground and passed through sieve.
• Calibrated dean and stark apparatus should be used.
• Correction factor for volume in calculation should be used while reporting the
• The apparatus should be properly cleaned and dried before use.
• Use calibrated receiver tube for collection of water.