In this article, we study the moisture content of dried fruit by Dean Stark distillation method. Drying fruits have a long history and has been conducted by traditional methods and sunlight. Drying is one of the best technics to preserve the beneficial properties and nutrients of fruits including vitamins and minerals while losing weight, improving shelf life, exporting, and making it easier to stay longer worldwide. Therefore, the moisture is reduced by drying in different ways, including various types of industrial dryers. an important point in the determination and protection of the remained moisture in the dried fruits is to keep a proper range of moisture content in them. One method to determine the moisture content of dried fruit is using the Dean Stark distillation method based on Standard 672 of the Iranian Standards Organization, AOAC 22.013, and ISO 665: 2000.
Dean Stark Distillation
After introducing the first method for determining the residual moisture in dried fruits, Dean-Stark isometric distillation method is used as follows. This method is used for all types of nuts and dried fruits.
In addition, you can use this method for fruits that have a lower sugar content and cannot be ground, for which the first method of measuring moisture content is not suitable.
Before examining the test method, we will define how the Dean-Stark distillation machine works, separates, and measure moisture.
As you can see in this figure, Stark Distillation is the name of a device used in chemical synthesis to collect water or other liquids from a reactor. Another name of Dean-Stark distillery is distillation trap. This method is used in the oil industry as well as in measuring the moisture content of bread.
Dean-Stark is a glass device with a balloon containing a sample, refrigerant, distillation column, burette, calibrated connector tube, thermometer, and fluid collecting chamber. This device can be used for immiscible liquids with different densities that are separated into different layers based on the difference in boiling point. The lower density fluid is at the top and the heavier fluid is at the bottom. Therefore, the volume of the aqueous phase can be obtained using a calibrated connector tube.
Required laboratory supplies for measuring by Dean Stark distillation
- Toluene with a degree of chemical purity
- Laboratory scales with an accuracy of 0.001
- Dean-Stark distillation machine (including 500 mm distillation machine, 10 ml moisture collection calibrated container based on 0.1 ml grade and other connections)
- Mill
- Heater
- Boiling stone
- Sample of dried fruit or nuts
Measuring moisture of dried fruit by Dean Stark distillation method
Chop, cut, or mill all kinds of dried fruit leaves, raisins, dates, etc. with a meat grinder, knife, and scissors, etc. Weigh the crushed samples immediately before absorbing ambient moisture. Transfer a certain amount of the weighted sample into the balloon of the Dean-Stark device.
– If the expected moisture is less than 17%, weigh 5±50 g of the sample, and
– If the expected moisture in the dried fruit is higher than 17%, weigh 30±5 g of the sample.
Place the toluene and the samples in the balloon in a way that toluene could cover all the samples and be 5 cm above the surface of the samples in the balloon. Add a few boiling stones to the balloon and shake to mix thoroughly. Connect the parts of the device to each other as shown in the figure, and fill the moisture collection container with some of the toluene that enters from the top of the refrigerant so that some of it overflows into the balloon after filling. Use cotton on the upper end of the refrigerant to prevent moisture from entering the distillation apparatus.
Water boils sooner and enters the collection chamber because toluene has a higher boiling point than water and lower density than water. Therefore, water is precipitated in the balloon and toluene forms an immiscible and separable phase above water.
Note that the end edge of the refrigerant should be about 25 mm higher than the moisture container. The distillation balloon and the tube connecting the collection balloon should be covered with asbestos or fiberglass.
You should wash the refrigerant from top to bottom with some toluene to collect the adhered moisture to the walls of the refrigerant. If the volume of the collected water does not change after 5 minutes, stop the distillation process and wait for the moisture collection balloon to reach ambient temperature.
If water droplets remain in toluene, transfer the water droplets into the aqueous phase by inserting a copper wire into the Dean-Stark distiller. The volume of the collected water should be read and recorded by the calibrated part of the connecting tube (with an accuracy of 0.1 ml). The percentage of moisture in dried fruit is calculated by the following formula:
In which, A is the recorded water volume from the calibrated part (the volume of the existed water from the dried fruit)
D= water density, considered 1 g/ml at all temperatures.
W= sample weight (g)
Ref: