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    Home > Chemicals Industry > Chemical Technology > Other methods for determining moisture in food (1)

    Other methods for determining moisture in food (1)

    • Last Update: 2021-07-25
    • Source: Internet
    • Author: User
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    1.
    Decompression drying method to determine the moisture in food

    (1) Principle: Using the principle of lowering the boiling point of water under low pressure, put the sample that is not suitable for drying at high temperature in a low pressure environment to evaporate the water at a lower temperature
    .


    Calculate the moisture content based on the mass lost after the sample is dried


    (2) Instruments

    ①Electric heating vacuum drying oven (with vacuum pump)
    .

    ② Dry bottle
    .

    ③Safety bottle
    .

    (3) Operation steps: Weigh the sample according to the requirements of the direct drying method, put it into the vacuum drying box, connect the drying box to the vacuum pump, and extract the air in the drying box to the required vacuum degree (generally 40~53kPa), and at the same time Heat to the required temperature (60±5)℃
    .


    Turn off the piston on the vacuum pump, stop pumping air, and keep a certain temperature and vacuum in the drying box.


    (4) Result calculation: same as direct drying method
    .

    (5) Tips

    ①Scope of application: This method is applicable to the determination of moisture in easily decomposable foods such as sugar and monosodium glutamate.
    It is not applicable to candies added with other raw materials, such as milk candies, soft candies, etc.
    , and does not apply to moisture content less than 0.
    5g/100g.
    Samples
    .

    ②When using the vacuum drying method to determine the moisture content, in order to remove the moisture volatilized from the sample during the drying process, and to prevent the moisture in the air from entering the oven when the oven returns to normal pressure in the later stage of drying, the accuracy of the measurement must be outside the vacuum oven.
    Set up a set of safety and buffer facilities, connect several drying bottles and a safety bottle
    .


    The entire equipment process is shown in Figure 3-1


    ③In this method, the pressure is generally 44~53kPa and the temperature is (60±5)℃
    .


    However, in practical applications, the pressure and temperature can be adjusted according to the nature of the sample and the pressure resistance of the drying oven


    Figure 3-1 Work flow chart of vacuum drying

    2.


    Determination of moisture in food by distillation method

    (1) Principle According to the principle of a binary system composed of two immiscible liquids, the boiling point of which is lower than that of any component, the water in the food is azeotropically distilled with toluene or xylene at a temperature lower than 100°C.
    Out, condense and collect the distillate
    .


    Due to the different densities of water and organic solvents, the distillate is stratified in a graduated receiving tube: according to the volume of water in the receiving tube, the moisture content of the sample is calculated


    (2) Reagent anhydrous toluene or xylene: take toluene or xylene, first saturated with water, separate the water layer, perform distillation, and collect the distillate for use
    .

    (3) Instrument moisture tester: as shown in Figure 3-2
    .

    (4) Operation steps Accurately weigh a proper amount of sample (with water content of about 2-5mL), put it into a 250mL conical flask, add 50-75mL of newly distilled toluene (or xylene ) (preferably immersed sample), and connect to condensation The pipe and the moisture receiving pipe are filled with toluene from the top of the condenser tube to fill the moisture receiving pipe
    .

    Heat and distill slowly to get 2 drops of distillate per second.
    After most of the water has evaporated, accelerate the distillation to about 4 drops per second.
    When all the water has evaporated, the volume of water in the receiving tube no longer increases.
    Add toluene from the top of the condenser to rinse
    .


    If water droplets are attached to the wall of the condenser tube, wipe it with a copper wire with a small rubber tip, and then distill for a while until the upper part of the receiving tube and the wall of the condenser tube are free of water droplets.



    Figure 3-2 Moisture measuring tube
    1-250mL Erlenmeyer flask 2-moisture receiving tube with graduation 3-condensing tube

    (5) Result calculation

    Where x——the moisture content of the sample, mL/100g (or the mass is calculated according to the density of water at 20℃: 0.


    9982g/mL)

    V——The volume of water in the receiving tube, mL

    m——the mass of the sample, g

    The calculation result retains three significant figures
    .

    The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 10% of the arithmetic mean
    .

    (6) Tips

    ①Scope of application: The distillation method is suitable for the determination of moisture in foods containing more volatile substances, such as oils and spices.


    Especially for spices, this method is the only and recognized moisture content analysis method


    ② Generally, use asbestos net when heating.
    If the sample contains high sugar content, it is better to use oil bath to heat
    .

    ③ Before measuring the volume of water in the collection tube, let the instrument cool down to ambient temperature
    .

    ④ When the sample is powdery or semi-fluid, fill the bottom of the bottle with refined sea sand, and then add the sample and distilled solvent
    .

    ⑤The organic solvent should be selected according to the nature of the sample
    .
    For foods that are thermally unstable, xylene is generally not used because of its high boiling point, and low-boiling organic solvents such as benzene are often used
    .
    For some samples containing sugar that can be decomposed to release moisture, such as dehydrated onions and dehydrated garlic, benzene can be used; for determination of cheese with mixed solvent of n-pentyl alcohol, dodecylbenzene (129~134℃) (1+1): toluene is used for determination Most spices; hexane is used to determine peppers, onions, garlic and other spices that contain a lot of sugar
    .

    Related Links: Determination of Moisture in Flour

     

     

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