What are the main methods for figuring out moisture?
ONE, commonly used moisture determination method:
1. Heat drying method:
①Atmospheric pressure drying method (this method is widely used);
② Vacuum drying method (used when some samples are heated and decomposed);
③ infrared drying method (this method is widely used);
④ vacuum drying method (desiccant method);
2. Distillation
3. Karl Fischer method
4. Determination of water activity AW
2. Heat drying method
1. Atmospheric pressure drying method
(1) Features and principles
Features: This method is the most widely used, the operation and equipment are simple, and has a very high accuracy.
Principle: Moisture in food generally refers to the substance lost by heating at about 100°C under atmospheric pressure. But what is actually lost at this temperature is the total amount of volatile substances, not all water.
(2) The drying method must meet the following conditions (for food):
Moisture is the only volatile component
Moisture must evaporate completely
The chemical changes of other ingredients in the food due to heat are negligible.
Not suitable for high-sugar and high-fat foods
As long as it meets the above three points, the oven drying method can be used. The oven drying method is generally carried out at 100-105°C.
(3) Determination points of oven drying method
Sampling (weighing): pay attention to prevent moisture changes
There are three factors in the selection of drying conditions: ① temperature; ② pressure (atmospheric pressure, vacuum) drying; ③ time.
(Generally, 70-105°C can be used for foods that are unstable to heat; 120-135°C can be used for foods that are stable to heat.)
(4) Operation method
Clean the weighing dish→dry to constant weight→weigh the sample→put into the oven with adjusted temperature (100～105℃)→dry for 1.5 hours→cool in desiccator→weigh→dry for another 0.5 hour→weigh to constant weight ( The difference between the two weights does not exceed 0.002g, which is the constant weight)
Calculation: Moisture = G2－G1/W
Solid content (%)=100－moisture %
G1——weighing dish weight after constant weight (g)
G2—weighing dish and sample weight after constant weight (g)
W - sample weight (g)
(5) Causes of errors in the oven drying method
The sample contains non-moisture volatile substances (alcohol, acetic acid, essential oils, phospholipids, etc.);
The combination of some components in the sample and water makes the measured results low (such as the hydrolysis of sucrose into two molecular monosaccharides), mainly to limit the volatilization of water;
The fat in the food is oxidized with the oxygen in the air, which increases the weight of the sample;
Decomposition of substances under high temperature conditions (fructose is sensitive to heat);
A hard shell is formed on the surface of the tested sample, which hinders the diffusion of water; especially for samples rich in sugar and starch;
Dry until the sample reabsorbs water.
2. Vacuum drying method
(1) Principle: Use lower temperature to dry under reduced pressure to remove moisture, and the reduced amount in the sample is the moisture content of the sample.
This law is applicable to foods that are easy to deteriorate when heated above 100°C and contain bound water that is difficult to remove. The measurement result is closer to the real moisture.
(2) Operation method
Accurately weigh 2.00-5.00g of sample→dry to constant weight weighing dish→to vacuum oven→70℃, vacuum degree 93.3-98.6KPa (700-740mmHg)→dry for 5 hours→cool in drying dish→weigh to constant weight
Calculation: Moisture = G/W
G——The weight loss after drying in the sample (g)
W - sample weight (g)
Vacuum drying method to measure moisture is generally used for samples above 100°C that are prone to deterioration, damage or difficult to remove bound water, such as syrup, monosodium glutamate, sugar, candy, honey, jam and dehydrated vegetables, etc., can be measured by vacuum drying method.
Determination of moisture by TWO and distillation (Dean-Stark)
Distillation was invented in the early twentieth century when it used boiling organic liquids to separate water from a sample and is still used today.
(1) Principle: Put the water-insoluble organic solvent and the sample into the distillation moisture measuring device to heat, the moisture in the sample evaporates together with the solvent vapor, and condenses such steam in the condenser tube, and the moisture content is determined by the water capacity. Get the moisture content of the sample.
(2) steps
Accurately weigh 2.00-5.00g of sample → put in a 250ml moisture determination retort → add about 50-75ml of organic solvent → connect to the distillation device → heat and distill slowly → until most of the water is evaporated → speed up the distillation → until the water volume in the scale tube is no longer increase → reading
(3) Calculate:
Moisture = V/W
V——The capacity of the water layer in the graduated tube ml
W - the weight of the sample (g)
(4) Commonly used organic solvents and selection basis
Commonly used organic solvents are clearer than water and heavier than water.
Benzene xylene CCl4
Density 0.880.860.861.59
Boiling point 80°C 80°C 140°C 76.8°C
(5) Selection basis: Xylene is generally not used for foods that are unstable to heat, because it has a high boiling point, and an organic solvent with a low boiling point, such as benzene, is often used. For some samples containing sugar, which can be decomposed to release water, such as dehydrated onion and dehydrated garlic, benzene can be used, and the organic solvent should be selected according to the nature of the sample.
(6) Advantages and disadvantages of distillation
Advantages: sufficient heat exchange; less chemical reaction after heating than gravimetric method; simple equipment and convenient management
Disadvantages: water and organic solvents are prone to emulsification; the water in the sample may not evaporate at all; the water sometimes attaches to the wall of the condenser tube, causing reading errors; the stratification is not ideal, causing reading errors, you can add a small amount of pentanol or iso Butanol prevents the formation of emulsions.
