Water is one of the indispensable substances for life activities, and moisture content testing is an important indicator for food analysis. The amount of moisture content in food is extremely important to ensure the quality of food. Control of food moisture content for maintaining the organoleptic properties of food, maintaining the balance of other components in the food, to ensure the stability of food are of great significance.
Forms of Water in Foods
Moisture in food exists in the form of both bound and unbound water. Bound water refers to crystallized water and adsorbed water, which are difficult to escape from the sample; unbound water includes wetting water, osmotic water and capillary water. This type of water is relatively easy to separate from the sample, usually in the food testing of moisture content is mainly unbound water. If the heating and drying time is prolonged without restriction in the determination of moisture, often the sample molecules will undergo certain chemical reactions, which will affect the results of the determination. Therefore, in the determination of moisture content must provide a certain temperature, time and operating conditions.
Methods of detecting moisture
1. Heat drying method:
Atmospheric pressure drying method (this method is widely used);
Using a conventional drying oven, the sample is placed at a certain temperature to evaporate the moisture. Moisture evaporates more slowly at atmospheric pressure and is suitable for samples with high moisture content. A blast drying oven is used to provide uniform heating, and an electronic balance accurately weighs the change in mass of the sample before and after drying. This method is simple and economical, but may not separate out the bound water in the sample.
Vacuum drying method (some samples are used when heating decomposition);
Drying under vacuum environment, reduce the high temperature decomposition when water evaporates. Suitable for some heat-sensitive substances to prevent high temperature damage. The vacuum drying oven controls the internal pressure to ensure that the samples are dried gradually at low temperatures; a vacuum pump maintains the low-pressure environment, and an electronic balance is used to accurately weigh the mass changes before and after drying. This method can better maintain the chemical composition of the samples and is especially suitable for samples that are susceptible to high temperatures.
Infrared drying method (this method is widely used);
The infrared drying method heats the sample by infrared radiation, causing rapid evaporation of moisture. This method determines moisture content in a short period of time and is suitable for rapid analysis. Infrared moisture meters measure moisture content accurately and often have built-in temperature control and automated weighing systems to simplify the process and increase efficiency.
Vacuum drying method (desiccant method);
Through the vacuum environment with desiccant (such as phosphorus pentoxide or sodium sulfate, etc.) to absorb moisture to dry the sample. It is suitable for some samples which are not resistant to high temperature and avoids the damage of high temperature to the samples. Vacuum drying oven is used to maintain the low-pressure state, and the desiccant is used to accelerate the absorption of moisture. Electronic balance is used to accurately weigh the change in mass of the sample before and after drying.
2. Distillation
Distillation is a method in which the water in a sample is evaporated and separated from the other components by heating the sample. The method is suitable for high moisture content or where the boiling point of the moisture in the sample differs significantly from that of the other components. It separates and determines moisture by evaporating the water and then recovering the water vapor through a condensation process.
The equipment required includes:
Distillation unit (e.g. Karl Fischer Distillation Unit): The distillation unit provides the heating and steam transfer required for moisture evaporation. Karl Fischer distillation units are specifically designed for moisture determination and are able to efficiently extract water from the sample by controlling the distillation process to avoid interference from other components.
Condenser: The condenser is used to cool the evaporated water vapor and condense it into liquid water for quantification. It is usually made of glass or high temperature resistant materials to ensure effective cooling.
Heater: The heater provides the heat required to vaporize the water in the sample. The heater is usually equipped with a temperature control system to ensure that the sample is heated at the proper temperature, thus avoiding excessive temperatures that could affect the composition of the sample.
3. Karl Fischer method:
The Karl Fischer method is the quantitative determination of water in a sample by means of a chemical reaction. The method is based on the reaction of water with a Karl Fischer reagent, which produces compounds, and the moisture content is calculated from the amount of reagent consumed. There are several variants of the Karl Fischer method depending on the moisture content and the type of sample:
Karl Fischer coulometric method (for trace moisture testing)
This method utilizes an electrolytic reaction in which iodine is generated by passing an electric current through a Karl Fischer reagent and the reaction binds to the moisture. This method is particularly suitable for the accurate detection of very small amounts of moisture and is often used to analyze samples with low moisture content. The Karl Fischer Coulometric Moisture Analyzer provides precise control of the electrolysis reaction and measurement of the electric current to determine trace amounts of moisture.
Karl Fischer Volumetric Method (for constant moisture detection)
This method determines moisture by adding a Karl Fischer reagent to the sample and titrating the reaction. It is suitable for samples with high or constant moisture content. The Karl Fischer volumetric titrator accurately determines the moisture content by means of an automatic titration and provides feedback on the results.
Karl Fischer Kjeldahl Oven Method (for dissolved and insoluble samples)
This method utilizes a Karl Fischer reagent to react with moisture in dissolved or insoluble samples. The Karl Fischer Oven Method is suitable for solid samples, especially large samples or samples that may decompose at high temperatures. The method provides highly accurate moisture determination and is widely used in chemical, pharmaceutical and food industries.
Determination of water activity (AW):
Moisture activity (AW) refers to the freedom of water in a sample and affects microbial growth and food preservation. Determination of water activity is carried out by means of a Moisture Activity Meter, which determines the water activity of a sample by measuring the saturation of water vapor on the surface of the sample. This method differs from traditional moisture content determination in that it focuses on the availability of moisture rather than the total amount, and is widely used for products that are sensitive to moisture activity, such as food and pharmaceuticals.
5. Chemical drying:
The chemical drying method absorbs moisture from the sample through the use of a chemical desiccant, such as phosphorus pentoxide or concentrated sulfuric acid. This method is suitable for samples that do not tolerate high temperatures, as the desiccant accelerates the absorption of moisture and ensures that the sample components are not damaged by high temperatures. The sample is mixed with the desiccant and left for a certain period of time, and the change in mass of the sample is monitored using weighing equipment to determine the moisture content. This method is commonly used to analyze sensitive materials and samples.
6. Microwave Method:
The microwave moisture determination method utilizes microwave heating of the sample, which results in rapid evaporation of water. This method is characterized by rapidity and accuracy and is widely used for moisture determination of food, agricultural products, environmental samples and so on. Microwave Moisture Tester is able to precisely control the microwave power and time, providing efficient testing in a short period of time. It deduces the moisture content from the mass loss of the sample or the change of microwave radiation with high efficiency and automation.
7. Infrared absorption spectrometry:
Infrared Absorption Spectroscopy (FTIR) is based on the absorption characteristics of moisture to infrared radiation for moisture determination. The sample is exposed to a certain wavelength of infrared light and the moisture absorbs a specific band of radiation. The instrument quantitatively analyzes the moisture content by measuring the absorption intensity. Accurate moisture content data is obtained in real time without destroying the sample, which is suitable for moisture detection of various solid and liquid samples, especially for multi-component complex samples.
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