For example, the rate of microbial growth in a food depends on the amount of bulk water present in a food, and not necessarily on the total amount of water present. There are analytical techniques available that can provide some information about the relative fractions of water in different molecular environments e.

Sample preparation. Selection of a representative sample, and prevention of changes in the properties of the sample prior to analysis, are two major potential sources of error in any food analysis procedure. When determining the moisture content of a food it is important to prevent any loss or gain of water.

For this reason, exposure of a sample to the atmosphere, and excessive temperature fluctuations, should be minimized. When samples are stored in containers it is common practice to fill the container to the top to prevent a large headspace, because this reduces changes in the sample due to equilibration with its environment.

The most important techniques developed to measure the moisture content of foods are discussed below. Evaporation methods. These methods rely on measuring the mass of water in a known mass of sample. The moisture content is determined by measuring the mass of a food before and after the water is removed by evaporation:.

Here, M INITIAL and M DRIED are the mass of the sample before and after drying, respectively. The basic principle of this technique is that water has a lower boiling point than the other major components within foods, e. Sometimes a related parameter, known as the total solids, is reported as a measure of the moisture content.

The total solids content is a measure of the amount of material remaining after all the water has been evaporated:. To obtain an accurate measurement of the moisture content or total solids of a food using evaporation methods it is necessary to remove all of the water molecules that were originally present in the food, without changing the mass of the food matrix.

This is often extremely difficult to achieve in practice because the high temperatures or long times required to remove all of the water molecules would lead to changes in the mass of the food matrix, e. For this reason, the drying conditions used in evaporation methods are usually standardized in terms of temperature and time so as to obtain results that are as accurate and reproducible as possible given the practical constraints.

Using a standard method of sample preparation and analysis helps to minimize sample-to-sample variations within and between laboratories.

The thermal energy used to evaporate the water from a food sample can be provided directly e. Convection and forced draft ovens. Weighed samples are placed in an oven for a specified time and temperature e. The thermal energy used to evaporate the water is applied directly to the sample via the shelf and air that surround it.

There are often considerable temperature variations within convection ovens, and so precise measurements are carried out using forced draft ovens that circulate the air so as to achieve a more uniform temperature distribution within the oven. Samples that contain significant quantities of carbohydrates that might undergo chemical changes or volatile materials other than water should not be dried in a convection or forced draft oven.

Many official methods of analysis are based on forced draft ovens. Vacuum oven. Weighed samples are placed under reduced pressure typically mm Hg in a vacuum oven for a specified time and temperature and their dried mass is determined. The thermal energy used to evaporate the water is applied directly to the sample via the metallic shelf that it sits upon.

There is an air inlet and outlet to carry the moisture lost from the sample out of the vacuum oven, which prevents the accumulation of moisture within the oven.

The boiling point of water is reduced when it is placed under vacuum. Drying foods in a vacuum oven therefore has a number of advantages over conventional oven drying techniques.

If the sample is heated at the same temperature, drying can be carried out much quicker. Alternatively, lower temperatures can be used to remove the moisture e. A number of vacuum oven methods are officially recognized.

Microwave oven. Weighed samples are placed in a microwave oven for a specified time and power-level and their dried mass is weighed. Alternatively, weighed samples may be dried until they reach a constant final mass - analytical microwave ovens containing balances to continuously monitor the weight of a food during drying are commercially available.

The water molecules in the food evaporate because they absorb microwave energy, which causes them to become thermally excited. The major advantage of microwave methods over other drying methods is that they are simple to use and rapid to carry out. Nevertheless, care must be taken to standardize the drying procedure and ensure that the microwave energy is applied evenly across the sample.

A number of microwave oven drying methods are officially recognized. Infrared lamp drying. The sample to be analyzed is placed under an infrared lamp and its mass is recorded as a function of time.

The water molecules in the food evaporate because they absorb infrared energy, which causes them to become thermally excited. One of the major advantages of infrared drying methods is that moisture contents can be determined rapidly using inexpensive equipment, e.

This is because the IR energy penetrates into the sample, rather than having to be conducted and convected inwards from the surface of the sample. To obtain reproducible measurements it is important to control the distance between the sample and the IR lamp and the dimensions of the sample. IR drying methods are not officially recognized for moisture content determinations because it is difficult to standardize the procedure.

Even so, it is widely used in industry because of its speed and ease of use. The water that is released by this reaction is not the water we are trying to measure and would lead to an overestimation of the true moisture content.

On the other hand, a number of chemical reactions that occur at elevated temperatures lead to water absorption, e. Foods that are particularly susceptible to thermal decomposition should be analyzed using alternative methods, e. chemical or physical. Distillation Methods.

Basically, distillation methods involve heating a weighed food sample M INITIAL in the presence of an organic solvent that is immiscible with water.

The water in the sample evaporates and is collected in a graduated glass tube where its mass is determined M WATER. Distillation methods are best illustrated by examining a specific example: the Dean and Stark method. A known weight of food is placed in a flask with an organic solvent such as xylene or toluene.

The organic solvent must be insoluble with water; have a higher boiling point than water; be less dense than water; and be safe to use.

The flask containing the sample and the organic solvent is attached to a condenser by a side arm and the mixture is heated. The water in the sample evaporates and moves up into the condenser where it is cooled and converted back into liquid water, which then trickles into the graduated tube.

When no more water is collected in the graduated tube, distillation is stopped and the volume of water is read from the tube. There are a number of practical factors that can lead to erroneous results: i emulsions can sometimes form between the water and the solvent which are difficult to separate; ii water droplets can adhere to the inside of the glassware, iii decomposition of thermally labile samples can occur at the elevated temperatures used.

Chemical Reaction Methods. Reactions between water and certain chemical reagents can be used as a basis for determining the concentration of moisture in foods. In these methods a chemical reagent is added to the food that reacts specifically with water to produce a measurable change in the properties of the system, e.

Measurable changes in the system are correlated to the moisture content using calibration curves. To make accurate measurements it is important that the chemical reagent reacts with all of the water molecules present, but not with any of the other components in the food matrix.

Two methods that are commonly used in the food industry are the Karl-Fisher titration and gas production methods. Chemical reaction methods do not usually involve the application of heat and so they are suitable for foods that contain thermally labile substances that would change the mass of the food matrix on heating e.

spices and herbs. The Karl-Fisher titration is often used for determining the moisture content of foods that have low water contents e.

dried fruits and vegetables, confectionary, coffee, oils and fats. It is based on the following reaction:. This reaction was originally used because HI is colorless, whereas I 2 is a dark reddish brown color, hence there is a measurable change in color when water reacts with the added chemical reagents.

Sulfur dioxide and iodine are gaseous and would normally be lost from solution. For this reason, the above reaction has been modified by adding solvents e. Direct methods involve removing the water from the product by drying, distillation, extraction, etc. then measuring the amount of water by weighing or titrating.

Direct methods provide the most reliable results, but are usually labor intensive and time consuming. Some examples include air oven-drying, vacuum oven-drying, freeze-drying, distillation, Karl Fischer, thermogravimetric analysis , chemical desiccation, and gas chromatography.

Indirect methods do not remove the water from the sample. Instead, they involve measuring some property of the food that changes as moisture content changes. These methods require calibration to a primary or direct method.

Their accuracy is limited by the accuracy of the primary method. Indirect methods are usually fast and require little sample preparation, but are less reliable than direct measurement methods.

Examples of indirect measurement methods include refractometry, IR absorption, NIR absorption, microwave adsorption, dielectric capacitance, conductivity, and ultrasonic absorption.

Any method that requires heating i. For example, if organic volatiles are present in a sample or if the sample decomposes while being dried, a Karl Fischer analysis, which is not susceptible to volatile loss or decomposition, will give different results than a loss-on drying analysis.

One answer to these problems is to simply use a consistent method and only compare values that have been obtained in the same way.

Unfortunately, consistency in measurement methods for moisture content analysis still will not eliminate all problems. Consider, for instance, loss-on drying. This method seems simple enough. A sample is weighed, and the weight is recorded. The sample is then transferred to an oven, allowed to dry, and the dry weight is measured.

The amount of water is determined by subtracting the dry weight from the initial weight, and the moisture content is then calculated as the amount of water divided by the dry weight or total weight, depending on the reporting method.

Even this simple loss-on-drying method is mined with potential variability traps. Instead, an arbitrary dryness that is reproducible has to be established for each sample. However, thermogravimetric graphs show that weight loss levels off at different temperatures for different products. Some testers use a balance and a hot plate, others a balance and an oven.

Today technology has made it easy to use an integrated balance and drying method to automate the procedure. The technique is often referred to as Loss-on Drying or LOD. Another system for moisture detection is based on the fact that many materials change electrical or.

dielectric characteristics depending the moisture level in the material. Most of these instruments measure changes in resistance , conductivity or capacitance. Because these techniques are measuring an indirect affect of moisture, calibration is necessary.

The calibration isaccomplished by comparing the dielectric reading to a known moisture for the sample. A graph is prepared and used to provide the translation of the electrical characteristic to the amount of moisture in the material sample. A third method is based on a chemical reaction and is known a Karl Fischer Titration.

The reagent reacts with water to eliminate the conductivity that was developed by the water in the test sample. This test is accomplished by injecting the material into a solvent and then adding the Karl Fischer Reagent.

The reagent is added until the reaction has converted all the water into a non conductive chemical. The amount of reagent used is measured and converted into units of water.

Karl Fischer techniques are advantageous for measuring small amounts of water, for samples that contain volatiles that degrade the accuracy of weight loss on drying techniques and for releasing bound water in some materials.

Application of the concepts of reflection and absorption of electromagnetic energy is becoming widely used, particularly for in-line or on-line measurement.

These techniques based on the finding that water as well as other chemicals has very specific absorption wavelengths.

A short video is executed to demonstrate the experiment procedure and sample calculation. Use the template provided to prepare your lab report for this experiment. Your report should include the following:.

Properties and Behavior of Soil - Online Lab Manual Copyright © by MD Sahadat Hossain, Ph. Skip to content Introduction The moisture content of soil also referred to as water content, is an indicator of the amount of water present in soil. Practical Application Almost all soil tests determine the natural moisture content of the soil, and it is essential knowledge for all studies of soil mechanics.

The natural moisture content provides an idea of the state of the soil in the field. Moisture content is one of the most important index properties used for the correlation of soil behavior and its index properties.

The moisture content of the soil is used to express the phase relationships of water, air, and solids in a given volume or weight of the material. For cohesive soil, the consistency of a given soil, along with its liquid and plastic limits is used to express its relative consistency.

It is important to note that moisture is not just water. Water specifically refers to molecules with the chemical formula H 2 O, however moisture is a wider term that includes anything that evaporates from a substance when heated, for example solvents, flavourings, fats and other substances.

There are 3 types of moisture content that all must be considered when measuring moisture content:. Surface moisture — Moisture adhered to the surface sample, sometimes referred to has hygroscopic, adhesive or free water. This is the least difficult moisture to remove.

Absorbed moisture — This moisture has travelled into the pores and capillaries of the sample. Sometimes called equilibrium moisture. Chemically bonded moisture — Moisture molecules are chemically bonded to the sample at a molecular level.

This is the most difficult type of moisture to remove. The moisture content of a substance can impact its overall quality, so measuring moisture content is vital to consistently uphold the high standards of products.

For example, in foods, moisture content can impact the taste, smell and shelf life of a product. Determining the moisture content of food can also help to reduce food waste. In agriculture, the moisture content of soil can help determine optimal growing conditions for plants and crops.

Most industries have laws to regulate the moisture content of substances. This makes a moisture content analyzer an essential piece of equipment for many research and development labs, as well as quality control facilities. Moisture Content Determination can be carried out in just 3 steps when using a Moisture Content Analyzer:.

If you would like to find out more about applications of our moisture analyzers, you can read these related articles:.

Product Finder. Laboratory Weighing Semi-Micro Balances Analytical Balances Premium Advanced Precision Balances Premium Advanced Entry. Industrial Scales. Ash Analyzers. Moisture Analyzers.

Special Products. View All Products. About Us News Events Blog Careers Partner Log-In. Blog Measuring Moisture Content in 3 Easy Steps. Measuring Moisture Content in 3 Easy Steps Moisture Determination is vital for many industries. There are 3 types of moisture content that all must be considered when measuring moisture content: Surface moisture — Moisture adhered to the surface sample, sometimes referred to has hygroscopic, adhesive or free water.

Reasons for Moisture Determination The moisture content of a substance can impact its overall quality, so measuring moisture content is vital to consistently uphold the high standards of products.

Moisture Determination In 3 Steps Moisture Content Determination can be carried out in just 3 steps when using a Moisture Content Analyzer: Find the initial weight of the sample by weighing the sample before drying Set parameters temperature, time and dry sample Weigh sample after drying and compare to the initial weight to calculate the Loss on Drying.

Related Articles If you would like to find out more about applications of our moisture analyzers, you can read these related articles: Application Note: Methods for Moisture Determination with XM60 Application Note: Moisture Determination of Different Plastic Granules Application Note: Moisture Determination of Liquid Soap Application Note: Moisture Content of Spaghetti Application Note: Moisture Determination of Cement Determining the Moisture Content of Medical Marijuana.

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