DETERMINATION OF MOISTURE CONTENT IN FOOD

What is moisture content?

Any water included in a food product is considered to have a moisture content. Many factors, including ambient moisture from the manufacturing and packaging locations, packaging techniques, and food storage, might introduce too much moisture into the food. The moisture in these areas should be carefully monitored and managed to maximize food quality and safety.

Why is moisture content analysis of food important?

The amount of moisture in food may have a significant impact on factors like taste, texture, appearance, shape, and weight. It affects legal and labeling requirements, requirements that are significant commercially, the shelf life of the food or food products, measures of food quality, and food processing processes. food product’s many attributes can be adversely affected by variations from the ideal moisture level, which can have an influence on both the product’s quality and safety. Because of this, moisture content analysis has become an essential part of the food sector.

Methods for determining moisture in foods

Methods for determination of moisture in food can be divided into four different classes.

1. Drying methods
2. Distillation procedure
3. Chemical assay
4. Physical procedure

Drying methods

Thermal drying methods are usually used in the procedures for determining the moisture content stipulated in food standards. The material is heated to a certain temperature, and the weight loss is used to calculate the sample’s moisture content. The type of oven, temperature, and drying time all affect the moisture value. As a result, the methods provide estimated moisture values at the same time rather than precise ones. The rate of water evaporation from a solid phase’s surface depends on the drying temperature and the water vapour pressure. Yet, practical considerations involve choosing temperatures that minimize the breakdown of organic components while keeping the time needed for quantitative drying at the chosen temperature reasonably short.

Moisture content %  

Advantage

• Drying methods are simple, relatively rapid, and permit the simultaneously analyzes of large number of samples.

Disadvantage

• Moreover, the irreversible thermal breakdown of an organic component results in the volatilization of a material and the creation of a gaseous product when a wet organic substance is heated.
• Further weight changes resulting from oxidation phenomenon(i.e. oxidation of oils) occur. 
• Creation of a crust on a dried sample that resists moisture evaporation from the center.

Distillation procedure

Methods for distillation are based on precise measurements of the water that has been evaporated from a food sample. In practice, evaporation methods rely on estimations of the water that has been evaporated from a food sample. In essence, distillation procedures entail heating weighted food samples while they are in the presence of an organic solvent that is insoluble in water. The sample’s water evaporates, is collected in a graduated glass tube, and its volume is calculated there.

 Dean and stark methods

Dean  and stark is the best method of distillation methods. In a flask, a known weight of food is combined with an organic solvent like xylene or toluene. The organic solvent must not dissolve in water, be less dense than water, have a higher boiling point than water, and be safe to use. The sample and organic solvent are combined in a flask that is heated and connected to a condenser by a side arm. The sample’s water evaporates, rises into the condenser, gets cooled, and is then transformed back into liquid water, trickling into the graduated tube. Distillation is halted and the amount of water is measured from the graduated tube when there is no longer any water remaining in it.

Advantage

• Suitable for application to foods with low moisture contents.
• Equipment is relatively cheap, easy to setup and operate.
• suitable for use on foods that include volatile oils, such as herbs or spices, because the oils stay dissolved in the organic solvent and do not affect the measurement of the water.

Disadvantages

• Destructive
• Not applicable to some types of foods.
• Involves the use of flammable solvents.

Chemical assay to determine moisture content in food

The content of moisture in foods may be calculated using the interactions between water and certain chemical reagents. In these procedures, the food is provided a chemical reagent that, when introduced to water, specifically reacts to modify the system’s mass, volume, pressure, pH, color, and conductivity. Calibration curves are used to connect measurable changes in the system to the moisture content. The chemical reagent must react with all of the existing water molecules in order to provide accurate results, but it must not interact with any of the other elements of the food matrix.

  Foods that contain volatile ingredients that could be lost by heating or foods that contain thermally labile substances that would change the mass of the food matrix on heating (such as foods with high sugar concentrations) are suitable for chemical reaction methods because they rarely involve the application of heat (e.g. spices and herbs)

Karl-Fisher method

The Karl-Fisher titration is often used for determining the moisture content of foods that have low water contents (e.g. dried fruits and vegetables, confectionary, coffee, oils and fats). It is based on the following reaction:

2 H₂O + SO₂ + I₂               H₂SO₄ + 2 HI

HI is colorless, but I₂ is a dark reddish brown hue, therefore there is a detectable change in color when water interacts with the additional chemical reagents. This is why the reaction was first used. Iodine and sulfur dioxide are gaseous substances that would typically evaporate out of solution. In order to retain the S₂O and I₂ in solution, solvents (such as C₅H₅N) have been added to the above mentioned reaction, however the essential principles of the procedure remain the same.

The food to be analyzed is placed in a beaker with solvent, and Karl Fisher reagent is then used to titrate the results (a solution that contains iodine). Iodine reacts with any water that is still present in the sample, leaving the solution colorless (HI), but if all the water has been consumed, any remaining iodine is seen as a dark red brown colour  (I2). Using a pre-made calibration curve, the amount of iodine solution needed to titrate the water is measured and related to the moisture content. Instead of concentrating on a color change, the process may be made more precise by using electrical methods to track the reaction’s end-point.

Advantage

• It is a method of choice for determination of water in many low moisture foods such as dried fruits and vegetables, candies, chocolate, roasted coffee, oils and fats.
• Viscous fluids or pastes are generally homogenized with a solvent.
• The excess of iodine that cannot react with water is in free form.
• Granular products must be pulverized.

Disadvantage

• The main difficulty in using the Karl Fischer method arises from the lack of complete water extraction.
• Formaldehyde is found to be a more rapid and versatile extractant of water from foods than methanol.
•  Modification of the extraction procedure is exemplified by a method for the water determination in dairy products, where in xylene or carbon tetrachloride is employed in mixed solvent systems with alcohol.

Physical procedure

A number of analytical methods have been developed to determine the moisture content of foods that are based on the fact that water has appreciably different bulk physical characteristics than the food matrix, e.g., density, electrical conductivity or refractive index. Physical methods are often only appropriate for food analysis when the ratio of water-to- food matrix changes but the composition of the food matrix does not change. Because both the density and electrical conductivity of water are much higher than those of oil, it is possible to measure the water content of oil-in-water emulsions.

 Many food compositions may give the same value for the physical property being measured; it may not be possible to reliably estimate the moisture content of the food if the food matrix’s composition varies along with its water content. Under these situations, it might be possible to establish the composition of the food using a combination of two or more physical approaches, such as density measurements combined with electrical conductivity measurements.

References

http://ecoursesonline.iasri.res.in/mod/page/view.php?id=857

https://www.news-medical.net/life-sciences/Why-is-Moisture-Content-Analysis-of-Food-Important.aspx

https://people.umass.edu/~mcclemen/581Moisture.html