Knowing how much moisture or water in a food is a critical value with ramifications for food testing, preservation and safety, and quality. Moisture content affects the way we perceive texture, taste and flavour, appearance and weight, and influences the shelf-life of a food product. If there is any deviation from a standard value for a food product it can adversely affect both the chemical and physical properties of the foodstuff.
Determination of moisture content is also a necessity when calculating the content of all other food constituents on a uniform basis which is usually a dry weight basis. All the dry matter remaining after moisture analysis is commonly referred to as total solids. Very often total solids is a more useful measure !
Moisture content is not required on any nutrition label but it must be known if the total carbohydrate content of a food is to be calculated. Likewise, when the ash content is measured, it can then be combined with the moisture determination to give other critical measures.
Summary Of Techniques
The main techniques for measuring water or moisture content in food are the following:-
- Chemical analysis – usually by the Karl Fischer titration or calcium carbide testing.
- Spectroscopic analysis – by infra-red spectroscopy, microwave spectroscopy, proton nuclear magnetic resonance spectroscopy
- Thermogravimetric methods and analysis (TGA) based on oven, halogen or IR drying, microwave drying.
- Other techniques include gas chromatography, density determination, refractometry and so on.
One of the most influential analytical references is by Nielsen (2010) which is worth purchasing as a reference work.
Regulatory & Legal Requirements
As we’ve already mentioned, moisture content is not needed on any nutritional label. However, some foods have legal limits for water content depending on the type of product. A cheddar cheese for example cannot be greater than 40 per cent by moisture.
Thermogravimetric methods
The most effective technique generally is to measure the moisture content of a food before and after drying. The value usually quoted as a percentage of the initial weight is the loss of product weight during drying by measuring the change in a mass of sample which is heated in a controlled manner until no further change in weight is observed.
The equation used is:-
% Moisture = (Initial Sample Weight – Final Sample Weight) / Initial Sample Weight)
-Balance & Drying Ovens
In oven drying, the food sample is heated under specified and defined conditions. The loss of weight is used to calculate the moisture content of the sample. The various types include:-
Forced Oven With Draft—The sample is weighed in a moisture pan and placed in a drying oven for a particular time. Very often the time and oven temperature are defined for a particular food when no standard method exists. Drying times for this technique are between 0.75 and 24 hours, depending on the food sample.
Vacuum Oven—Drying is performed under reduced pressure (25-100 mm Hg) which allows a more thorough removal of water and volatiles without decomposition and within 3 to 6 hours of drying time.
Microwave Oven—A precise and rapid technique that allows some segments of the food industry to make in-process adjustments of moisture content before final packaging. In vacuum microwaves, a drying time of 10 minutes can yield results equivalent to those of five hours in a standard vacuum oven.
Infrared Drying—Relies on the penetration of heat into the sample being dried, as compared to heat conductivity and convection as with conventional ovens. Required drying time can be as little as 10-25 minutes. It is suitable for qualitative use when checking changes during a process, but is not approved by the AOAC.
References
Isengard, H.D. (2001) Water content, one of the most important properties of food. Food Control. 12(7) pp. 395-400
James, C.S., (1996). Analytical Chemistry Foods, New York, Blackie Academics. USA
Mulvaney, T.R. (1995) In: Cunniff P, ed. Official Methods of Analysis of AOAC International. 16th ed. Arlington, Va.; pp. 42-1-42-2.
Nielsen S.S. (2010) Determination of Moisture Content. In: Nielsen S.S. (eds) Food Analysis Laboratory Manual. Food Science Texts Series. Springer, Boston, MA.
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