Determining Moisture Content in Foods
Oven Drying Methods
With oven drying, the sample is heated under specified conditions, and the loss of weight is used to calculate the moisture content of the sample.
Forced Oven DraftSample is rapidly weighed into a moisture pan and placed in the oven for an arbitrarily selected time if no standard method exists. Drying time periods for this method are 0.75-24 hours, depending on the food sample.
Vacuum OvenDrying under reduced pressure (25-100 mm Hg) allows a more complete removal of water and volatiles with-out decomposition within 3-6 hr drying time.
Microwave OvenA 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 DryingEmploys 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. (Suited for qualitative in-process use, but not approved by the AOAC.)
Distillation techniques involve codistilling the moisture in a food sample with a high boiling point solvent that is immiscible in water, collecting the mixture that distills off, and then measuring the volume of water. Includes direct and reflux distillation.
Chemical MethodsKarl Fischer Titration
This technique is particularly suited to food products that show erratic results when heated or submitted to a vacuum. It is the method of choice for low-moisture foods such as dried fruits and vegetables, candies, chocolate, roasted coffee, oils and fats, and low-moisture foods high in sugar or protein.
Electric (dielectric or conductivity)Moisture content is determined by measuring the change in capacitance or resistance to an electric current passed through a sample.
HydrometryUsed to determine moisture/ solid content of beverages and sugar solutions. Measuring the specific gravity or density of the sample via one of the
Pycnometer: used to compare the weights
of equal volumes of a liquid and water. Yields density of the liquid compared to water.
Hydrometer: a standard weight on the end
of a spindle which displaces a weight of liquid equal to its own weight. In a low-density liquid, weight will sink to a greater depth.
Westphal Balance: functions on the principle that the plummet on the balance will be buoyed by the weight of liquid equal to the volume displaced.
RefractometryMeasures moisture content of oils and syrups as a function of the degree of refraction of a light beam as it passes through the sample.
Infrared AnalysisMeasures the energy
that is reflected or transmitted by the sample when exposed to infrared light.
Freezing PointMeasures the solutes present by determining the freezing point of the sample. Used principally to measure for added water content
Moisture Content Calculations
(For oven-drying methodologies)
|% Moisture =
|(wt of wet sample - wt of dry sample)
|wt of wet sample
Moisture Content of Selected Foods
From USDA Nutrient Database for Standard Reference, with modification. Release 11-1 (Aug. 1997)
|Food item ||
(wet wt basis)
|Cereals and pasta
|Wheat flour, whole grain ||
|Corn flakes cereal ||
|Macaroni, dry, enriched ||
|Yogurt, plain, low fat ||
|Cheddar cheese ||
|Ice cream, vanilla ||
|Fats and oils
|Butter, salted ||
|Oil, soybean ||
|Fruits and vegetables
|Watermelon, raw ||
|Apples, raw, with skin ||
|Cucumbers, with peel, raw ||
|Potatoes, raw, flesh and skin ||
|Meat, poultry, and fish
|Beef, ground, extra lean, raw ||
|Chicken, light meat, meat and skin, raw ||
|Finfish, flounder/sole, raw ||
|Egg, whole, raw, fresh ||
Table of Contents
Resources and Regulations
Independent Testing Labs
Technical Data Documents