Determination of moisture content in Foods

1. 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 Draft—Sample 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 Oven—Drying 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 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—Employs 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.)

2. Distillation Methods 

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.

3. Chemical Methods  (Karl 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.

4. Physical Methods

Electric (dielectric or conductivity)—Moisture content is determined by measuring the change in capacitance or resistance to an electric current passed through a sample.

Hydrometry—Used to determine moisture/ solid content of beverages and sugar solutions. Measuring the specific gravity or density of the sample via one of the following instruments:

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.

Refractometry—Measures 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 Analysis—Measures the energy that is reflected or transmitted by the sample when exposed to infrared light.

Freezing Point—Measures 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]/100

Moisture Content of Selected Foods

Food item   % moisture (wet wt basis)

Cereals and pasta

Wheat flour, whole grain 10.3

Corn flakes cereal 03.0

Macaroni, dry, enriched 10.2

Dairy products

Yogurt, plain, low fat 89.0

Cheddar cheese 37.5

Ice cream, vanilla 61.0

Fats and oils

Butter, salted 16.9

Oil, soybean 00.0

Fruits and vegetables

Watermelon, raw 91.5

Apples, raw, with skin

Raisins 15.4

Cucumbers, with peel, raw 96.0

Potatoes, raw, flesh and skin 79.0

Meat, poultry, and fish

Beef, ground, extra lean, raw 63.2

Chicken, light meat, meat and skin, raw 68.6

Finfish, flounder/sole, raw 79.1

Egg, whole, raw, fresh 75.3