Determining Moisture Content of Foods

Food processors need to know the moisture content of their products, and a number of methods are available for determining it, in both the laboratory and the processing plant.

According to Bobbie McManus, Process Product Manager for CEM Corp., Matthews, N.C. (phone 800-726-3331), “Effective moisture control is important in the food industry for both quality control and the efficient use of raw materials. In many cases, the process involves performing a rapid moisture/solids test, making an adjustment, and retesting. Quick response in the process allows plants to operate at their maximum profit level.”

A wide range of sample types can benefit from a rapid moisture analysis, she said, including dairy products, meats, spray-dried cheese, cake mixes, candy, condiments, and snack foods, to name a few. “In the dairy industry, efficient moisture/solids control maximizes yield and can save thousands of dollars per year. Dairies buy their milk and cream based on solids content, and different levels of solids are better suited for different products.”

At the same time, she added, rapid moisture testing also enables companies to easily maintain or improve their product consistency and quality. “For instance, moisture analysis is performed on licorice for mouthfeel and ease of extrusion. If the licorice is too dry, it will taste old. If it is too wet, it will be sticky and will not extrude properly for packaging.”

Here are some of the methods for determining the moisture content of foods and beverages:

• Oven Drying. This is the standard against which other methods are compared. The difference in weight of a sample after drying tells how much moisture the sample originally contained. The method is accurate but time consuming. “Standard oven drying,” McManus said, “can take 8–16 hr.”

• Microwaves. “Since the late 1970s,” McManus added, “the preferred method of choice has been the microwave moisture analyzer, first developed by CEM Corp. scientists. Microwave moisture analysis quickly yields accurate results and has been an AOAC-approved method for many years.”

“Microwave moisture analysis is as accurate as traditional oven drying,” she explained. “The big advantage is the speed. Quality assurance/quality control technicians can run a test in minutes and still have enough time to make adjustments to their process, optimizing their yields and reducing the amount of out-of-specification product.” The company’s Smart System Microwave Moisture Analyzer, she said, is an easy-to-use production-environment system with laboratory precision in accurately measuring solids, liquids, powders, and slurries.

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• Karl Fisher Titration/Homogenization. Another method for determining moisture content is the Karl Fisher titration method and a homogenization technique, according to Tore Fossom, Technical Specialist at the Instrument Div. of Mettler Toledo, Inc., Columbus, Ohio (phone 800-638-8537). “It is important to know the moisture content of food and candies, especially soft, chewy candies. If there is too much water, the product becomes too soft and sticky, and if there is too little, it becomes hard. Analyzing water by loss on drying is difficult because the product may not release all of the water, and it may caramelize or alter under heating. Another consideration with loss on drying is that some of the volatiles may be not water, but flavoring ingredients.”

Determining moisture by Karl Fischer titration is an efficient, rapid, and specific way to determine the water content of a food product, Fossom said. “Using a Mettler Toledo DL38/HS280 Water Determination Titrator, the water is mechanically broken out of the solid sample and determined by a specific chemical reaction which quantifies only the water. The design of the HS280 Homogenizer, with a rotating knife within a stator with teeth, pulverizes the sample far more efficiently than older style rotating blade turbo units.” 

An appropriate size sample, typically 1 g, is weighed into a small container. The operator selects “Sample” on the titrator, inserts an identification code if desired, then drops the sample into the port on the top of the vessel. The homogenizer switches from stir mode, where the knife rotates at 1,500 rpm, to homogenize mode at 9,000 rpm. The sample is pulverized for 1–3 min, then the homogenizer switches back to stir mode and the titration for water commences. The titrator adds a measured amount of an iodine-containing solution which reacts with the water released from the sample. The operator transfers the weight of the sample from the balance to the titrator. When the water is all reacted, the titrator computes the water content and prints a report.

The homogenization technique can effectively determine water in a variety of foods, Fossom said. The solvent mixture maximizes efficiency of extraction. Fat-containing foods such as chocolates are best extracted with a mixture of methanol and chloroform. Sugar-containing foods such as candies are best extracted with a mixture of methanol and formamide. The formamide mixture extracts carbohydrates and proteins efficiently, as well.

Usually several samples can be analyzed before the solvent needs to be changed, he said. Changing the solvent is easy. The solvent suction tube is lowered to the bottom of the vessel and the pump started. After the spent solution has been removed, the operator pulls the tube back up and pushes the fill button. Fresh solvent comes over from the solvent bottle. The operator restarts the titrator to dry the solvent, and shortly the vessel is ready for the next set of samples.

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• Near-Infrared Spectrometry. Another method, used in both the laboratory and the processing plant, is near-infrared spectrometry. “Near-infrared photometric analyzers use discrete NIR wavelengths to measure one or more constituents in a variety of food products,” according to Imogen Swanton (phone +011-44-1536-408066), United Kingdom–based Technical Services Manager for Process Sensors Corp., headquartered in Milford, Mass. (phone 508-473-9901). “The measurement is nondestructive, noncontact, and instantaneous. Consequently, it is the ideal measurement solution for process control.”

NIR, she explained, is typically used in the food industry to measure moisture, oil, and protein content, but it can also be used indirectly to measure added salt and flavorings in snack foods. Molecular bonds—principally O–H in water, C–H in oil, and N–H in protein—absorb NIR energy at well-defined wavelengths. The absorbance at the specific wavelength is directly proportional to the quantity of that constituent.

The MCT Series moisture analyzers generate the absorbance wavelength and reference (non-absorbing) wavelengths using a rotating wheel containing narrowbandpass filters. Light emitted from a quartz halogen lamp passes through each filter in turn and illuminates the product. Nonabsorbed light is backscattered, collected, and focused onto a single temperature-controlled detector. The electrical signals generated by the detector are mathematically treated in ratio-based algorithms to provide a value that is proportional to the concentration of the measured constituent.” 

The MCT 600 Bench Top NIR Tester is operator friendly and simple to use, Swanton said. Easy-to-follow software guides the user through calibration and sample testing. Samples, straight from the production line, are placed in the sample dish, and a touch of the button initiates the test. In less than 10 sec, the constituent measurements are displayed and the unit becomes ready for additional samples.

Typical applications include moisture and oil in chips and corn-based snack foods, and moisture in cookies, cereals, flour, soy meal, coffee, tea, milk powders, and pet foods.

• Freezing-Point Depression/Cryoscopy. Dairy laboratories use freezing-point depression to measure the water content in milk from cow, sheep, goat, buffalo, and other milk-producing animals, said Ken Micciche, Director of Marketing, Advanced Instruments, Inc., Norwood, Mass. (phone 781-471-2145). It is also used by the beverage industry to confirm formulation and production consistency for carbonated beverages, fruit juices, and other beverages requiring fermentation. 

When raw milk is delivered to a processing plant, a cryoscope is used to determine water content. “Since milk is sold by weight, processors abhor paying for excess water,” Micciche said, “because it can affect shelf life by harboring bacteria and the excess water makes it more difficult to produce a consistent, quality product. Samples are tested before raw milk is unloaded into storage, and finished products also are tested for water content for quality assurance purposes.”

by Dean Duxbury,
Contributing Editor
Consultant, Oak Brook, Ill.
[email protected]