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A moisture assay can be one of the most important analyses performed on a food product, yet one of the most difficult from which to obtain accurate and precise data, says Robert Bradley ([email protected]), Professor Emeritus, Dept. of Food Science, University of Wisconsin.
Moisture levels control many properties of food, including water activity (Aw), shelf stability, standards of identity, crystallization, and pumpability. Conversely, the type of packaging used can influence and control moisture content.
"The storage stability of dry or dried foods and animal feeds is definitely influenced by moisture level," Bradley says. "What’s more, the computation of nutritional values requires an accurate assessment of moisture content. Moisture analysis is as important for consumables with long-established production records as it is for newly developed food items and beverages. Without this valuable number—the moisture content—manufacturers, processors, scientists, nutritionists, or whoever is working with a specific food cannot proceed."
Successful and accurate moisture analysis depends on paying attention to basic principles such as sample collection, handling, storage, and preparation, Bradley says. "These procedures are perhaps the greatest potential source of error in any analysis. Precautions must be taken to minimize inadvertent moisture losses or gains that occur during these steps. Obviously, any exposure of a sample to the open atmosphere should be as short as possible."
Any heating of a sample by friction generated during grinding should be minimized, Bradley continues. Moreover, there should be zero headspace in the sample storage container. That’s because moisture migrates from the sample to the colder container, forming moisture droplets in the void space that may not reincorporate into the sample.
"To control potential errors, one should remove the entire sample from the original container, reblend quickly, then remove a test portion," Bradley advises.
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Moisture Analysis Methods
Several methods to analyze moisture in food are available, including oven drying (convection, forced-draft, vacuum, and microwave), rapid moisture analyzer, distillation, chemical (Karl Fischer titration), and physical (electrical, hydrometry, refractometry, infrared analysis, and freezing point).
Bradley prefers the oven drying method, and uses a vacuum oven, specifically a Lab-Line model, as well as a Lab-Line forced-draft oven, which are currently manufactured by Fisher Thermo Scientific (phone 800-446-6060, www.thermo.com).
"The vacuum variety is not the speediest oven available, but it is recognized by AOAC International in many official methods, as is the forced draft oven," Bradley points out. "Moisture analysis using the microwave oven has been given peer-review approval and that distinction is less than AOAC approval. Because of accuracy variables, no method involving infrared heat lamps has been AOAC approved or approved by peer review."
When using any of the oven-drying methods mentioned above, the sample is heated under specified conditions, and the loss of weight during heating is used to calculate the moisture content of the sample as a percentage of the initial weight.
Variables to Consider
"The amount of moisture determined is highly dependent on the type of oven used, conditions within the oven, and the time and temperature of drying," Bradley says. "Vacuum, forced-draft, and convection ovens allow for simultaneous analysis of large numbers of samples. The time required may be from a few hours to more than 24 hours."
Moisture removal from a product is sometimes achieved in a two-stage process. Liquid products such as juices and milk are commonly predried over a steam bath before drying in an oven. Products such as bread and field-dried grain are often air dried, then ground and oven dried. The moisture content is calculated from the moisture loss at both the air and oven drying steps. (Note: % moisture = dry weight x 100 divided by wet weight; % solids = 100 - % moisture.)
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Regardless of the technique employed, the particle size, particle size distribution, sample size, and sample surface area during drying influence the rate and efficiency of moisture removal.
Each of the various drying methods has specified drying temperatures and times, which must be carefully controlled.
"There may be considerable variability of temperatures within one style of oven," Bradley says. "Therefore, it’s important to determine the extent of that variation before relying on data collected from an oven’s use."
The greatest temperature variation exists in a convection oven. A 10°C temperature differential across a convection oven is not unusual. That’s because hot air slowly circulates without the aid of a fan. Moreover, air movement in the oven is obstructed further by pans that hold the food samples.
"A convection oven should not be used when precise and accurate measurements are required," Bradley advises.
Forced-draft ovens have the least temperature differential across their interior, usually not greater than 1°C. A fan circulates the air within the oven.
"Forced-draft ovens with air-distribution manifolds aligned in the shelves appear to have added benefit where air movement is horizontal across shelving," Bradley mentions. "So no matter whether the oven shelves are filled completely with sample pans or only half filled, the result would be the same for a particular sample."
Two features of some vacuum ovens contribute to a wider temperature spread across the oven. The first is a glass panel in the door, which acts as a heat sink, giving a lower internal oven temperature near the glass panel. The second is the way air is bled into the oven. "If the air inlet and discharge are on opposite sides, conduct of air is virtually straight across the oven," Bradley explains.
Some newer models have air inlet and discharge manifolds mounted top to bottom. In these vacuum ovens, the air circulates upward from the front, then backward to the discharge in a broad sweep. This minimizes cold spots and exhaust moisture in the interior air.
"If a vacuum oven generates too much variation when measuring temperature, greater than plus or minus 2°C, don’t buy it," Bradley cautions. "A decision to purchase should be based on consistency of results."
A vacuum oven facilitates drying under reduced pressure (25–100 mm Hg), and it also offers the advantage of more complete removal of water and volatiles without sample decomposition within a 3- to 6-hr drying time compared to a forced-draft oven.
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Key Points About Vacuum Ovens
There are several important points to keep in mind when using a vacuum drying oven, Bradley says.
"The temperature within any vacuum oven depends on the product," he relates. "Products with high sugar content require reduced oven temperatures, and even with a reduction to 65°C there can be some decomposition of the sugar, resulting in a false moisture value. If the product to be assayed has a high concentration of volatiles, consider using a correction factor to compensate for the loss, which again will calculate as moisture. Although correction factors are non-analytical, it may be the only route to go."
Additionally, heat is not conducted well in a vacuum, so the pans that hold the samples must be placed directly on the metal shelves to conduct heat. Also, drying time is a function of the total moisture present, nature of the food, surface area per unit weight of sample, whether sand is used as a dispersant, and the relative concentration of sugars and other substances capable of retaining moisture or decomposing. Given these variables, the drying interval (e.g., 3 hr ±15 min with a vacuum oven) is determined experimentally to provide reproducible results.
"No practical method ever removes 100% of the moisture," Bradley emphasizes. "We strive for steady-state conditions within a reasonable time interval and with a minimum of decomposition."
The firm is an authorized dealer for Sartorius (www.sartorius.com), A&D Weighing (www.andweighing.com), and Adamlab (www.adamequipment.com) moisture balances. A moisture balance is a type of moisture analyzer that uses infrared drying principles and records directly in percent moisture.
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"Sartorius offers a wide selection of infrared heat sources, such as a metal tube, halogen lamp, COR quartz glass heater, and ceramic heating element," Shane says.
"What moisture analyzers have in common is that, these days, many people shop the Internet for the best price and personnel knowledgeable about the product and technical expertise don’t seem to be in huge demand when people call us. Instead, it’s all about price, price, price."
What good is any piece of instrumentation if it can’t do the job it was purchased for? Shane wonders. "There are many different moisture balances available and each model is targeted to a specific market," he says. "No moisture balance can cover every single moisture determination application."
There are a number of different moisture balances available for purchase online and probably many more to come to the marketplace, Shane predicts. "Also, there are many Web sites advertising moisture balances but, typically, these companies aren’t authorized dealers. As a result, technical support is lacking in our industry. An astute purveyor of moisture analysis equipment should make certain customers know what they need, not just sell them a product."
Before buying any moisture analyzer online, it’s best to call the vendors and see if they are authorized dealers and if they know anything about the product advertised on their Web site, Shane advises.
"Before you purchase any infrared oven online, ask the vendor to answer these questions, and if he can’t you shouldn’t consider him a viable supplier," Shane emphasizes.
• What is the purpose of the temperature calibration kit? Answer: All instruments should be calibrated to a traceable standard, especially when working within regulated industries such as the food and pharmaceutical industries. Moisture analyzers are no different. Moisture analyzers should be calibrated for both the accuracy of reading the weight and temperature of the food sample.
• What are the benefits of halogen, ceramic, and quartz as heat sources? Answer: Each of these different heating elements will emit the infrared energy at a different wavelength. Knowledge of the sample color, texture, and moisture content can help determine which type of heating element will work best for that sample.
• What do you do with a filter pad? Answer: Filter pads are used for several reasons, including to evenly disperse liquid or pasty samples and to prevent the burning of dark-colored samples.
by Linda L. Leake, Contributing Editor,
Food Safety Consultant,