[FOOD SAFETY & QUALITY] by Neil H. Mermelstein
Instruments for Moisture & Water Activity Determination
Metrohm analyst adds a sample to the recently introduced 885 Compact Oven Sample Changer for KF titration for moisture determination. The sample is heated in an oven, and a carrier gas transfers the released water to the titration cell, where it is then determined by Karl Fischer (KF) titration. Because only the water enters the KF cell and the sample itself does not come into contact with the KF reagent, unwanted side reactions and matrix effects are eliminated.
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ow much moisture is present in a product (the moisture content) and how much of it is available to microorganisms to use for growth (the
water activity) are important measurements in the food industry. J. Peter Clark’s October 2009 column and my November 2009 column provided in-depth coverage of these issues. This column contains upto-date information about the instruments that suppliers offer for measurement of moisture content and water activity.
Determining Moisture Content The moisture content of a sample is generally determined by weight loss upon drying or by Karl Fischer (KF) titration. In the loss-on-drying method, a sample is placed on a balance within an oven, and the weight is measured as drying proceeds until the sample reaches equilibrium. The loss in weight represents the moisture content of the sample. The
process time can be decreased by use of infrared, halogen, or microwave heating. In the KF method, the amount of water in a sample is determined directly by titration with KF reagent. In coulometric titration, the sample is added to the reagent, and electrolytic oxidation causes the production of iodine. The amount of iodine produced is proportional to the electricity used, which in turn is proportional to the amount of water in the sample. In volumetric titration, a sample is added to a moisture-free solvent and titrated with the reagent, whose titer is known. The titration endpoint, determined by a constant-current polarization voltage method, represents the moisture content of the sample. Among the companies providing instruments for moisture measurement are the following: • Arizona Instrument LLC (www.azic.com). According to Shari Moore, Marketing Manager, the company offers the Computrac line of moisture/ solids/ash analyzers, introduced to the food industry in 1981. Significant improvements have been made since then for increased throughput, decreased test times, and greater accuracy and repeatability. The Computrac Max analyzers use rapid losson-drying technology to provide real-time moisture measurements. The metal heating element used with the Max analyzers eliminates hot and cold spots in samples, and the temperature controller allows for more precision and control throughout the testing process. The most recent introduction was the Max 4000XL in January 2009. The Computrac Vapor Pro analyzers use relative humidity (RH) sensor-based technology. They use no reagents and are simple to operate in laboratory and production settings. They are not only more environmentally friendly, Moore said, but also safer, more durable, and easier to use than KF titration. The results correlate with the coulometric KF titration method in precision and accuracy. • Decagon Devices Inc. (www.decagon.com). The company introduced its AquaLab moisture analyzer in 1987 and its AquaLab 4TE Duo dew point moisture analyzer as its replacement in 2007. Brady
Carter, Head Research Scientist, said that most moisture analyzers use chemicals or high temperatures to determine moisture content, but the Duo uses the dew point method, in which an infrared beam focused on a tiny mirror determines the precise dew point temperature of the sample. That dew point temperature is then translated into water activity, which is then translated into moisture content, based on a product-specific calibration model. Most products with less than 40% moisture can be analyzed using the dew point method. In this range, moisture changes affect water activity readings more strongly, yielding high precision and repeatability. Products with moisture levels above 40% usually have such uniformly high water activity values that other moisture analyzers are a better fit, he said. Precision varies with the product tested, but for many products this method has up to 10 times the precision of a typical moisture meter. Volatiles are a challenge for the dew point method, Carter added. He would like to see a way to make a primary measurement on samples that contain volatiles without resorting to a capacitance (RH) sensor, which causes a drop in accuracy and speed. Also, in measuring moisture content, everyone is looking for a primary method that is specific to water and works quickly for all samples without destroying any sample material in the process. • Metrohm USA Inc. (www. metrohmusa.com). George Porter III, Product Manager, Titration, said that the company offers coulometric KF titrators for low levels of moisture (a few ppm to 0.5%), including the 831 Coulometer, 756 Coulometer, 851 Coulometric Titrando, and 852
Coulometric Titrando. Metrohm also offers volumetric KF titrators for higher levels, including the 870 Titrino Plus, the 890 Titrando, and the 901 Titrando. The first KF titrators were launched in the 1960s; the 831 and 756 in the late 1990s; the 870 in 2007; the 890 in 2008; and the 851, 852, and 901 in 2009—each superseding its predecessor. Among recent improvements is use of the Dosino™ reagent-dosing system, which removes the drawbacks experienced with the classical automatic burette, Porter said. It sits on top of the reagent bottle and doses reagent from the top down, preventing leaks and air bubbles and increasing accuracy and precision. And the newly introduced 885 Compact Oven Sample Changer automates KF titration. The instruments are also designed with safety in mind, he added. They feature no-touch reagent exchange and overfill and overflow prevention. They are also designed to work with the newest generations of green KF reagents that are free from substances such as pyridine and methanol. And the sample preparation is geared to complex matrices, such as food, with a variety of solutions for automatic analysis and preparation, such as homogenization, oven-assisted water determination by KF, and automatic weighing of samples. The challenge with moisture measurement, he said, will continue to be educating customers on the need for exact moisture determination, not just estimation, and the fact that KF is not the messy technique with badsmelling chemicals that people remember from the past. • Ohaus Corp. (www.ohaus.com). Silvestro Rocchio, Market Manager,
Shimadzu’s MOC63u moisture analyzer, introduced to the U.S. food industry in April 2011, uses the loss-on-drying method to determine moisture content and features a halogen lamp as a heat source.
said that the company introduced mechanical moisture balances more than 40 years ago and now offers three halogen moisture analyzers— the MB25, MB35, and MB45 —and one infrared moisture analyzer, the MB23. The MB45 was introduced in 1999, and the most recent introductions, the MB25 and MB23, in 2009. The analyzers feature high-precision electronic weighing technology and an integrated heat source to provide fast, clean, and inexpensive loss-ondrying results, Rocchio said. The halogen heating technology begins to dry the sample in seconds, and the reflective gold plating creates a uniform distribution of heat, improving the accuracy. Although no new moisture analyzers are planned for the near future, Rocchio said, the company is continuously monitoring the food industry for enhancements, efficiencies, and solutions it can provide. As for challenges ahead, he said that the 10.11 • www.ift.org
[FOOD SAFETY & QUALITY]
Instruments for Moisture & Water Activity Determination continued... Novasina’s LabSwift portable instrument uses the principle of equilibrium relative humidity to measure water activity. It features a temperaturestabilized measurement chamber and a sensing element that uses an electrolyte whose resistivity changes with changes in air humidity.
increasing need for product innovations include improved accuracy, shorter test times, and enhanced operator efficiency. Future products must be able to quickly adapt to new customer requirements, regulations, and unique industry needs while providing high-quality, competitive, and cost-effective solutions. • Shimadzu Scientific Instruments (www.ssi.shimadzu. com). Kevin McLaughlin, Senior Marketing Coordinator, said that the company introduced its MOC63u moisture analyzer in Japan in February 2011 and the United States in March, first marketing it to the food industry in April. The analyzer utilizes
Static vs Dynamic Moisture Isotherms
he relationship between moisture content and water activity of a product is described by its moisture sorption isotherms— curves relating them to each other at specific temperatures. There are several methods for determining moisture isotherms. In the traditional static method, a sample of food is exposed to various constant-humidity atmospheres produced by means of saturated salt solutions in a series of sealed chambers, all maintained at the same temperature. After equilibrium is reached, the moisture content is generally determined by weight loss or gain. In the dynamic vapor sorption method, a sample is subjected to select conditions of humidity and temperature, and the weight loss or gain is measured. Mass flow controllers, one for dry air and another for air saturated with water, generate desired ratios of wet to dry air and can provide nearly any relative humidity from 0 to 98%. Decagon Devices’ Vapor Sorption Analyzer, or VSA (originally called AquaSorp), generates dynamic dew point isotherms by exposing a sample first to wet air, then to a desiccant. By pg
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measuring the sample weight and the water activity dynamically as the sample changes, the VSA can generate high-resolution isotherms. The company is developing a second-generation VSA, scheduled to be released in September 2011, that incorporates both dynamic and static isotherms in one instrument. In addition to dynamic dew point isotherms, it offers users the option of performing traditional vapor sorption analysis, in which water activity is held constant and the instrument waits for the sample to come to equilibrium with that water activity. This will give users the ability to find crucial water activity values for their products using the dynamic function and then perform kinetic moisture sorption studies at those values using the traditional static function. In May 2011, IFT presented the webcast “Dynamic Isotherms vs Static Isotherms— Which Is Better?” The presenters were Brady Carter of Decagon Devices and Shelly Schmidt of the University of Illinois. The webcast was recorded and is available on IFT’s website (www.ift.org).
the loss-on-drying method. This method requires virtually no reagents or pretreatment, the instruments are not expensive, and the measuring range extends from 0.01% to 100%. The drying chamber is located above a balance, and a halogen lamp is used as the heat source to dry the sample. The sample mass is continuously measured as it is heated from room temperature until the water evaporates. The fully dry mass is used to calculate the moisture content. The halogen heating method offers high power and can reach a high temperature in a short time, McLaughlin said. The disadvantage is that the temperature of the heating element increases at high power, which rapidly applies high temperatures to the sample and causes decomposition of part of the sample surface. Consequently, the optimal temperature and temperature program can differ depending on the sample color and decomposition temperature. To overcome these problems, the company incorporated several temperature control settings into the instrument—automatic ending, timed ending, rapid drying, slow drying, and step drying—allowing users to choose the best one for their application and sample. For example, he said, the slow-drying mode gently heats samples that might solidify at the surface or samples that reduce under high temperature whereas the step-drying mode allows step-bystep changes in drying conditions, useful when measuring samples that contain a large amount of water. Since the instrument is so new, the company is primarily working on ways to improve measurement and analysis by running applications and developing countermeasures to moisture content discrepancies so that users will have a better understanding of how to prepare samples and how to adjust conditions to obtain the most accurate moisture content. McLaughlin said that finding a way to minimize trial-and-error procedures and thereby make sample
preparation and analysis easier remains a major challenge to address. • TEWS Elektronik GmbH (www. tews-elektronik.com). According to Dietrich Strobel, Sales Engineer, for moisture determination the company offers the laboratory instruments MW 4300, MW 4310, and MW 1150; portable meters MW 1100 and 1100S; and online systems. The company introduced its first generation of instruments for fast measurement of food samples 22 years ago and its first online systems 13 years ago. All the instruments are based on microwave resonance measurement. Using a low-energy microwave field that penetrates the product sample, the instruments analyze the complex permittivity of a sample, a property highly influenced by the water content. The resulting moisture values are independent of sample quantity and density. Therefore, Strobel said, the method is more accurate than other electromagnetic methods and doesn’t require sample preparation such as weighing or grinding. Scanning of microwave resonance curves and calculation of moisture readings is fast enough to provide instant readings, and online monitoring of moisture is possible with several hundred measurements per second. The company is manufacturing the third generation of microwave measuring systems. New types of sensors have been developed to meet the requirements of specific applications. Late last year, the company introduced a new sensor for automated measurement of moisture and bulk density in extruded pet food pellets, ensuring that a given mass of product fits properly into a package. Also, the company is working on a new microwave sensor for real-time monitoring of fluid-bed drying processes. Like other industries, Strobel said, the food industry has more and more tightened requirements regarding quality control and automation. This means continuous measurement rather than occasional measurement
TEWS Elektronik’s moisture instruments, based on microwave resonance measurement, use a low-energy microwave field that penetrates a product sample to analyze the permittivity of the sample, a property highly influenced by water content.
of samples, 100% control rather than spot tests. He added that the company is receiving more inquiries about automated, fast, nondestructive, incore moisture measurement in solid food products such as whole candy, whole cookies, or whole pretzel sticks. Such products are a challenge because of their shape, handling, or non-homogeneous composition or even because of substances that simply defy the use of specific measuring methods. • Others. Among other companies offering moisture analyzers are Mettler Toledo Inc. (http://us.mt. com), MoistTech (www.MoistTech. com), Perten Instruments (www. perten.com), and Sartorius Mechatronics Corp. (www.sartoriusmechatronics.com).
Measuring Water Activity Water activity (aW ) is a measure of how much of the water present in a product is available to microorganisms to use for growth. Microorganisms will not grow below a certain water activity level. While other factors influence whether an
organism will grow in a product, water activity is often the most important. It is measured by placing the sample into a closed chamber and equilibrating the liquid-phase water in the sample with the vapor-phase water in the headspace and measuring the relative humidity of the headspace. Since water activity is the ratio of the water vapor pressure of the sample to the water vapor pressure of pure water under the same conditions, it is also equal to the equilibrium relative humidity (ERH), expressed as a percentage, divided by 100. Two types of instruments are used to measure water activity. Capacitance sensors consist of two charged plates separated by a hygroscopic polymer membrane. As the membrane adsorbs water, its ability to hold a charge increases. The change in capacitance is approximately proportional to the water activity as determined by a sensor calibrated against known saturated salt standards. The sensor gives a signal relative to the ERH and thus the water activity. »» 10.11 • www.ift.org
[FOOD SAFETY & QUALITY]
Instruments for Moisture & Water Activity Determination continued...
Rotronics’ HygroPalm HP-23Aw Set includes a portable water activity meter with interchangeable probes for measuring water activity, humidity, and temperature. The instrument features a quick mode for measurement of water activity in 4–6 min.
The chilled-mirror dew point system consists of a dew point sensor, an infrared thermometer, and a fan. A sample is placed in a cup, which is then sealed against the sensor block. As air is passed over a chilled surface whose temperature is gradually reduced, the temperature at which the water vapor begins to condense on the surface is the dew point temperature, which is directly related to the vapor pressure of the air. The dew point sensor measures the dew point temperature of the air, and the infrared thermometer measures the sample temperature. The relative humidity of the headspace is calculated as the ratio of dew point temperature saturation vapor pressure to saturation vapor pressure at the sample temperature. When the water activity of the sample and the relative humidity of the air are in equilibrium, measurement of the headspace humidity gives the water activity of the sample. Among the companies providing instruments for determining water activity are the following: • Decagon Devices Inc. (www. decagon.com). Brady Carter said that pg
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the AquaLab Series 4TE benchtop water activity meter measures water activity in 5 min or less with an accuracy of ±0.003 aW. The measurements are repeatable, giving the same result regardless of the user or the location. Users put a 7.5 mL product sample in a disposable cup, place the cup in the water activity meter, seal the sample chamber lid over the sample, and wait for vapor equilibrium. An infrared beam focused on a tiny mirror determines the precise dew point temperature of the sample, which is translated into water activity. The instrument displays both water activity and moisture content. Among the improvements that have been made, he said, is a complete body redesign. The sample drawer has been replaced by a flip-top sample chamber that makes the sensor easy to see and keep clean. The volatiles sensor has been completely integrated so users can measure either normal or volatile samples without changing the sensor head. The instrument also has a larger data screen and comfortable navigation buttons. And internal data management options have been added. Carter said that the company is always looking for a faster measurement that provides more accuracy. Water activity instruments will always need enough time to bring the sample to equilibrium. Since it is difficult to make the measurements any faster, future improvements may include the ability to read multiple samples at once. Volatiles are also a challenge for the dew point method, he said. He would like to see a way to make a primary measurement on samples that contain volatiles without resorting to a capacitance sensor, which causes a drop in accuracy and speed. Also, in measuring moisture content, he said, everyone is looking for a primary method that is specific to water and works quickly for all samples without destroying any sample material. • Novasina AG (www.novasina. com). Markus Bernasconi, Sales
Engineer & Product Manager, said that the company offers its LabMaster high-end instrument with temperature-stabilized measurement chamber and its LabSwift portable instrument, first marketed to the food industry several years ago. Both work on the basic principle of ERH determination. The instruments feature a temperaturestabilized measurement chamber and a sensing element that uses Novalyte, the company’s electrolyte whose resistivity changes with changes in air humidity. Reusable Sal-T salt tablets are used for easy calibration. Bernasconi said that one of the challenges ahead is that water activity measurement is still influenced by volatiles. The company uses a protective filter which has to be replaced after a certain length of time. A chemically inert sensing element would be better, he said. Another challenge is to get customers to understand the difference between water activity and moisture. Water activity is still an underestimated parameter. • Rotronic Instrument Corp. (www.rotronic-usa.com). The company offers water activity instruments in the HygroLab series as well as a portable instrument. David Love, Vice-President of Sales, said that the most recent introductions were the HygroLab 3 Set, first marketed to the food industry in early 2000, and the HygroPalm HP-23Aw Set, introduced in early 2010. Both operate on the principle of ERH. He added that since then, the accuracy has been improved to ±0.008 aW. The company is developing a new benchtop unit using the same technology as the HP23Aw. The HP23-Aw Set measures aW, humidity, and temperature; features two interchangeable probes; and offers a quick mode for measurement of water activity in 4–6 min. FT Neil H. Mermelstein, a Fellow of IFT, is Editor Emeritus of Food Technology • [email protected]