Food Quality & Safety Farm to Fork Safety Tue, 21 Feb 2017 13:01:28 +0000 en-US hourly 1 Determining Moisture Content Fri, 03 Feb 2017 11:30:27 +0000 A comparison of different thermogravimetric methods that follow the principle of differential weighing

The post Determining Moisture Content appeared first on Food Quality & Safety.



Moisture content influences the taste, texture, weight, appearance, and shelf life of foodstuffs. Even a slight deviation from a defined standard can adversely impact the physical properties of a food material. For example, substances which are too dry could affect the consistency of the end product. Conversely, excess moisture may cause food material to agglomerate or become trapped in the piping systems during production. Also, the rate of microbial growth increases with total water content, possibly resulting in spoiled batches that need to be disposed of. However, water is also an inexpensive ingredient adding to the weight of the final product. Hence, obtaining an optimal analytical value for moisture is of great economic importance to a food manufacturer. For these reasons, food analysts engage in the delicate balancing of moisture and total solids to ensure consistent product quality, safety, and profitability.

Legal Requirements

International and national standards define the permitted thresholds for moisture content in commercially sold products. Regulatory bodies such as the BRC (British Retail Consortium), IFS (International Featured Standards), or GFSI (Global Food Safety Initiative) heavily influence the production, processing, and sale of foods. For food manufacturers, this translates into increased workload around quality assurance and the development of efficient and cost-effective solutions. According to the stated legal requirements, methods of analysis and procedures must be clearly described and tested. Many food producers themselves have strict criteria for measurement accuracy, reliability, and traceability to ensure the consistent quality of their products. These standard operating procedures encompass the entire measurement process, including sample volume, number of required measurements, maximum tolerable deviation, and procedures for correcting errors.

Water Properties in Food

As mentioned in chapter 6 of Food Analysis by S. Suzanne Nielsen, official methods and procedures for moisture analysis are important since the method used to determine moisture may lead to varying results for moisture content, depending on the form of the water present in a food. In the simplest scenario, water retains its properties by existing “freely,” i.e. it is only surrounded by other water molecules. Free water (also known as bulk water) can be adsorbed on surface particles, held in narrow capillaries, or stored in the pore systems deep within the food material. For instance, dried fruit or meats have complex cellular structures where water is bound by adsorption to the surface or transported deep within the cells by capillary action. Adsorbed water can also become physically bound to other elements present in the food material such as proteins, or exist as chemically bound water (e.g. certain salts such as Na2SO4·10H2O). In a bound state with other molecules, water most often evaporates at a higher temperature compared to free water molecules. Consequently, physically or chemically bound water takes on varying physicochemical properties, making it very challenging for the food analyst to accurately measure the moisture content.

Technologies for Moisture Analysis

A summary of technologies used for moisture determination are listed below.

  • Thermogravimetric analysis (oven drying, halogen/IR drying, microwave drying, etc.)
  • Chemical analysis (Karl Fischer titration, calcium carbide testing)
  • Spectroscopic analysis (IR spectroscopy, microwave spectroscopy, proton nuclear magnetic resonance spectroscopy)
  • Other (e.g. gas chromatography, density determination, refractometry, etc.)

This article focuses on thermogravimetric analysis (TGA). Moisture content is derived from the loss of product weight during drying by measuring the change in mass of a sample while being heated at a controlled rate until no more change in weight is observed.

Balance and Drying Oven

The drying oven, commonly used for commercial purposes, is the established reference method for loss on drying (LoD) by TGA. In this procedure, a sample is weighed and subsequently heated to allow for the release of moisture. Following this, the sample is cooled in the desiccator before reweighing. Moisture content is calculated by the difference in wet and dry weight. In this process, measuring accuracy and the resolution of the balance are extremely important. Careful consideration must also be given to maintain identical conditions, where temperature and duration are vital for generating precise and reproducible results.

Table 1 (click to enlarge)

Advantages. Two important advantages gained from using a drying oven are sample throughput and flexibility in regards to sample volumes/sizes. This method also produces very precise results while being cost effective (see Table 1).

Disadvantages. This method requires extended heating periods and cooling phases, meaning it usually takes hours to produce results. Procedures are laborious and tedious, involving many manual steps. Therefore, the potential for error is high, since weighing is performed manually and in separate stages during the drying process as opposed to a moisture analyzer with an integrated precision balance which allows for the continuous and automatic recording of results. Typical pitfalls include mixing up samples, manual transcription errors or the miscalculation of weighing results. The risk of committing such errors increases with large sample volumes (see Table 1).

Moisture Analyzer

Moisture results can be obtained more rapidly using a moisture analyzer. The measurement principle does not differ from that of the thermogravimetric method. The main distinction lies with the type of heat source used: In the oven, samples are heated by convection while a moisture analyzer heats samples via the absorption of infrared energy.

Advantages. The most important advantage is the rapid measurement time, thanks to the efficient heat source. Results can be obtained within 2–10 minutes. Samples are heated quickly and evenly, and obtained measurements show good repeatability. Handling is also straightforward and the risk of error is reduced (see Table 1).

Disadvantages. All thermogravimetric methods, including the moisture analyzer, carry the risk of decomposing constituents or the loss of volatile components during heating. This results in a further decrease in weight, which is not explained by the release of water. Finally, samples can only be measured one at a time and the automation of measurements is not feasible (see Table 1).

Halogen Technology

The technology of halogen drying can measure moisture content in virtually any substance. Halogen technology uses a halogen heating device in combination with an integrated precision balance for the measurement and recording of sample weight before, during, and after the release of moisture. Thanks to its innovative heating technology, halogen moisture analyzers (HMAs) are capable of producing fast and precise measurements.

Furthermore, automated moisture determination eliminates transcription and calculation errors. Most HMAs offer a number of predefined methods, which can be stored and easily accessed via the display menu. Some manufacturers also allow users to set individual user rights to ensure that quality criteria are met. The calculated results are stored in the instruments or can be printed out or transferred to PC via USB or other interfaces.

Reference Methods

Table 2 (click to enlarge)

Reference methods are of much use to food manufacturers who must comply with legal requirements for the maximum or minimum amount of water present in diverse foods. Up until now, moisture content determination in a drying oven is the established reference method. Values determined by other methods must, therefore, always be referenced against the LoD method in the drying oven.

METTLER TOLEDO, as an example, has a library of validated measurement methods for over 100 food products saving users time in developing methods for different food specimens. If a substance is not included in the library, it is possible to adapt a method from a comparable food sample. For instance, Table 2 compares procedures and results for moisture analysis in ground hazelnut using a drying oven and METTLER TOLEDO HMA. Based on six measurements, the mean value of the moisture result was calculated. The results reveal that a moisture analyzer produces identical results to a drying oven. In addition, the standard deviation for both methods is comparable and very small.


Moisture content is a critical indicator of food quality, safety, and shelf life, thus moisture analysis serves an important quality control function in various stages of the food production chain, from raw material testing in the laboratory to incoming goods inspection. Several analytical procedures are available to measure moisture content in diverse food samples. Selecting the correct procedure for a particular sample or application is pertinent to the food industry’s success since the accuracy of moisture measurements are highly dependent on the analytical method used.

Compared to the traditional drying oven, faster determination of moisture content via LoD can be achieved using alternative methods. For example, an HMA is straightforward to operate and produces reliable results in just 5-15 minutes, compared to 2-4 hours when using a drying oven. In addition, the automation of weighing measurements and calculations allows for fully compliant and reliable results.

Dr. Appoldt is head of strategic product group moisture at METTLER TOLEDO. Reach her at Raihani is global lab marketing at METTLER TOLEDO. Reach her at


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Complete Moisture Analysis for Wheat Mon, 15 Feb 2016 16:31:11 +0000 Though moisture content has purpose as a standard of identity, water activity can be more effective in evaluating moisture in grain and flour to provide a consistently stable product

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Fotolia_WheatDuring the summer of 2015, Alltech tested the North American wheat crop and found an average of 3.2 mycotoxins per sample. Mold and the mycotoxin it produces represent a serious economic threat to wheat growers and flour producers. Molds are spread throughout the crop as grain mixes in storage. Often, pockets of high moisture develop, allowing mold to grow. Moisture continues to be an issue as grain is tempered, processed into flour, and stored.

Moisture content is monitored at various points in this process. However, susceptibility to mold and microbial growth is better correlated to water activity than to moisture content because fungi cannot grow (and mycotoxin cannot be produced) in grain and flour below a 0.70 water activity. Moisture content is not directly related to mold and microbial growth rates, and the currently suggested moisture content levels for flour and farina correspond with water activity levels right at the cutoff point for mold growth. Any uncertainty in the moisture content measurement can allow contamination of the product.

Water activity is a measure of microbial susceptibility accepted by the USDA and a key component in many Hazard Analysis and Critical Control Point plans. To prevent mold and microbial contamination, it behooves the flour industry to consider including a water activity specification to ensure microbial safety.

How Does Contamination Occur?

According to Cereal Foods World’s study “The Case for Water Activity as a Specification for Wheat Tempering and Flour Production”, it appears that through trial and error, those who have set current recommended moisture levels for grains and flour have fortunately, if unknowingly, pinpointed the right water activity level to maximize stability. However, as current moisture content specs result in water activities very close to the microbial growth limit, problems can result from even small uncertainties in the moisture content measurement.

In fact, because flour is hygroscopic, it can easily take on moisture that pushes its water activity above the microbial growth limit. If flour or farina is exposed to high humidity (more than 70 percent), its water activity will quickly become unacceptable. Soft flour will reach unsafe water activity levels the fastest, but all products will have reached unacceptable water activities and moisture levels in only 24 hours. (This change in water activity and moisture content only applies to product exposed to ambient conditions. Moisture movement through a mass of bulk stored product once the surface has come to equilibrium and the subsequent changes in water activity and moisture content are not part of this discussion.)

Using the Right Measurement for Every Step

Weighing. Wheat products, both flour and grain, are sold on a weight basis. When you buy grain you don’t want to pay for water. Thus, there are restrictions about what moisture contents are allowed, and these restrictions have become a useful standard of identity for grain and flour to make sure that purchasers are buying what they expect. Moisture content is commonly required for any flour specification sheet, with 13.5 percent ideal for soft wheat and 14 percent ideal for hard wheat.

Tempering. Before it is milled, grain must be tempered with moisture to soften the endosperm and toughen the bran, which facilitates grinding of the grain and separation of the bran and germ from white flour. Liquid water is added to raise the moisture level of the grain from 12 percent to 17 percent. The grain then equilibrates for 16 to 24 hours before it is milled. However, standard practices do not include a test to determine if moisture equilibrium has been achieved, but instead depend on pre-set soaking times. The 16 to 24 hour hold time for tempered wheat is typically not long enough for mold to grow. Longer hold times, however, are likely to cause tempered wheat with water activities higher than 0.70 to experience mold growth. Since mold, microbial spoilage, and lower rates of rancidity are better correlated with lower water activities than moisture contents, it would make more sense to focus on optimizing the water activity level and then confirm that the moisture content is acceptable rather than to rely on just a moisture content specification.


Shelf life. To have value as an ingredient, flour must maintain its quality during storage. The degradative reactions that could potentially end its shelf life include microbial spoilage, caking and clumping, nutritional loss, color loss, and rancidity. The two extrinsic influences that will most significantly impact its rate of shelf life loss are temperature and moisture. Wheat and flour producers currently use moisture content as an indication of whether their product is going to be in danger of molding. However, water activity is not only a well-established predictor of mold and microbial growth, it is also more closely correlated with caking and clumping, nutritional loss, color loss, and rancidity. Water activity measurements are more repeatable than moisture content analysis, and can be verified using saturated or unsaturated salt solutions. In addition, water activity helps form the basis for the FDA’s definition of potentially hazardous foods. Consequently, including water activity in flour specifications is more critical to ensuring quality and shelf life than moisture content.

Moisture Content Methods for Wheat and Flour

The moisture content is a measure of the quantity of water in a product reported on either a wet or dry basis. In theory, moisture content determination is simply a comparison of the amount of water in a product to the weight of everything else in the product. In practice, it is an extremely complex process to actually obtain reliable results.

There are many ways to analyze how much water is in a product. The Association of Official Agricultural Chemists, or AOAC, lists 35 different methods for measuring moisture content. These are classified as either direct or indirect measurement methods. Direct moisture content methods either force water out of a sample at elevated temperatures and track the weight change or involve a chemical reaction with water and titration. The most common direct moisture methods include air-oven drying and Karl Fischer titration.

Indirect methods do not remove the water from the sample but instead try to predict the moisture content based on either testing under accelerated heat conditions or by correlating another measured property to the moisture content. These secondary methods require calibration to a primary or direct method. Examples of indirect measurement methods include halogen or infrared-based moisture balances, near infrared absorption, and dielectric capacitance.

The advantage of direct methods is that they are a primary measurement typically with superior precision, but they are also labor intensive and have long analysis times. Indirect methods are typically much faster than direct methods, but are not primary measurements based on accepted standards and consequently suffer in reliability. Due to the absence of a scientific definition of “dry,” all moisture methods lack a standard that would allow the comparison of methods or determination of accuracy. These issues can cause problems when determining shelf life because current wheat moisture content specs are so close to the mold-growth limit.

Loss-on-drying methods are also affected by the ambient conditions under which samples are analyzed. It is difficult to obtain reproducible results with all loss-on-drying methods, including both the moisture balance and convection oven, because relative humidity in the lab environment affects the moisture content measurement. See Table 1.

The ideal moisture method would combine high throughput testing with a primary measurement, eliminate variability due to changing ambient conditions, and provide a scientific standard for dry. Table 1 shows how much more accurate moisture content measurement can be when using this type of method.

Which Water Activity Specs Should Be Used?

Mold. A suggested water activity specification for flour and farina would be 0.62-0.68 aw. As indicated in “The Case for Water Activity as a Specification for Wheat Tempering and Flour Production” study, this water activity range corresponds with ideal moisture levels for hard and soft flour. In addition, this water activity range would prevent mold growth and minimize the rate of rancidity.

Tempering. Based on the connection between the water activity of whole grain and current tempering moisture levels, it would be feasible to temper to a constant water activity rather than to a moisture level. This water activity level would achieve tempering conditions similar to those currently being used, but with more consistency since water activity is more precise than moisture content and can be easily monitored with instrumentation to determine when tempering is complete.

Finally, grain could be tempered to 75 percent relative humidity using saturated sodium chloride and vapor equilibration techniques. Grain would achieve a uniform water activity of 0.75 without the issues caused by adding liquid water. The equilibration time should be similar to current tempering hold times, so mold growth would not have time to begin. Finally, having a consistent starting water activity for tempered wheat will consistently result in flour or farina water activities less than 0.70 aw, preventing mold growth and reducing the rate of rancidity.

Combining Moisture Content and Water Activity

The flour industry has used moisture content levels to control flour and farina quality for many years. Although this procedure has avoided major disasters, it unknowingly placed the specification close to dangerous limits. The addition of a water activity specification would check this possible problem by clearly identifying where the flour or farina is with respect to the growth limits, all with a test that takes less than five minutes.

Further, use of water activity will improve other processes like tempering, where grain can be brought to appropriate relative humidity values that will not only allow excellent processing qualities, but avoid mold growth and other issues. In practice, wheat producers should temper their grain to a specific water activity, then after it has been milled, re-assess the water activity of the flour to make sure it will be safe. A moisture content measurement should then be taken so growers can sell their flour on a weight basis.

Dr. Carter is senior research scientist at Decagon Devices. Reach him at

Funding to Improve Low-Moisture Foods

In 2015, a team from the University of Nebraska-Lincoln (UNL) received a portion of a $5 million USDA food safety grant to enhance low-moisture food safety by improving development and implementation of pasteurization technologies. UNL will receive $943,617 over five years.

According to UNL, low-moisture foods, such as nuts, spices, and peanut butter, have been considered at low risk for foodborne illness because they are consumed in a dry state. While microbial growth isn’t possible, the bacteria can survive and stay on the food product for a long time. Foodborne pathogens, such as Salmonella, can cause illness even at very low levels.

UNL research will focus on improving and developing technologies that can kill the bacteria while not affecting food quality. Research includes looking at radio frequency and extrusion processing of the low-moisture products to inactivate the bacteria. Traditional heating heats the product from outside to inside, whereas technology using electromagnetic waves, such as radio frequency waves, volumetrically heats the product. With these methods, according to UNL, the impact on food quality is minimal because the food can be more evenly heated throughout the product to inactivate bacteria.

Research findings will be disseminated to local food processors in Nebraska and across the country.

In addition to UNL, principal co-investigators on the project are Michigan State University, Washington State University, Illinois Institute of Technology, and North Carolina State University.—FQ&S


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