Food water content determining methods

Isopiestic determination is one of the most commonly used methods for measuring food aw. In this method a sample of known mass is stored in a closed chamber and allowed to reach equilibrium with an atmosphere of known ERH (or equilibrate with a standard of known aw). In the first protocol (see Basic Protocol), a standard salt solution, for which aw is well established, is used to control this atmosphere. The aw of the sample is then determined by equilibration with the resulting atmosphere. In the second protocol (see Alternate Protocol), the isopiestic determination of aw is accomplished by equilibration of the sample with a reference material, for which the relationship between water content and aw is known. The condition of equilibrium is determined by reweighing the sample at intervals until constant mass is reached. The moisture content of the sample is then determined either directly or by calculation from the reference material s original moisture content and change in mass. Unsaturated salt solutions of known ERH can also be used to equilibrate the samples however, this requires estimation of the ERH of the jars at the end of the equilibration by measuring the exact concentration of the salt solution, which may be tedious. 

This method involves preparation of a standard curve by equilibration of a specific amount of dry standard material in duplicate or triplicate over different saturated salt solutions. The standard curve is a plot of aw versus water content of the standard material. The standard must be stable during reuse of the material, and the chamber used should be exactly the same as will be used later. For the measurement, the size of each standard should be in a controlled narrow range, (e.g., —1.6 0.1 g). Once the standard curve is made, dry aliquots of the same mass of standard are equilibrated over a large quantity of the food material (-10 to 20 g). The moisture content of the standard material is then measured (e.g., by mass gain) and aw is estimated from the standard curve (Vos and Labuza, 1974). This method avoids preparation or storing of saturated salts for each determination moreover, use of a standard shortens the equilibration, thus requiring less time for measurement, and less abuse of temperature for sample as well as standard. 

Moisture. The water content of lecithin products is usually less than 1.0%. As a consequence of lecithin s essentially moisture-free state, lecithin products have very low water activity and do not adversely contribute to the microbiological profile of most food systems. Most lecithin products are preserved well in storage. Higher moisture levels usually indicate a greater potential for spoilage or chemical degradation. Moisture is determined by AOCS Official Method Ja 2b-87 (77). A less accurate moisture level can also be determined by azeotropic toluene distillation (AOCS Official Method Ja 2-46) (77). One cannot determine lecithin moisture by vacuum oven methods. These methods are known to degrade lecithin products and yield false moisture levels. 

Isengard, H.-D., Kling, R., and Reh, C.T. Proposal of a new reference method to determine the water content of dried dairy products, Food Chem., 2006, (in press). 

The assessment of the stable isotope ratio of foods is a convenient method that was developed for the determination of fraud in the food products industry. Determination of D and 0 content is a method utilized in the determination of watering of fruit juices and wines [10], Stable isotope ratios of 0/ 0, and H/ H are useful in the discrimination of other adulteration schemes... 

The analysis of water content in foods is one of the most frequently and routinely performed analyses of foods. Methods used in the analysis of water are outlined in Figure 1.1. The great variability in water contents of food materials requires that the results of other analyses of foods are to be reported on a dry weight basis. This is one of the main reasons for the routine determination of water content in the food laboratory. While there is ongoing research to achieve improvements in the methods of water analysis [19,20], recent interest is also forwarded to the economic aspects of water determination in foodstuffs [21],... 

FIGURE 1.1 Methods for determining water content of foods. (Based on Isengard, H.-D., Food Control, 12, 395, 2001.)... 

Freezing, drying, and increasing the content of solutes in a food system are practical methods of decreasing and increasing shelf life. The curve can be used to determine the critical value for water activity ( critical a ") equivalent to the bound or monolayer water activity value. Plots... 

Water content is defined as the amount of water lost by a food when it reaches the true equilibrium against zero water vapor pressure. From this definition arises a quantification method of water content called the absolute reference method, a determination that is only possible in specialized laboratories. The food industry only uses practical reference methods, calibrated against the absolute reference method. Moreover, water content standards are not available because a product s water content depends on the humidity and temperature of the environment, which makes participating in intercomparison exercises between laboratories essential for detecting possible experimental errors. 

The determination of the water content is one of the most frequently used methods in laboratories for foods around the world. One method is the drying method using drying ovens... 

The headspace technique is very flexible and can be applied to the most widely differing sample qualities. Liquid or solid sample matrices are generally used, but gaseous samples can also be analysed readily and precisely using this method. Even the water content in food or pharmaceutical products can be determined by headspace GC (Kolb, 1993). Both qualitative and, in particular, quantitative determinations are carried out coupled to GC-MS systems. 

Direct or indirect methods may be used to determine moisture in dehydrated foods. Indirect methods must be calibrated in terms of direct methods—the most common of which are the oven, distillation, and Fischer methods. Accuracy of the direct methods is difficult to evaluate except by comparison with a chosen reference method. Several reference methods are reviewed, but none can be given an unqualified recommendation as most practical and suitable for all foods. An indirect measure of moisture is the equilibrium vapor pressure of water, which can be measured easily and accurately. Arguments are presented to show that vapor pressure may be a better index of the stability of dehydrated foods than the moisture content, which has been frequently used for this purpose. 

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