Water Determination in Food

Water determination in foods is a deceptively simple theme. Defining the quantity to be measured identifies the inherent complexity. Three separate types of measure may be appropriate: a gravimetric measure; a measure related to vapor pressure; and a measure of classes of water. The ubiquitous nature of water in our environment provides additional complexity in the challenge of preventing transfer of water between sample and environment. The earliest measures of amount of water were all gravimetric, determining the weight fraction of water in the food. These methods range from simple direct weighing using a difference technique, to more complex methods where the amount of water is determined by spectroscopic methods or by chemical assay. A wide range of methods have been developed and are in daily use because gravimetric water content is important for formulation and for labeling purposes. This measure, however, is of little value for the prediction of the stability of a food, even though water plays a critical role in determining the stability characteristics of foods. For a measure of the amount of water relevant to stability concerns, the vapor pressure or its related thermodynamic parameters is more relevant. Determination of vapor pressure uses methods developed from thermodynamic roots, although if the product is not at true equilibrium the measured quantity is not a thermodynamic descriptor of the product but it is still a measure of a product characteristic. Rather than describing necessarily the thermodynamic state of the water in the food, these measures all determine the vapor pressure of the atmosphere in contact with the food. Whether this is the equilibrium vapor pressure, and therefore the thermodynamic description, has to be established on a case by case basis. The vapor pressure per se is an important parameter. Vapor pressure can be measured directly in a manometric system or can be inferred from the dew point temperature (at which the relative humidity just reaches 100%). It can also be inferred from the electrical characteristics of sensors that have electrical conductance or capacitance that is a function of the relative humidity of their immediate environment, once sensor and immediate environment are equilibrated. A special set of applications measure both the gravimetric water content and the vapor pressure that corresponds to this water content. The result is a sorption isotherm. In this case, as an alternative to measuring the vapor pressure, a stable, known vapor pressure can be established and the product allowed to equilibrate to constant weight. The establishment of an atmosphere of known vapor pressure/relative humidity is the essential component of isopiestic techniques. On the assumption of equilibrium, analysis of sorption isotherms according to various models can lead to categorizing various classes of water and identifying the amount of water in each class. Spectroscopic measurements can also lead, more directly, to the identification of the amount of water in different classes. Many different labels, such as bound water, unfreezable water and restricted water, are attached to the categories of water, depending upon the method used to distinguish between the classes. For direct and indirect estimation of vapor pressures it is critically important that the temperature of the sample be stable and accurately known, and also that the temperature in the region where the measurement that will translate into vapor pressure is to be made is accurately known. This is because the driver for many properties that depend upon water content is the relative vapor pressure, and the vapor pressure of pure water has a steep dependence upon temperature. It is also important that sufficient time be taken to allow for the establishment of a uniform and steady vapor pressure throughout the vapor spaces of the system, otherwise moisture transfer will occur, driven by the pressure gradients, changing the characteristics of the materials.