But how do you measure water? It seems relatively trivial at first sight (water is water, it is H2O, right?), but measuring it in food ingredients may be complex. There are many methods for measuring water, but the way we use them can vary depending if we want to measure water activity or water content. Due to a variety of technical reasons, it is not easy to measure water activity in a reproducible and robust manner and test results depend even on the kind of equipment used.
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Explore This IssueApril/May 2014
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Water content can be measured by a simple method called loss on drying, which is performed as simply as it sounds. An amount of the given sample is weighed, put in an oven at a certain temperature (typically slightly above the boiling point of water) for several hours, weighed again, and the process is repeated until two subsequent weightings do not indicate further weight loss. The assumption is that all evaporated material is water. This method is not specific because any weight loss is counted as water, even if it is due to flavors, fragrances, and other volatile substances that evaporate. In this specific method, all weight losses are counted as water. The challenge with certain heat-sensitive ingredients, such as milk powder, is that a chemical reaction takes place during the heating process and volatile substances are liberated, of which one is actually water, but not water that has been freely available in the sample, and, therefore, available for microbiological growth. It is water that became available after a chemical reaction took place. These types of heat-sensitive samples may actually continue to lose weight as the process continues and the measurement needs to be terminated after a fixed amount of time that is set by convention (e.g. after two hours in the oven).
Another method is a chemical reaction based on a Karl Fischer titration, which determines all water content that is present in a sample. While this method is capable of measuring all water in certain ingredients, the method requires complete dissolution of the sample, and that is not always possible to achieve. Besides, just as with the loss on drying method, there is a question of availability of water for microbiological growth, which may not be adequately addressed with this method (e.g. certain salt crystals have crystallization water that is not freely available, but part of the crystal structure, which would not be liberated using the loss on drying method, but it would be liberated and measured as water in the Karl Fischer titration).
So which water do we want to measure?
All these methods are valid and widely used, but they may return different results if applied to the same sample. So, it is important to agree upfront on what is the most scientifically-sound way to measure this one residue. Which method is considered the most appropriate often involves a discussion of regulatory requirements, scientific considerations, ease of use, cost, speed of analysis, and availability of instruments.
The examples above illustrate how complex it is to choose even one type of test to help establish food ingredient integrity. When complex ingredients come into play, especially those derived from biological sources, a multicomponent system needs to be considered. A food ingredient monograph many times suffices to establish the integrity of a particular ingredient and the FCC contains more than 1,200 of them. However, different approaches may be necessary for ingredients that are closer to raw agricultural products, such as pomegranate juice and other fruit juice concentrates.
Some of the components of pomegranate juice include sugars, polyphenols, acids, minerals, and water that present natural variability that is influenced by the species of pomegranate as well as environmental conditions (region where the fruit is grown, climate, harvest, and processing conditions, etc.).