Water can exist in a product either in bound or free state.
Free water is water adsorbed from the environment and is not chemically bound to the product. It is easily removed by evaporation or controlled heating.
Water of hydration is the amount of water bound chemically to the product. It has significant effect on the molecule structure and appearance of the material. An example is CuSO4. 5 H2O which has 5 molecules of water bound to each molecule of water.
Moisture measurements assume significance in analysis of pharmaceuticals and food products mainly because of:
- Product purity – water content should be within specified limits
- Shelf life – it is related to the exposure of material to different climatic conditions with variable temperature and humidity
- Texture – texture quality changes with absorption of water by the products
- Weight- bulk consignment is affected by moisture content of adsorbed water
Bound water can be estimated by gravimetric studies such as loss on drying and Karl Fischer titration. On the other hand water of hydration can be estimated, in terms of number of molecules bound to each molecule of product, by techniques such as DSC which measures heat flow into or from the sample under heating, cooling or isothermal conditions. Different water molecules are given off in the 2:2:1 sequence for CuSO4. 5H2O.
Water activity is the amount of water that is available for microbial growth in the product. It is defined as the ratio of P and P0 where P is the vapour pressure of water in the substance and P0 the vapour pressure of pure water at same temperature. Water activities are based on a scale of 0 to 1.0 where pure water has a water activity of 1.0. High water activity promotes growth of microorganisms. Bacteria usually require at least a water activity of 0.91 and fungi at least 0.7. Foods with lower water activities have a longer shelf life. When water activity is high foods tend to decay faster thereby posing several health problems to the consumer.
Some key ingredients such as sugar, salt and pectin can be introduced which make water unavailable for growth of microorganisms. In such products there is higher moisture content but water activity is low due to limited availability of water for growth of microorganisms. Jams and jellies are good examples of such products. These contain around 50 to 60% water but the water activity usually lies between 0.7 – 0.75. Sugar and pectin in such products binds water and makes it unavailable for growth of microorganisms.
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