Pawkit
Theory
39
Water activity
Water activity (a
w
) is a measurement of the energy
status of the water in a system. It indicates how
tightly water is bound, structurally or chemically,
within a substance. Water activity is the relative
humidity of air in equilibrium with a sample in a
sealed measurement chamber. The concept of water
activity is of particular importance in determining
product quality and safety. Water activity influences
color, odor, flavor, texture and shelf-life of many
products. Most importantly, it predicts product
safety and stability with respect to microbial growth,
chemical and biochemical reaction rates, and physi-
cal properties.
Therefore, water activity is a far better indicator of
perishability than water content. Figure 1 shows
how the relative activity of microorganisms, lipids
and enzymes relate to water activity. While other
factors, such as nutrient availability and tempera-
ture, can affect the relationships, water activity is
the best single measure of how water affects these
processes.
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