Measuring Water

Determining the relationship between relative humidity, moisture content, and water activity.

On Measuring Water

Water covers over 70% of the Earth's surface, but it doesn't just sit around. It goes through a process called the water cycle where it evaporates from bodies of water like lakes, rivers, and oceans, then rises into the atmosphere where it condenses and eventually falls back down as precipitation like rain. The amount of water that evaporates depends on how much moisture the air can hold, which is influenced by temperature.

When water evaporates, it happens because of something called the saturation pressure. This pressure is also known as the vapor pressure of water or saturation vapor pressure. It basically tells us how much water vapor can be in the air as a gas, and this amount is affected by temperature. So, the hotter it is, the more water vapor can exist as a gas in the air.

What is Relative Humidity?

Relative humidity refers to the amount of water vapor pressure that is in the air compared to the maximum amount it can hold (which is called saturation vapor pressure). If the relative humidity goes over 100%, it means there is too much water in the air and it will start to condense, forming water droplets.

What is Water Activity?

Water activity is a way to measure how much "free" water is in a substance. It's not just any water, but the water that isn't chemically bound to anything. The official definition says that water activity is the water's vapor pressure in a substance when it's in a perfect balance with the air around it and the vapor pressure of distilled water in the same conditions. If the water activity is 0.70, that means the vapor pressure is 70% of what it would be in pure water. Water activity increases when things get hotter.

To understand water activity, imagine a glass of water and a sponge. If you place the sponge in the glass of water, it will absorb some water and become saturated. However, if you take the sponge out of the water and leave it in the open air, it will start to lose moisture and eventually reach an equilibrium with the surrounding air. The amount of moisture the sponge loses depends on the humidity of the air. If the air is very dry, the sponge will lose more moisture and become dry faster. On the other hand, if the air is humid, the sponge will lose less moisture and remain damp for longer. This is because the humidity of the air affects the amount of moisture the sponge can hold onto.

The formula for water activity is aw = p/ps, where p is the water vapor pressure above the product's surface and ps is the water vapor pressure above pure water's surface. Both of these are measured at the same temperature as the product. Water activity is dependent on temperature because water interacts differently with things depending on how hot or cold they are. The value of water activity is always between 0 and 1.

What Is Moisture Content?

Moisture content refers to the amount of water present in a substance, typically expressed as a percentage of the total weight of the substance. This is a common measurement used in many industries, including agriculture, food processing, construction, and manufacturing. Moisture content can affect the properties and quality of a substance, such as its strength, durability, texture, and shelf life.

How Are These Things Related?

In general, as the relative humidity of the surrounding environment increases, so does the moisture content of a material, because the material absorbs more moisture from the air. However, the water activity may or may not change, depending on the temperature and the water binding properties of the material.

Moisture content and water activity are directly related. As the moisture content of a material increases, its water activity also increases. However, the relationship is not linear and depends on the water binding properties of the material.

How About an Example?

Let's take dried flower as an example. Relative humidity (RH) is the measure of the amount of moisture in the air relative to the maximum amount of moisture the air can hold at a given temperature. For example, if the air can hold 10 grams of moisture at a certain temperature and it currently holds 5 grams, then the RH is 50%.

Moisture content (MC) is the measure of the amount of water within the flower itself. For example, if a bag of flowers weighs 100 grams and 30 grams of that is water, then the moisture content is 30%.

Water activity (aw) is the measure of the amount of water available for microbial growth in the flower. It is a dimensionless number between 0 and 1, where 1 represents pure water. For example, if the water activity of the flower is 0.85, then it means that there is 85% of the water available for microbial growth compared to pure water.

Now, let's see how these are related. The moisture content of flower affects the water activity, which in turn affects the growth of microorganisms. If the moisture content of the flower is high, the water activity will also be high, making it a suitable environment for microbial growth. On the other hand, if the moisture content of the flower is low, the water activity will also be low, making it difficult for microorganisms to grow.

Relative humidity is related to both moisture content and water activity. When the relative humidity is high, the moisture content of the flower will increase due to the water in the air being absorbed by the flower. This, in turn, will increase the water activity of the flower, making it a more favorable environment for microbial growth. When the relative humidity is low, the moisture content of the flower will decrease, resulting in a lower water activity and less favorable conditions for microbial growth.