Is Water Activity and Moisture Content Same?

Summary

This video explains the differences and relationships between water activity and moisture content in foods. Using examples like fresh and dried apricots, honey, cookies, salt, and sugar, it demonstrates that water content and water activity are related but distinct. The video highlights key factors affecting water activity, including water content, solids composition, and solute properties, and explains their measurement methods. Water activity reflects the energy and reactivity of water molecules, while moisture content measures total water present. By understanding these concepts, food scientists can manipulate water and solids to achieve desired stability, shelf life, and quality in various food products.

Takeaways

• 😀 Water activity (aw) and moisture content are related but not the same; water activity measures water's energy state while moisture content measures total water present.
• 😀 Decreasing water content in a food generally decreases its water activity, as seen in fresh vs. dried apricots.
• 😀 Foods with the same water activity can have different moisture contents depending on the type of solids, e.g., honey vs. cookie.
• 😀 Foods with the same moisture content can have different water activities due to differences in solid interactions, e.g., salt vs. sugar.
• 😀 Water activity is influenced by water, solids, and properties of solids such as molecular weight and hydrophilic/hydrophobic characteristics.
• 😀 Moisture content is a compositional measure, calculated as mass of water per total mass (wet basis) or per dry solids (dry basis).
• 😀 Water activity originates from equilibrium thermodynamics and is related to the chemical potential, fugacity, and vapor pressure of water.
• 😀 Water activity has no unit and is measured using standardized methods, often with sensors in equilibrium chambers.
• 😀 Moisture content requires no standard measurement; it is commonly determined by drying the sample until mass stabilizes.
• 😀 By manipulating water content or the properties of solids in a food, it is possible to achieve a desired water activity.
• 😀 Water molecules exist in different energy states: low (less reactive), medium, and high (more reactive), which affects water activity.

Q & A

Are water activity and moisture content the same in foods?

No, they are not the same. Water activity indicates the energy state and availability of water in a food, while moisture content measures the total amount of water present.

How does water content affect water activity?

Generally, decreasing water content decreases water activity. For example, fresh apricots with 80% water have a water activity of 0.93, while dried apricots with 22% water have a water activity of 0.62.

Can two foods have the same water activity but different moisture content?

Yes. For instance, honey (18% moisture) and cookies (5% moisture) both have a water activity of 0.65. The difference arises from the type of solids in the food.

Can two foods have the same moisture content but different water activity?

Yes. Salt and sugar both may have 16% moisture content, but their water activities differ (0.900 for salt and 0.998 for sugar) due to solute properties and interactions.

What are the main factors affecting water activity in foods?

Water activity is influenced by water content, type and characteristics of solids (such as molecular weight, chemical structure, hydrophilic or hydrophobic nature), and ice formation through freezing.

What is the main difference between the driving forces of water activity and moisture content?

Water activity is driven by the chemical potential of water, whereas moisture content has no driving force; it is purely a measure of quantity.

What units are used to express water activity and moisture content?

Water activity is a dimensionless number with no units, while moisture content is expressed as a percentage, either on a wet basis or dry basis.

How is water activity measured commercially?

Water activity is measured using a sensor in a small closed chamber. The sample reaches equilibrium with the air, and the water activity is determined by the relative humidity of the surrounding air.

How is moisture content determined?

Moisture content is measured by drying a food sample in an oven until the mass no longer changes. It is calculated as the mass of water divided by the total mass (wet basis) or by the dry solids (dry basis).

What is the origin of water activity compared to moisture content?

Water activity originates from equilibrium thermodynamics and reflects the energy state of water, while moisture content originates from the compositional amount of water in the food.

How can food scientists manipulate water activity?

Water activity can be controlled by adjusting water content, changing the type or properties of solids (hydrophilic or hydrophobic), or through freezing to form ice.