Moisture Content and Water Activity

Moisture Content

• Refers to the amount of water in a sample.
• Can be expressed as:
  • Percentage of mass
  • Other types of ratios

Water Activity

• Represents the amount of unbound water present in a sample.
• Unbound Water: Part of the water not chemically bound to molecules, allowing it to move freely.
• Important for understanding:
  • Microbial growth
  • Chemical stability
  • Physical stability

Differences Between Moisture Content and Water Activity

• Moisture Content: Total water present, both bound and unbound.
• Water Activity: Only considers unbound water that can exert vapor pressure.

Water Activity Scale

• Scale ranges from 0 to 1.
  • Pure water = 1
• Defined as the ratio:
  • Vapor pressure of water in sample / Vapor pressure of pure water at the same temperature.

Implications of Water Activity

• Microbial Growth:
  • Lower water activity (<0.5) = Low likelihood of microbial growth.
• Example:
  • Majority of food samples with a water activity of 0.5 have low risk of microbial spoilage.