water activity

Physical States of Water

• Vapor

• Liquid: interaction with food components– Hydrogen bonding– Covalent bonding– Salt linkages– Van der waal interactions

• Ice

Crystallization of Water

• Nucleation: formation of crystalline nuclei– Low temperature and fast rate are important for

the size and number

• Crystal growth– Small with low temperature– Large at high temperatures

• Recrystallization– Problems with freeze-thaw cycles (e.g., frozen

foods)

Problems of Crystal Growth

• Large crystals are more stable than small ones

• Small crystals melt and water used for growth of large crystals

• Tissues can rupture with crystal growth– Physical changes– Enzyme activation– Chemical destruction

• Storage is important

Decompartmentalization

Freezing Property Changes

• Changes in pH (due to salt precipitation)• Increase in ionic strength• Increase in viscosity• Increase in osmotic pressure• Decrease in vapor pressure• Decrease in freezing point• Increase in surface potential• Change in oxidation-reduction potential

Changes in pH Upon Freezing(e.g., phosphate buffer)

• Monobasic: MH2PO4 (acidic)

– When M = Na+, this form precipitates first and the pH increases

• Dibasic: M2HPO4 (basic)

– When M = K+, this form precipitates first and the pH decreases

Changes in pH, brought about by freezing can affect biochemical and chemical reactions, and microbial growth

Water Activity (aw)

• The amount of water available (unbound) for chemical and biochemical reactions, and for microbial growth to occur.

• Knowledge of water activity allows us to make predictions about food quality.

• Processing – freezing, dehydration, concentration, salting,

sugaring)

Water Activity

• Determines direction of moisture transfer• Most reaction rates increase with increasing

water activity• Most rates correlate better with water activity

than moisture content• Moisture sorption isotherms are useful

Water Activity (aw) DefinitionFor an ideal solution:

n1 p ERH n1 + n2 p0 100

n1 = moles of solvent (water)n2 = moles of solute p = vapor pressure of solutionp0 = vapor pressure of solventERH = equilibrium relative humidity

aw = = =

Water Activity of Selected Foods

Food Product aw

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Moisture Sorption Isotherm

Type I: tightly bound H2O (monolayer)

Type II: hard to remove H2O (H-bonding)

Type III: loosely bound H2O (available)

Instruments to Measure Water Activity

Experimental Determination of aw

Hysteresis: loss of H20 binding sites

Hysteresis

Effect of Temperature on the Sorption Isotherm

Consider products with more than one component• Oreo cookie, Twinkies, Pizza with the works

Moisture Equilibration Between Components(Water Migration)

Factors Influencing Water Activity

• Solute interactions

• Capillary suction forces

• Surface force interactions

Control of aw in Foods

• Understand moisture sorption isotherms• Equilibrate with atmosphere of lower or higher

equilibrium relative humidity (ERH)• Formulation approaches

– Add solute(s) (e.g., humectants)– Anticaking agents (e.g., calcium silicate)– Remove or add water

• Packaging approaches– Select to minimize water permeation– Resealable packages

• Handling instructions– Change temperature

Solutes and Humectants

• Sodium chloride

• Sugars (e.g., sucrose, glucose, fructose)

• Sorbitol

• Glycerol

• Propylene glycol

Halophilic- likes a salty environmentXerophilic- likes a dry environmentOsmophilic- likes high osmotic pressure