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) Definition For an ideal solution: n1 p ERH n1 + n2 p0 100 n1 = moles of solvent (water) n2 = moles of solute p = vapor pressure of solution p0 = vapor pressure of solvent ERH = equilibrium relative humidity aw = = =

Water Activity of Selected Foods Food Product aw

Moisture Sorption Isotherm Type I: tightly bound H2O (monolayer) Type II: hard to remove H2O (H-bonding) Type III: loosely bound H2O (available) aw Moisture content

Instruments to Measure Water Activity

Experimental Determination of aw

Hysteresis Hysteresis: loss of H20 binding sites

Effect of Temperature on the Sorption Isotherm

Moisture Equilibration Between Components (Water Migration) Consider products with more than one component Oreo cookie, Twinkies, Pizza with the works

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 environment Xerophilic- likes a dry environment Osmophilic- likes high osmotic pressure