Lecture 2: Water Freeze Drying and Water Activity

Vocabulary flashcards covering key terms from the notes on water, freeze-drying processes, and water activity concepts.

Sublimation

Phase change where a solid (ice) turns directly into a gas, a key process in freeze-drying.

Critical Point

Pressure–temperature combination where liquid water and water vapor cease to exist as distinct phases.

Fast Freeze

Rapid freezing that forms small, regular ice crystals and helps preserve cell structure and quality.

Density of Ice vs. Water

Ice is less dense than liquid water; freezing reduces density and causes ice to float.

Hydrogen Bonding

Intermolecular attraction that makes water unique and contributes to high specific heat and structural cohesion.

Specific Heat (Water)

Water’s high heat capacity due to hydrogen bonding, allowing it to absorb/releases large amounts of heat with little temperature change.

Enthalpy of Vaporization

Energy required to convert liquid water to vapor (approximately 10.5 kcal/mol in the notes).

Dielectric Constant

Property measuring a material’s ability to store electrical energy in an electric field; relates to solvent power.

Solubility

Ability of a substance to dissolve in a solvent.

Denaturing of Proteins

Temperature increase can denature proteins and alter hydrophilicity (as noted in the lecture).

Syneresis

Expulsion of interconnected liquid (e.g., whey) from a gel network.

Colligative Properties

Properties of solutions that depend on solute concentration, not on solute identity (e.g., boiling point elevation, freezing point depression).

Boiling Point Elevation

Raising the boiling point of a solvent by adding a solute to a solution.

Freezing Point Depression

Lowering of the freezing point of a solvent when a solute is dissolved.

Surface Tension

Cohesive forces at the surface of a liquid that give it a measurable surface tension (water has high surface tension).

Bound Water

Water chemically bound to materials or absorbed to surfaces; not freely available as a solvent.

Free Water

Water weakly bound or not bound to the matrix; can be eliminated and is available as a solvent.

Water Activity (aw)

Measure of the free energy status of water in a system; relates to microbial, chemical, and physical properties and to water content.

Moisture Content

Amount of water in a material; an extensive property that does not depend on temperature.

Equilibrium Relative Humidity (ERH)

The relative humidity at which a material in a given state is in equilibrium with its environment.

Raoult’s/Baoult’s Law

Partial pressure of a solvent is proportional to its mole fraction in a solution; relates to water activity concepts.

Hygrometers (Types)

Instruments to measure humidity: resistive electrolytic, capacitance, and dew point hygrometers.

Bottled Water Regulation

Bottled water is regulated by the FDA; labeling and minerals/vitamins can be subject to regulatory standards (EPA involvement mentioned).

Water Content vs. Water Activity

Water Content is the total amount of water (extensive); Water Activity measures the availability of free water and its energy status (not solely a function of temperature).

pH

Scale measuring hydrogen ion concentration: pH = -log10[H+]; notes mention pH can change with temperature.