MH

Lecture 2: Water Freeze Drying and Water Activity

Water Freeze Drying and Key Concepts

  • Freeze-drying purpose and mechanism
    • Process involves sublimation of ice under controlled, low-pressure conditions to remove water from foods without phase-changing to liquid water during drying.
    • Food preservation heavily depends on the amount of water present; the goal is normally the removal of water to inhibit microbial growth and enzymatic reactions.
    • Critical Point: a specific combination of pressure and temperature where the distinction between liquid water and gas ceases to exist.
  • Water structure and bonding (overview from transcript)
    • Water is highly reactive and carries nutrients; understanding its bonding helps explain stability and phase behavior.
    • Molecular structure notes referenced in transcript:
    • O–H bond energy (approximate): E_{O-H} \approx 110\ \text{kcal/mol}
    • O–H bond distance: r_{O-H} \approx 0.96\ \text{Å}
    • Bond angle: transcript lists ~1890 (likely a transcription error; typical value is ~104.5° for H–O–H).{{NOTE: transcript reports 1890°, actual value ~104.5°.}}
  • Solid ice vs liquid water (structural implications)
    • Ice has different O–O distances and bond angles compared to liquid water; the transcript notes the distance is closer in solid ice and that this structural difference reduces density relative to liquid water when ice forms.
  • Fast Freeze and its role in quality preservation
    • Fast Freeze creates small, regular ice crystals.
    • This preservation helps maintain cell structure and overall quality during subsequent processing.
    • Density changes: transcript indicates density decreases by about ≈18% below 0°C.
  • pH and temperature relationships
    • pH is defined as: \mathrm{pH} = -\log([H^+])
    • As temperature increases, pH is reported to decrease (the transcript states this, and implies changes in bond angles or interactions that affect acidity/basicity).
  • Bond types and their energy scales (summary from transcript)
    • Strong bonds
    • Ionic: roughly \approx 200\ \text{kcal/mol} to break.
    • Covalent: roughly between 50-150\ \text{kcal/mol} to break.
    • Weak bonds
    • Dipole–Dipole: approximately 1-5\ \text{kcal/mol}.
    • Van der Waals interactions: contribute to holding together proteins, lipids, starch; include induced dipole effects due to quantum fluctuations.
    • Hydrogen bonding
    • Hydrogen bonds give water its special properties; they are weaker than covalent/ionic bonds but enable structural cohesion and interactions with solutes.
    • Contributes to high specific heat capacity of water.
  • Thermodynamic properties of water in solutions
    • Enthalpy of vaporization: \Delta H_{\text{vap}} \approx 10.5\ \text{kcal/mol}
    • Dielectric constant: describes a material’s ability to store electrical energy in an electric field; relates to solubility and solvent polarity.
    • Dipole moment: symbolized as \mu (m).
  • Solubility, protein behavior, and other effects
    • Temperature rise can denature proteins and alter hydrophilicity/hydrophobicity balance.
    • Syneresis: expulsion of liquid (often whey) from a gel network as it contracts or reorganizes.
  • Colligative properties (general concepts from transcript)
    • Boiling Point Elevation: a solution boils at a higher temperature than the pure solvent due to solute presence.
    • Freezing Point Depression: a solution freezes at a lower temperature than the pure solvent.
    • Both effects depend on solute concentration and do not depend on the identity of the solute.
  • Surface tension in water
    • Water exhibits relatively high surface tension due to cohesive hydrogen bonding.

Bound Water vs Free Water

  • Bound Water
    • Chemically bound to other materials or adsorbed to surfaces.
  • Free Water
    • Weakly bound or not bound to the matrix; readily eliminated and available as a solvent for microbial activity during development.
  • Practical implications
    • Bound water contributes to water-holding capacity and stability; free water governs microbial growth potential.

Water Characteristics Recap and Examples

  • Recap of key aspects:
    • H-bonds are central to high specific heat and the overall unique properties of water.
    • Intermolecular bonds include hydrogen bonding; intramolecular bonds also influence water’s properties.
  • Sorbet example (transcript reference)
    • Mango sorbet variations:
    • Mango sorbet with 5% sucrose
    • Mango sorbet with 10% sucrose
  • Colligative properties (reprise)
    • Emphasize that boiling point elevation and freezing point depression depend on solute concentration.
  • Bottled water regulation and labeling
    • Bottled water is regulated by FDA and EPA.
    • Labels typically include information on added minerals and vitamins; regulatory emphasis on safety and labeling accuracy.
  • Water content vs. water activity: core concepts
    • Water content (extensive parameter): amount of water is proportional to the amount of material; does not depend on temperature.
    • Water activity (a_w): measure of the free energy status of water in a system; more closely tied to microbial, chemical, and physical stability than water content alone.
    • Relationship to perishability: higher a_w generally correlates with higher potential for microbial growth and chemical activity.

Water Activity Fundamentals

  • Relationship and definitions
    • Water activity a_w is a measure of the free energy state of water, not just the total water present.
    • a_w correlates with the tendency of water to participate in chemical/biological processes in foods.
  • Raoult’s law (transcript calls it Baoult’s Law)
    • Partial pressure of a solvent in a solution is proportional to the mole fraction of the solvent:
      p{solvent} = x{solvent} \; p_{solvent}^0
    • This underpins how solutes reduce water’s vapor pressure and thus influence a_w.
  • Practical meaning in foods
    • a_w provides a more relevant metric than moisture content for predicting microbial stability, texture, color, and shelf life.
  • Standards and measurement approaches
    • Empirical measurement and standards are used to calibrate a_w against target stability profiles.
    • Common measurement methods include resistive electrochemical hygrometers, capacitance hygrometers, and dew point hygrometers.

Equilibrium Relative Humidity (ERH) and Calculations (Illustrative Examples)

  • ERH definition
    • ERH is the relative humidity at equilibrium with a food’s moisture content.
    • It is expressed as a percentage.
  • Example (transcript-provided, though some values are garbled in the source)
    • Example 1: Flour with 18% moisture and 20% protein content.
    • At 5% moisture, protein level (Dry Basis) ≈ 21.15\%.
    • Wet basis protein level ≈ 22.2\%.
    • Recap: a_w ≈ 0.78; ERH at the stated condition would be reported in the transcript as a related percentage (78%).
    • Lipids example (illustrative, content from transcript is garbled):
    • There are references to moisture levels 5%, 10%, 15% and solids percentages (e.g., 95%, 90%), with calculated lipid fractions given as 4.7% in one case, but the exact interpretation is unclear due to transcription errors.
    • Takeaway: These examples illustrate how moisture content, composition, and ERH/a_w interact, and why precise calculations require careful framing of the basis (dry basis vs wet basis) and temperature.
  • How to reduce water activity (summary from transcript)
    • Increase solute concentration (e.g., salt or sugar) to lower a_w.
    • Remove water from the system or bind it more strongly to reduce free water.
  • Measurement approaches mentioned
    • Resistive electrolytic hygrometers
    • Capacitance hygrometers
    • Dew point hygrometers

Guest Lecture: Water Activity – Weight Basis vs. Water Activity Concepts

  • Why is water expressed on a weight basis?
    • Water Content (extensive parameter): amount of water scales with the amount of material and does not depend on temperature.
  • Water Activity (a_w)
    • Measures the free energy status of water in a system, not just the total amount of water.
    • a_w is a critical predictor of food stability, including microbial inhibition, texture changes, and spoilage risks, beyond what water content alone can predict.
  • Practical implications for food processing and storage
    • Controlling a_w through formulation, packaging, and processing is essential for shelf-life optimization.
    • Understanding the distinction helps in designing drying, curing, and storage strategies that target microbial control and quality retention.

Quick reference: Key equations and definitions cited in the transcript

  • Water structure and bonding (values mentioned in transcript):
    • O–H bond energy: E_{O-H} \approx 110\ \text{kcal/mol}
    • O–H bond distance: r_{O-H} \approx 0.96\ \text{Å}
    • Bond angle: transcript lists 1890° (note: actual ~104.5°; transcript shows likely error)
  • Enthalpy of vaporization: \Delta H_{\text{vap}} \approx 10.5\ \text{kcal/mol}
  • Strong/weak bond energy categories (approximate ranges from transcript)
    • Ionic: ~200\ \text{kcal/mol} to break
    • Covalent: 50-150\ \text{kcal/mol} to break
    • Dipole–Dipole: ~1-5\ \text{kcal/mol}
  • Temperature and pH relation
    • \mathrm{pH} = -\log[H^+]
  • Boiling Point Elevation (colligative property)
    • \Delta Tb = i \cdot Kb \cdot m
  • Freezing Point Depression (colligative property)
    • \Delta Tf = i \cdot Kf \cdot m
  • Raoult’s Law (as referenced in transcript under Baoult’s Law)
    • p{solvent} = x{solvent} \cdot p_{solvent}^0
  • Water activity concept
    • a_w is the measure of free water available for chemical/biological processes; related to microbial stability and product quality.
  • ERH (Equilibrium Relative Humidity)
    • ERH is the relative humidity at which a product is in equilibrium with ambient air for its given moisture content.
  • Key definitions
    • Bound water: chemically bound or adsorbed to matrices
    • Free water: water not bound to the matrix; readily available for reactions and microorganisms

Notes source content includes several transcription artifacts and typographical errors (e.g., bond angle values, some arithmetic in ERH examples). The above notes reproduce the ideas and equations as presented in the transcript, indicating explicitly where numbers or statements appear to be transcription errors. When studying, treat the numbers that seem inconsistent with standard chemistry as items to verify against authoritative sources or lectures.