Water in Food Preparation Lecture Power Point Slides Part 1

Water

• Essential to life, food preparation, and safety.

Water Activity

• Water Activity (Aw): measure of the availability of water in a food product.

• Difference between moisture content and water activity.

• Reference video: Water Activity Explanation

Water Content of Foods

• Many foods contain more water than any other nutrient.

• Beverages typically > 75% water.

• Most fruits and vegetables are also > 75% water.

Nature of Water

• Polarity: Water is unique due to uneven distribution of + and - charges.

• Structure:

  • H+ – O- – H+

  • Forms 4 hydrogen bonds.

  • Dipolar characteristic.

Molecular Attraction

• Water molecules exhibit special attraction to each other due to hydrogen bonding.

• High boiling point for small size due to molecular attraction.

Bound vs. Free Water

• Different functions and availability:

  • Bound Water:

    • Tightly held by complex molecules (e.g., carbohydrates, fats, proteins).

    • Low mobility, resistant to freezing, boiling.

    • Unavailable for microbial growth.

  • Free Water:

    • Largest amount present in food.

    • Easily separated from food.

Heat Capacity of Water

• In liquid state, water has a large capacity to absorb heat.

• Standard for measuring heat capacities of other substances (specific heat of 1 calorie).

• Represents calories needed to increase the temperature of 1 gram of substance by 1°C.

Other Specific Heats for Comparison

• Olive Oil: 0.471

• Aluminum: 0.214

• Cast Iron: 0.119

• Uses of double boilers and water baths to manage heating.

Determining kcals in Food

• Requires a thermometer, wire for ignition, insulation, and a chamber for the food with air space and water.

Vapor Pressure

• Water evaporates when heated; condenses when cooled.

• Relationship between evaporation and condensation in closed systems.

Vapor Pressure in Closed Systems

• Equilibrium is reached when evaporation equals condensation.

• Vapor pressure relates to boiling points in different conditions.

Boiling Point Considerations

• Vapor pressure is pressure above the surface of a liquid.

• Vapor pressure must equal atmospheric pressure for boiling.

• Lower atmospheric pressure lowers boiling point; higher pressure raises boiling point.

Effects on Cooking

• Reduced atmospheric pressure (vacuum or higher altitude) decreases boiling point and cooking times.

• Increased pressure (pressure cooker or lower altitudes) raises boiling point.

Factors Affecting Vapor Pressure

• Temperature: Increased temperature raises vapor pressure.

• Concentration of solution: Higher concentration (e.g., sugar) decreases vapor pressure and increases boiling point.

Boiling Point in Tucson

• Elevation: Tucson is at 2000’.

• Boiling point decreases by 1°F for every 500’ elevation increase.

Freezing a Liquid

• Freezing occurs when enough heat is removed to create equilibrium between liquid and solid states (0°C; 32°F).

• Water expands on freezing, causing cell rupture.

• Slow freezing leads to larger ice crystals.

Solutes and Freezing

• Adding solutes decreases freezing point (e.g., salt on ice).

• Can lower the temperature by about 20°C with enough salt.

Osmosis

• Defined as water passing through semi-permeable membranes from lower solute concentration to higher concentration.

• Water follows solutes like salt and sugar.

Osmotic Pressure

• Force that draws water from lower to higher concentrations.

• Higher levels of solutes lead to greater osmotic pressure and corresponding lower water activity.

• Low water activity is significant for food preservation.

Functions of Water in Food

• Heat Transfer: Moderates effects of heat.

• Moist-Heat Cooking Methods: Utilizes water for cooking.

Dispersions

• Mixtures of particles (dispersed phase) and a medium (continuous phase).

• Each phase may be a liquid, solid, or gas.

Phases of Dispersions

• Dispersed Phase: Particles suspended in a medium.

• Continuous Phase: Material holding the dispersed particles.

Types of Dispersions by Particle Size

1. Solutions (smallest particles)

2. Colloidal Dispersions

3. Coarse Dispersions (largest particles)

Solutions

• Water as the universal solvent; particles dissolve completely.

• Heating increases solubility in solutions.

Crystallization in Solutions

• Evaporation can lead to supersaturated solutions, resulting in crystallization (e.g., in candy making).

Colloids

• Unstable systems of small particles suspended in a dispersing medium.

• Examples include emulsions (liquid in liquid) and gels (liquid in solid).

Food Constituents in Colloidal State

• Important to note many food constituents are found as colloids instead of true solutions.

• Examples of colloids: emulsions like milk and foams like whipped cream.

Colloidal Chemistry

• Surface chemistry due to large molecules in colloids attracting and holding other substances.

• Adsorption: Ability to hold molecules on the surface, promoting stability.

Suspensions

• Unstable mixtures with large dispersed particles (e.g., starch in water).

• Gravity affects the separation of particles.

Water Hardness

• Two types:

  • Temporary: Ca, Mg, & Fe bicarbonates precipitate upon boiling.

  • Permanent: These do not precipitate and affect soap's cleaning ability.

Effects of Hard Water on Cooking

• Calcium salts affect rehydration and cooking of legumes.

• Hard water may be alkaline and alter the color of vegetables; it can cloud tea due to precipitation of polyphenols.

Softening Hard Water

• Employing ion exchange to swap out Ca and Mg ions for Na ions increases sodium content.

• Water-softening agents can precipitate Ca and Mg salts, enabling softer water for cooking.