BASIC FOODS 1: Water

Water Properties

• Free water: lightly entrapped but easily pressed, easily separated from food, measures water activity, example: fruits and vegetables, baked bread, then toast (evaporates water)
• Bound water: incorporated into the chemical structure of other nutrients, not easily removed and resistant to freezing or drying, not available to act as a medium for dissolving salts, practically unavailable for microbiological activities

Measuring Heat Energy (Calories)

• Calories: measurement of heat energy, 1 calorie = the amount of energy required to raise 1 gram of water 1°C (measured between 14.5°C and 15.5°C at normal atmospheric pressure)
• Melting: to melt 1 gram of ice requires 80 calories (heat is absorbed)
• Freezing: this process will release 80 calories per gram (heat is released/lost)

Specific Heat

• Water's high specific heat makes it unique among compounds on earth
• Hydrogen bonding between water molecules means it takes more energy to heat water than any other substance
• Example: same temperature, oil (burning hot) vs. water (lukewarm)
• High water content enables animals to withstand hot and cold temperatures

Freezing Point

• Temperature at which a liquid changes to a solid
• When temperature drops to freezing (32°F/0°C at normal atmospheric temperature), ice can form
• Heat of solidification occurs when at least 80 calories (0.08kcal) of heat are lost per gram of water
• Water expands and becomes less dense when completely frozen (ice floats)
• Frozen foods rupture the cells of plants and cells, diminishing texture quality

Melting Point

• Temperature at which a solid changes to a liquid
• Returning the same calories (80 calories) of heat to a gram of ice will cause it to reach its melting point and turn back into water
• While the ice absorbs the 80 calories, there is no increase in temperature (latent heat)

Boiling Point

• Boiling point: vapor pressure = atmospheric pressure
• Water requires 540 calories (0.54 kcal) of energy per gram to boil and vaporize - heat is absorbed
• Boiling point is reached when the pressure produced by steam equals the pressure of the atmosphere pushing down the earth
• Reaches 212°F (100°C) at sea level

Dispersion Types

• Colloidal dispersion: particles not settling out of suspension, example: gravy, jellies
• Suspension: will eventually settle out, mechanical agitation is necessary, example: cornstarch mixed in water
• Emulsion: liquid dispersed in another liquid, example: milk, butter, mayonnaise
• Foam: gas dispersed in a liquid, example: whipped cream, meringue

Dispersion Destabilization

• Solutions are stable, but unstable colloidal and coarse dispersions can separate out
• Emulsions – can either cream or coalesce
• Suspensions undergo either sedimentation or flocculation

Water Activity

• Measure of the amount of available free water in foods, ranges from 0 to 1
• Below 0.85, do not support bacterial growth
• High in water activity supports bacterial growth, example: milk, meat, vegetables, and fruits
• Dehydration preserves food by reducing water activity

Osmosis and Osmotic Pressure

• Osmosis: movement of a solvent through a semi-permeable membrane to the side with the higher solute concentration, equalizing the solute concentration on both sides of the membrane
• Osmotic pressure: the pressure or pull that develops when two solutions of different solute concentration are on either side of a permeable membrane
• Salting used as preserving foods – it draws water out of food and to itself