WATER (Vapor pressure (• The water does not boil, yet individual water…
• The water does not boil, yet individual water molecules gain enough energy to escape from the liquid as vapor
• The vapor (gaseous) molecules that have escaped from the liquid state exert a pressure on the surface of the liquid known as the vapor pressure.
• When the vapor pressure is high…-the liquid evaporates (is vaporized) easily - many molecules exist in the vapor state -the boiling point is low
Effects of temperature on water vapor
• The vapor pressure increases with increasing temperature
• At higher temperatures,
• - molecules have more energy;
-easier for them to overcome the forces holding them within the liquid and to vaporize
-Increase molecules in the vapor state
Effects of solute
• The vapor pressure decreases with addition of solutes, such as salt or sugars
• Less water molecules available for vaporization, fewer molecules in the vapor state, vapor pressure will be lower
• Attraction to the solute - Limit water evaporation
• In a sugar solution (e.g. ice cream), the initial freezing point of the solution is lower than 0° C due to these dissolved sugars
• As ice crystallization begins and water freezes out in its pure form, the concentration of the remaining solution of sugar is increased due to water removal and hence the freezing point is further lowered.
• The effect of sweeteners on freezing characteristics of ice cream mixes is demonstrated by the plot shown on the ice cream freezing curve.
Effects of atmospheric pressure
• If the external pressure is decreased, for example, at high altitude (low atmospheric pressure), water boils at a lower temperature and so food may require a longer time to cook.
• Deposition:a process where a gas changes phase and
turns directly into solid without passing through the liquid
- Example: snow that formed in clouds. Water vapor changes directly to ice without first becoming a liquid
- If a puddle of water is left on the ground for a day or two, it will dry up because the liquid evaporates
• When ice is subjected to vacuum and then heated, it is converted into vapor without going through the liquid phase.
• This phenomenon is known as sublimation, and is the basis for the food processing method known as freeze drying
1.1 Moisture in foods
• The presence of water in food is described as the moisture content or water activity (aw) of the food
• Moisture refers to the amount of water present in food
• Water in food which is not bound to food molecules can support the growth of bacteria, yeasts and molds (fungi). The term water activity (aw) refers to this unbound water.
Production & Preparation
- Universal Solvent
- Heat transfer medium
- Product / Ingredients
- Reactant in chemical rxn
• - The temperature at which a given material changes from a liquid to a gas
• The boiling point of a liquid is the temperature at which its vapor pressure is equal to the pressure of the gas above it
• Anything that lowers the vapor pressure increases the boiling point (e.g. addition of solutes)
• a change in the physical properties of a substance when the temperature or pressure are changed.
• Temperature decrease – less random motion, the molecules tend to line up in an orderly manner, held together in a rigid crystal
• If the molecules have more thermal energy, they shake around and break free of their neighbors.
Soft water vs Hard water
• Hard water is water that has a high content of dissolved minerals (salts of calcium and magnesium carbonate and/or calcium and magnesium sulfate)
• The degree of hardness becomes greater as the amount of dissolved minerals increases
• Soft water is treated water in which the only ion is sodium
Problems associated with hard water:
• Will cause precipitation which evidenced by a powdery metallic precipitate that builds up if a pan is used for this purpose frequently
• Causes cloudiness in tea & coffee because of interactions between these salts and the tannins in the beverages
• Softening of dried legumes is impeded (delay) significantly when hard water is used for rehydrating and cooking them
• Hair washed in hard water may feel sticky and look dull
• Certain minerals in the hard water interact with the soap or detergent, which keep it from interacting with soils, stains and dirty dishes
How to treat hard water
• The water may be softened by boiling (soluble bicarbonates precipitate when boiled and leave deposits or scales) and insoluble carbonates may be removed from the water
• Permanently hard water cannot be softened by boiling as it contains either calcium or magnesium sulfates (CaSO4 or MgSO4) as well as other salts that are not precipitated by boiling.
• Permanent hard water may only be softened by the use of chemical softeners
The two most common ways to treat hard water:
• Ion Exchange Water Softener: a technique that involves using minerals to perform a chemical reaction with the minerals in the water & alter them enough that they will not precipitate
• Polyphosphate feeders are dissolves into the water and coats the iron, calcium and magnesium in it, making it impossible for these agents to precipitate out of the water
• The low viscosity of water is important in its role as the solvent for life's chemical reactions & for blood circulation