Chapter 5 - water
What is the heat of vaporization for water at 100 degrees Celsius?
540 calories/gram of water
What is the heat of fusion for water at 0 degrees Celsius?
80 calories/gram of water
Which microorganism requires the highest range of water activity to grow?
Bacteria - .85-1.0
What is free water?
Free flowing and easily separated from food
What is hard water?
Has more calcium and magnesium; leaves deposits
What is soft water?
Has more sodium chloride
What happens at the molecular level as water is heated?
Hydrogen bonds break and reform as molecules move more rapidly; some molecules overcome their attraction and escape as vapor
What is bound water?
In structure of proteins, carbohydrates, and fat; not easily removed; resistant to freezing and drying; not available to dissolve salts, sugars, acids
How do solutes affect vapor pressure?
It lowers the vapor pressure and interferes with evaporation; decrease water molecules at the surface, make ionic bonds and keep water molecules in liquid
How does using a pressure cooker affect boiling point?
It raises atmospheric pressure and makes the boiling point higher; shorter cooking time needed
How does adding a soluble substance or substance that ionizers affect the boiling point?
It reduces vapor pressure and makes the boiling point higher; adding sugar or salt to water raises its boiling point
How does evaporation occur?
Kinetic motion enables hydrogen bonds b/w water molecules to break
At a low temperature, how much solute dissolves?
Less solute dissolves the lower the temperature is
No microbes can reproduce at what level of water activity?
Less than .5
What happens if you add energy?
Melting, boiling, and evaporation
Which microorganism require the lowest range of water activity to grow?
Molds - .80-.87; .60-.75
The higher the water activity range, the _______ favorable conditions are for microorganisms
More
What is diffusion?
Movement of solute molecules along a concentration gradient
What is osmosis?
Movement of water/solvent through a semi-permeable membrane to an area of lower water concentration
Which heats faster - water or oil?
Oil - lower specific heat
For every 500 feet, how many degrees does the boiling point lower?
One degree fahrenheit
What is water content?
Percentage of food that is water
What are some properties of water?
Polarity; high surface tension; neutral pH; colligative properties - solvent; high specific heat; latent heat; solvent; dispersing medium; essential nutrient
What is vapor pressure?
Produced when molecules evaporate, forming a vapor over the liquid/solid they evaporated from; rises as temperature increases; varies b/w substances
What are colligative properties?
Properties of a solvent that are affected by the number of solute molecules present; include vapor pressure, boiling point, freezing point
What is surface tension?
Properties of the surface of a liquid that causes it to decrease surface area and allows it to resist an external force; due to hydrogen bonding
What is adsorption?
Resorption - food is taking up water from the environment
What are the states of water?
Solid - ice; liquid - water; and gas - water vapor
What affects freezing temperature?
Solute - interfere with the ability of hydrogen bonds to form permanent structures; lowers the freezing point of water and reduces vapor pressure
What are some functions of water in food?
Solvent; dispersing medium; hydrates; promotes chemical reactions - hydrolysis, ionization; heat transfer/cooking medium; microorganisms
Why is water a good solvent?
Substances, such as sodium chloride, ionize/dissociate in water; salt is the solute and water is the solvent
What factors affect water activity?
Temperature - effects vary by food; other food ingredients - effects vary; preservation methods - drying or freezing; adding solute - adding sugar or salt
What is specific heat?
The amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius; expressed as calories/gram; takes 1 calorie of water to raise 1 gram of water 1 degree Celsius - 1 calorie/gram
What is pH?
The measure of hydrogen ion concentration in a solution
What happens when water is less than 4 degrees Celsius?
The molecules separate slightly, causing an increase in volume
As water cools, what happens to the molecules?
They move slower and slower
What is a salt?
Union of positive and negative ions
What happens at the boiling point of water?
Vapor pressure of the liquid overcomes the atmospheric pressure and attraction of hydrogen bonds
How is water activity calculated?
Vapor pressure of water in sample divided by vapor pressure of pure water
What is desorption?
Water is lost from a food - drying
1 gram of sugar added to 1 liter of water lowers the freezing point by how much?
1.86 degrees Celsius - soluble; application - frozen desserts w/sugar take longer to freeze
1 gram of salt added to 1 liter of water lowers the freezing point by how much?
3.72 degrees Celsius - ionizers; applications - salt on freezing roads and making ice cream
What happens when water is at 0 degrees Celsius?
Actual freezing occurs; the ice lattice expands and is less dense than water; structure is referred to as a crystal lattice
What is hydrolysis?
Adding water to molecules to break bonds
What happens to water at 4 degrees Celsius?
An organized bonding pattern b/w the molecules begins to appear; water is at its greatest density
How does atmospheric pressure influence boiling point?
As barometric - atmospheric - pressure increases, the temperature at which water boils become higher - and vice versa
What does evaporation mean?
Changing from liquid to gas; absorbs surrounding heat and has a cooling effect
What is bound water unavailable for?
Chemical/microbial activity; important factor in food safety/spoilage
What happens if you remove energy?
Condensation, freezing
What two types of bonds are found in and b/w water molecules?
Covalent and hydrogen bonds. Hydrogen bonds attract the molecules to each other - structural tension
What does water activity express?
Degree to which water is bound or free within food; ranges from 0 - no free water to one - pure water
What does polarity of water mean?
Electrons concentrate around the oxygen atom, causing electrical asymmetry - water is a dipole; hydrogen atoms are at a 104.5 degree angle
What is bound water important for?
Food texture; gelatin/pectin
How can you convert hard water to soft water?
With a water softener; ion exchanger - exchanges calcium and magnesium with sodium
Do we need water to live?
Yes
What are the types of latent heat?
heat of fusion - solidification; heat of vaporization
What is latent heat?
heat released or absorbed during a change in state; amount of heat required to cause a change in state of matter
Which do you add to keep beverages cold longer - ice or water?
ice
Atmosphere pressure decreases as altitude __________.
increases
Less atmospheric pressure = _______ vapor pressure needed to overcome it
less
Lower temperature of boiling point = ___________ cooking times needed
longer
Water is an essential ___________
nutrient
