2: Water Flashcards
How do you calculate density?
Density= mass/volume
What are the main/unusual properties of water?
- Expands when frozen
- Ice has a lower density than liquid water, so icebergs will float. Water at 4 degrees is at the bottom of the lake, with ice at the surface.
- Unusually large surface tension (property of a fluid to resist external force) due to H bonding
- Dielectric constant (relative permittivity) : relative amount of electrical energy stored in a material vs vacuum. Very polar solvent & allows salts to dissolve
- High heat capacity : amount of heat required to raise temp by 1 degree C without changing phase
- High latent heat of vapourisation : amount of heat energy required to convert liquid to vapour without changing temp (e.g. from liquid to gas = still 100*C)
Explain bonding in water
The unpaired electrons on oxygen each pair up with a single electron from hydrogen to form permanent covalent bonds. The unbonded pairs of electrons on oxygen are available to form temporary hydrogen bonds with other molecules.
Water is dipolar. Explain what this means
Oxygen is more electronegative (electron-attracting) than hydrogen. Within the water molecule the oxygen is slightly negative and the hydrogens are slightly positive.
Means that:
-It forms H bonds with other dipolar & ionic groups, but not with non-polar molecules, e.g. fats
Explain the process of determination of water content
- Weigh a dry container (dish) and record weight
- Grind/mince foodstuff
- Weigh sample in container and record weight
- Heat at specified temp for specified time (or freeze dry)
- Weigh sample & container and record weight
- Repeat heating/weighing cycle to constant mass
- Calculate mass loss
Mass loss varies with heating conditions
Loss of substances other than water (volatiles)
Incomplete removal of ‘bound’ water
How do you calculate percentage of water on a fresh weight basis (fwb)?
%water = mass loss/original food mass x100
(%water fwb is always less than 100%)
Mass loss = mass of food & dish - mass of dish & contents after drying
Original food mass = mass of food & dish - mass of dish
How do you calculate percentage of water on a dry weight basis (dwb)?
%water dry basis = mass loss/food mass after drying x100
Mass loss = mass of food & dish - mass of dish & contents after drying
Food mass after drying = mass of dish & contents after drying - mass of dish
How does water content affect food stability?
Foods with high water content spoil more rapidly.
- Can inhibit deterioration by making water unavailable for chemical, enzymatic & microbial reactions
How can water be made unavailable in foods, to inhibit deterioration?
- Remove water e.g. drying or concentrating (milk powder, sultanas)
- Immobilise water e.g. freezing or adding sugar or salt (jams, cured fish/meat)
Explain strongly bound water, and how this impacts the water activity (aw)
Strongly bound water has a lower water activity than weakly bound water.
Strongly bound water:
* Does not move easily within a food
* Does not freeze (cannot move to form ice crystals)
* Does not act as a solvent
* Is not available to take part in spoilage reactions
* Does not escape easily into the atmosphere i.e. has low vapour pressure
Give the formula for water activity
aw = p/p0
p = vapour pressure of water in equilibrium with sample
p0 = vapour pressure of pure water at the same temp
aw x 100 = Equilibrium Relative Humidity (ERH %)
Define vapour pressure
The tendency of molecules to escape from a liquid
Give the subdivisions of water based on the strength of its binding
- Free/bulk water
- Entrapped water
- Bound/multilayer water
- Vicinal/monolayer water (closest to food surface)
Why might aw increase
With increased temperature. Water moves from regions of high to low aw
Describe vicinal/monolayer water
- Most strongly bound, closest to food surface
- Largest binding energy
- Removal of heat is difficult and irreversible (as binding surface damaged)
- Does not freeze (cannot move into crystal lattice)
- Not a solvent
- Associated with proteins, polysaccharides, salts, especially insoluble structures
- aw = 0.25 approx
Describe bound/multilayer water
- Successive layers bound by hydrogen bonding
- Strength of binding declines progressively
- Water held in smaller capillaries
- aw = 0.8 approx
- Freezing point decreased substantially (doesn’t freeze until below -40 degrees C)
Describe entrapped water
- Physically held / entrapped in the food matrix (behind membranes, in capillaries, prevented from flowing freely by gel structure)
- Up to 96% of total water of a typical high moisture food
- Solvent capacity, freezing point, water mobility all reduced slightly vs free water. Can move short distances
Describe free / bulk water
- Behaves like pure water
- Up to 96% of total water of a typical high moisture food
- Held within structure, but readily squeezed out by pressure
- Solvent capacity and freezing point are normal
What is the cause of temporary water hardness, and how can it be removed?
Due to bicarbonates. Removed by boiling
What is the cause of permanent hardness, and how can it be removed / softened?
Caused by sulphates and chlorides.
2 methods: lime softening & ion exchange cartidges
Describe the method of lime softening
Add slaked lime and soda ash to water. Calcium will precipitate to become calcium carbonate; & magnesium will precipitate to become magnesium hydroxide.
Softened water can be used in heat transfer. Process also kills bacteria
Describe the method of using ion exchange cartridges to soften water.
Resins in cartridges have sodium ions temporarily associated. Hard water replaces sodium ions with calcium & magnesium ions inside exchanger.
Soft water with sodium ions leaves exchanger. Sodium ions restored by adding NaCl to exchanger
Name 5 examples of waterborne pathogens (diseases) caused by faecal bacteria
- VTEC (verocytotixin E. coli)
- Salmonella
- Shigella
- Vibrio cholera (cholera)
- Campylobacter
Give 2 examples of enteric viruses caused by waterborne pathogens
Hepatitis A
Norovirus
