BMS1064: Wk2 - Water Flashcards
How do you calculate density?
Density = Mass/Volume
At what temp is water most dense?
Whats unusual about water?
Around 4 degrees C.
It expands when freezing (less dense as solid).
SO water at 4C is at bottom of lake with ice floating on top.
Water has an unusually great: (what 5 things?)
- Density
- Surface tension
- Dielectric Constant
- Heat Capacity (energy to raise by 1 degree)
- Latent Heat Capacity (energy to convert liquid to gas without temp change)
Which are stronger? Hydrogen or Covalent bonds?
Covalent (Intramolecular)
How do you determine the water content of a sample?
- Weigh dry container
- Grind food, add to container and weight container again (with sample)
- Heat at specified temp for specified time
- Weigh sample again
- Reheat untul constant mass
- Calculate mass lossW
When determining water content of a sample, what could cause variations in mass loss?
Incomplete removal of ‘bound’ water molecules.
Loss of substances other than water.
What is the equation to calculate % of water in a sample on a FRESH WEIGHT basis?
% water = (Mass loss / original food mass) x 100
What is the equation to calculate % of water in a sample on a DRY WEIGHT basis?
% water = (Mass loss / food mass after drying) x 100
When answering Qs where you need to calculate things, how should you answer?
Write out equation
Identify components in Q
Put numbers into equation
Calculate answer
How can you increase the shelf-life of food in relation to water and why would you do this?
Remove water (by drying or concentrating) or Immobilise water (by freezing or adding sugar - unavailable for reactions).
Food with high water content spoil more rapidly.
What is Water Activity?
How ‘available’ water is to take place in chemical reactions.
aw is an average for all the water in the food.
More strongly bound water has lower Water Activity.
Water Activity is more important than just the water content of food.
What can strongly bound water not do?
- Does not move easily within a food.
- Does not freeze (cannot move to form ice crystals).
- Does not act as a solvent - i.e. dissolve substances.
- Is not available to take part in spoilage reactions.
- Does not escape easily into the atmosphere (low vapour pressure).
What is Vapour Pressure?
The tendency of molecules to escape from a liquid (i.e. evaporate)
What is the equation for Water Activity?
aw = p / p0
Water Activity = vapour pressure of water in equilibrium with sample / vapour pressure of pure water at the same temperature
How can you calculate Equilibrium Relative Humidity from Water Activity?
aw x 100 = Equilibrium Relative Humidity (ERH %)
What are the catagories that water can be divided into based on the strength of its binding?
*Free water
*Entrapped water
*Bound/multilayer water
*Vicinal water (closest to food surface)
How does aw change with temp?
And how does water move according to aw?
aw increases with increasing temperature.
Water moves from regions of high aw to regions of lower aw.
Describe Vicinal/monolayer water.
Found closest to the food surface.
Most strongly bound - removal by heat difficult, does not freeze, not a solvent.
Associated with proteins, polysaccharides and salts.
Present at 0.5 ±0.4% of total water in high moisture food.
The layer of water is complete at aw approx 0.25.
Describe bound/multilayer water.
- Successive layers of water bound by hydrogen bonding.
- Strength of binding declines progressively.
*Water held in smaller capillaries <1 μm diameter
*Does not freeze until < -40C - 3 ±2% of water of typical high moisture food.
*Layers complete at awabout 0.8
Describe entrapped water.
Water physically held/entrapped in the food matrix:
- behind membranes, in capillaries > 1 μm diameter
- prevented from flowing freely by gel structure
Solvent capacity, freezing point, water mobility all slightly less than free water
- Can move short distances
*Up to 96% of total water of a typical high moisture food
Describe Free/bulk Water
Behaves like pure water.
Held within structure, but readily squeezed out by pressure.
Solvent capacity and freezing point are normal.
- Up to 96% of total water of a typical high moisture food
What causes transfer of water in food?
If atmosphere and food have same aw - no water movement.
If aw(food) > aw(atmosphere) then the food loses water.
If aw(food) < aw(atmosphere) then the food gains water
- Also applies to foods in contact with each other e.g. cucumber sandwiches
How do you calculate water activity using moles of solvent and solute?
aw = mol of solvent (water) / (mol of solute + mol of solvent)
aw= nw/(ns + nw)
What are the units for aw? What is the largest number it can be?
No units for aw!
aw will always be <1.
