Water in Food Flashcards
What is the equation for density?
Mass / Volume
What are the properties of water?
Large surface tension
High heat capacity
High latent heat of vaporisation
High dielectric constant
What is dielectric constant?
The relative amount of electrical energy stored in a material vs the amount of electrical energy of free space
What bonding occurs in water?
Covalent = between oxygen and hydrogen within a water molecule Hydrogen = between hydrogen and oxygens of different water molecules
How can you determine the water content of a food?
- Weigh dry container
- Weigh sample in the dry container after grinding
- Heat at a specified temperature
- Weigh sample
- Repeat heating and weigh process until a constant mass is obtained
- Calculate the mass loss
What are the disadvantages of determining the water content of food by heating and weighing?
Mass loss varies with heating conditions
There may be loss of substances other than water
There may be an incomplete removal of bonded water within the food
What is the calculation for percentage of water of a food on a fresh weight basis?
(Mass loss / Original food mass) x 100
What is the calculation for percentage of water of a food on a dry weight basis?
(Mass loss / Food mass after drying) x 100
Do foods with a high water content spoil more or less easily than foods with a lower water content?
More easily
How can foods with a high water content be prevented from spoiling?
By making water unavailable
How is water made unavailable in foods with a high water content?
Remove water = drying or concentrating
Immobilise water = freezing, adding salt, adding sugar
What are the properties of strongly bonded water?
Low water activity Does not move easily Does not freeze Does not act as a solvent Cannot partake in spoilage reactions Low vapour pressure
What is the equation of water activity?
p / po
p = vapour pressure of water in equilibrium with sample
po = vapour pressure of pure water at the same temperature
How is equilibrium relative humidity calculated?
Aw x 100
How is water in food categorised?
Depending on the strength of its bonding
What are the four layers of water found in food?
Free / bulk water
Entrapped water
Bound water
Vicinal water
How does water activity change as temperature increases?
Increases
Water moves from regions of _______ water activity to regions of _______ water activity
High
Low
What are the properties of vicinal water?
Most strongly bonded
Removal by heat is difficult and non-reversible
Does not freeze
Does not act as a solvent
What is the estimated water acitivty of vicinal water?
0.25
What are the properties of bound water?
Layers are bound by hydrogen bonds
Water is held in capillaries
What is the estimated water activity of bound water?
0.8
What are the properties of entrapped water?
Water is held in tissue matrix (behind membranes of in capillaries) which prevents flow
Lower freezing point than free water
More likely to act as a solvent compared to bound water
What are the properties of free / bulk water?
Behaves like pure water
Readily squeezed out by pressure
What happens to a food when the water activity of the food is greater than the water activity of the atmosphere?
Food loses water
What happens to a food when the water activity of the food is less than the water activity of the atmosphere?
Food gains water
Does water movement occur between foods that are in contact with each other?
Yes
What is Raoults law?
Water activity = Nw / (Ns + Nw)
Nw = moles of solvent
Ns = moles of solute
