Water - week 2 Flashcards
What’s the equation of Density
Mass/volume
At What temperature is water at its heaviest density
4 degrees (this is when there is ice on the surface of the water and underneath it is liquid) due to its expanding when freezing
What properties does water have?
-large surface tension
-large dielectric constant(relative permittivity)
-large heat capacity
-Large latent heat of vaporisation
Define what surface tension is
Property of a fluid to resist an external force which is due to HYDROGEN BONDING
Define Dielectric constant
The relative amount of electrical energy stored in a material vs vacuum, its a very polar solvent and so it allows ionic salts to dissolve
Define heat capacity
Amount of heat energy required to raise the temperature by 1degree WITHOUT changing phase (e.g water vaporising)
Define latent heat of vaporisation
Amount of heat energy required to convert liquid to vapour without changing temperature (e.g 100 degree water into 100 degree steam)
Explain the bonding in water
Then paired electrons on Oxygen each pair up with a single electron from hydrogen to form COVALENT BONDS
The unbounded pairs of electrons on the oxygen are available to form temporary HYDROGEN BONDS with other molecules - (forms cohesion)
How do you determine the water content of a certain food
1) Weigh a dry container and record weight
2)Grind or mince food (to give it enough surface area)
3)Weigh sample into container and record weight
4)Heat at specified temperature for a specified time (or freeze dry)
5)Weigh sample and container and record weight
6)repeat heating/weighing cycle until constant MASS
7)calculate mass loss
What are the 2 reasons why mass loss varies with heating conditions when determining the water content of food
1)Loss of substances other than water (volatiles)
2)Incomplete removal of “bound” water
What is the formula to calculate the percentage of water on a FRESH weight basis
% water= (mass loss/original food mass) x 100
% water is always less than 100- as no fresh food is made up of 100% of water
What is the formal to calculate the percentage of water on a DRY weight basis
% water dry basis= (mass loss/food mass after dying) x 100
Sometimes this will give you an answer greater than 100% (yoghurt,jelly, etc)
What can you do to inhibit deterioration
By making water unavailable for chemical, enzyme can and microbial reactions
- remove water e.g drying or concentrating (mil powder,condensed milk ,cheese)
-immobilise water e.g freezing or adding sugar (jams), or slat
This shows that water activity is more important than water content itself
This is because foods with high water content seem to spoil more rapidly than foods with lower water content
Define/express the availability of water (equations and definitions of the terms)
aw = p/p0
p, vapour pressure of water in equilibrium with sample
p0, vapour pressure of pure water at the same temperature
(Vapour pressure relates to the tendency of molecules to escape from a liquid)
aw x 100 = Equilibrium Relative Humidity (ERH %)
Whats the simplified version of what water activity is ?
aw is an average value for water in the food- some molecules are bound more strongly than others.
What are the subdivisions that water can be categorised based on the strength of its binding
- free/bulk water
-entrapped water
-Bound/multilayer water
-vicinal water (closest to food surface)
What are the properties of vicinal water (also known as mono layer or Langmuir water)
-mostly strongly bound
-present at 0.5 ± 0.4 % of total water (of a typical high moisture food)
‒ Largest binding energy
‒ Removal by heat is difficult and not reversible (binding surface damaged)
‒ Does not freeze (cannot move to crystal lattice)
‒ Not a solvent
‒ Associated with proteins, polysaccharides, salts, especially the insoluble structures
‒ The layer of water is complete at aw approx 0.25
What are the properties of bound or multilayer water
‒ 3 ± 2% of water of a typical high moisture food
‒ Successive layers bound by hydrogen bonding
‒ Strength of binding declines progressively, negligible when n>3 ‒ Water held in smaller capillaries <1 μm diameter
‒ Layers complete at aw about 0.8
‒ Freezing point decreased substantially • Does not freeze until <-40C
What are the properties of entrapped water
‒ Water physically held/entrapped in the food matrix
• behindmembranes
• in capillaries > 1 μm diameter
• 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
What are the properties of free or 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 are the reasons why foods can change their water content
» If the atmosphere and a food have the same aw - no overall transfer of water
» If aw(food) > aw(atmosphere) then the food loses water
- cakes and cheeses go dry, fruits shrink and wrinkle
» If aw(food) < aw(atmosphere) then the food gains water
- sugar, instant coffee, milk powder etc become sticky and cake together
» Also applies to foods in contact with each other e.g. cucumber sandwiches
This happens because water moves from regions of higher aw(water activity) to regions of lower aw
How do you estimate the water activity by using Raoults law- derivation
aw = nw/(ns + nw)
ns=moles of solute
nw=moles of solvent (water)
what is the difference between Temporary hardness and permanent hardness
Temporary hardness- bicarbonates- removed by boiling
Permanent hardness- sulphites and chlorides- can’t be removed by boiling
What are the 2 methods of softening water
Lime softening
Use of ion exchange cartridges
