Water
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Which equation is used to determine water density?
Density= mass/ volume
What is the reason for the high surface tension of water?
The hydrogen bonding
What type of bonds are present in water?
Covalent bonding- O and H - 2S and 2P sub shells hydrides to form four 2SP3 orbitals
Outline the relation between food stability and water.
If there is a high water content in foods, these foods will spoil more quickly
What methods can be used to inhibit food deterioration?
We can inhibit food deterioration but reducing the availability of water, this is done by freezing, drying and adding salt or sugar.
Which equation is used to determine water activity?
aw = P/P0 aw = water activity P = partial pressure of water above the material P0 = partial pressure of pure water at the same temperature
How is Equilibrium relative humidity (ERH) expressed?
Equilibrium relative humidity is water activity expressed as a percentage; ERH = aw x 100
What variable can we increase to increase water activity?
Temperature can be increased to increase water activity.
What are the four ‘levels’ of water?
Vicinal Bound Entrapped Free
What are the key features of vicinal water?
Strongly bound Hard to remove with heat Does not freeze Not a solvent aw= 0.25
What are the key features of bound water?
Freezes at -40 degrees 1-5% of the water in high moisture food aw= 0.8
What are the key features of entrapped water?
Water that is being physically held by tissues 96% of the water in high moisture food
What are the key features of free water?
Has normal solvent capacity and freezing point96% of the water in high moisture food
What is the name of the law that can be used to estimate water activity?
Raoult’s Law
What is the equation of Raoult’s Law?
aw = nw / (ns + nw)
What does this refer to: ‘a law stating that the vapour pressure of an ideal solution is proportional to the mole fraction of solvent’?
Raoult’s Law
What two minerals does hard water contain?
Hard water contains dissolved magnesium and calcium ions
What causes temporary hardness (water)?
Temporary hardness is caused by dissolved calcium hydrogencarbonate
What causes permanent hardness (water)?
Permanent hardness is caused by dissolved calcium sulfate
How can temporary hardness resolved?
Dissolved calcium hydrogencarbonate can be removed by boiling
Can permanent hardness be resolved?
No it is permanent- dissolved calcium sulfate cannot be removed by boiling
What may be the consequence of a dry atmosphere on food?
It will cause a loss of water and therefore dried out food e.g. stale food
What may be the consequence of a moist atmosphere on food?
It will cause a gain of water and therefore ‘sticky’ food e.g. sugar clumps
Give an example of a water borne pathogen that is spread through faecal contamination
Salmonella