Week 2 - Water Flashcards
Importance of water
Diet - drink 2 litres/day
Commodity - bottled water businesses
Hydrolysis - breaking down fats
Solvent - allowing things dissolve
Food productions - crops require water, washing & cleaning, steam for cooking
H2O
Two hydrogen bonds covalently bonded to oxygen
Water properties
High MP: 0 degrees
Large BP: 100 degrees
3 physical states - liquid, gas, solid
Density - Max at 4 degrees, expands when frozen
Large Surface tension - fluid resists external force bc of H bonds
Large dielectric constant - electrical energy stored, allows salts dissolve
Large heat capacity - amount of heat to raise temp by 1 degree
Large latent heat of evaporation - amount of heat to go from liquid to vapour
Bonding in water
Covalent bonds: Oxygen unpaired electrons + Hydrogen electrons (permanent)
Hydrogen bonds: Left over oxygen electrons bond with other molecules
- O2 has higher electronegativity than Hydrogen
Determine water content
- Weigh + record
- Ground/mince food
- weigh and record
- Heat/freeze or dry
- Weigh and record
- Repeat steps 4-5
- Calc mass loss
Percentage water on a fresh weight basis (fwb)
% water (fwb) = (Mass loss/ original food mass) X 100
Mass loss =
(Mass of dish + mass of food) - (mass of dish + mass of food after drying)
Original mass=
(Mass of dish + mass of food) - (mass of dish)
*% water always less than 100%
Percentage water on a dry weight basis (dwb)
% water (dry basis) = (mass loss/ food mass after drying) X 100
Mass loss =
(Mass of dish + mass of food) - (mass of dish + mass of food after drying)
Food mass after drying=
(mass of dish + mass of contents after drying) - (mass of dish)
*can be over 100% - yoghurt/jelly
Water and food stability
High water = more spoilage
Solution:
Dry or concentrating. E.g: milk powder, condensed milk, cheese, sultanas
Immobilize water. E.g. freeze or add sugar (jam), or add salt (cured meat)
**water content and water activity are different
Define water activity
aw= p/p0
water activity = vapour pressure of water in equilibrium with sample / vapour pressure of pure water at the same temperature
ERH % = aw x 100
Equilibrium Relative Humidity
Water categories - in terms of binding strength
- Free/bulk water
- Entrapped water
- Bound/multilayer water
- Vicinal water - closest to food surface
Vicinal water
Closest to foods surface
- aw increases with high temps
- Water moves from a region of high aw to low aw
- most strongly bound, largest binding energy
- Difficult to remove heat and irreversible - binding surface damages
- cannot freeze bc cannot move to crystal lattice
- associated with proteins, salts & polysaccharides
Bound/ multilayer water
- Low freezing point. Does no freeze until 3
Entrapped water
- Water trapped in food matrix; behind membranes, in capillaries, can’t flow because of gel structure
- ~ 96% of total water of a typical high moisture food
- Slightly less Solvent capacity, freezing point, water mobility vs free water
Free / bulk water
- Behaves like pure water
- ~ 96% of total water of a typical high moisture food
- Held in structure but squeezed out with pressure
- Solvent capacity and freezing point normal
Relationship between: water activity, water content and spoilage
If aw of food and atmosphere = same, then no transfer of water
If aw (food) > aw (atmosphere), then food loses water. (E.g. Cake/cheese = dry. Fruit = wrinkly/shrinks)
If aw (food) < aw (atmosphere), then food gains water. (E.g. sugar/instant coffee/ milk powder = sticky/ lumps together. Cucumber sandwiches too)
