Water - week 2

Mass/volume

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

-large surface tension -large dielectric constant(relative permittivity) -large heat capacity -Large latent heat of vaporisation

Property of a fluid to resist an external force which is due to HYDROGEN BONDING

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

Amount of heat energy required to raise the temperature by 1degree WITHOUT changing phase (e.g water vaporising)

Amount of heat energy required to convert liquid to vapour without changing temperature (e.g 100 degree water into 100 degree steam)

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)

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

1)Loss of substances other than water (volatiles) 2)Incomplete removal of “bound” water

% 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

% water dry basis= (mass loss/food mass after dying) x 100 Sometimes this will give you an answer greater than 100% (yoghurt,jelly, etc)

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

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 %)

aw is an average value for water in the food- some molecules are bound more strongly than others.

• free/bulk water -entrapped water -Bound/multilayer water -vicinal water (closest to food surface)

-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

‒ 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 <-40C

‒ 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

‒ 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

» 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

aw = nw/(ns + nw) ns=moles of solute nw=moles of solvent (water)

Temporary hardness- bicarbonates- removed by boiling Permanent hardness- sulphites and chlorides- can’t be removed by boiling

Lime softening Use of ion exchange cartridges