Flashcards in Detailed notes Deck (32)
Percentage of water making up the body, raw meat and vegetables?
Raw meat- 75%
Water states in food ca be classified into?
bound water (0.1 to 0.9%) that acts with specific hydrophilic groups
bound water (1% to 5%) that forms several additional layers around hydrophilic
free water (5% - 96%) where the flow is unimpeded
free water (5% to 96%) held within matrix or gel
Water is the only substance that is....?
The only substance that is abundant in solid, liquid and vapour form. It is a universal solvent and dispersing agent; seems mild and inert...but a very reactive compound. It is the most anomalous of chemical compounds!
The water molecule?
-Is made up of 2 H and 1 O forming covalent bonds.
- Electrically neutral but the distribution of charges in the molecules make the O atom slightly more negative
-Though electrical neutral is also dipolar
-H Bonding is a case of dipole - dipole interaction.
-Attraction between the negative O and positive Hs leads to H bonding
- The oxygen of one water molecule can form a hydrogen bond with hydrogen atoms on two other water molecules. Every water molecule is H bonded to four
other water molecules.
- Water can exist in 3 forms depending on the extent to which the molecules are H-bonded.
Due to water molecules’ ability to engage in extensive, 3-dimensional H-bonding water has a high?
- High melting and boiling point
- High phase transition enthalpies
- High surface tension
- High heat capacity
Three existing phases of water
Solid, Liquid and gass
Triple point of water?
A single combination os pressure and temp at which pure water, ice, and vapour can coexist in a stable equilibrium. Occurs at 273.17 or 0.01 and a pressure of 611.73 pascals.
At this point is is possible to change all of the substance to ice, water or vapour by making extremely small changes in pressure and temp
Melting curve or fusion curve of ice/water?
Special because is has a negative slope - Ice is less dense than liquid water and melts at a lower temp and high pressure. This allows for ice skating (layer of liquid is formed between the skate and ice which acts as a slippery lubricant)
Why is water a better solvent than most liquids?
Due to polatiry
Why does water have a high melting and boiling point?
Due to H bonding
Why is ice less dense than water?
Due to shape of the water molecules
Why does water have a high surface tension?
Due to H bonding
Functions of water in food preparation?
- Enables ionisation of acid/bases
- Enables hydrolysis of table sugar to simple sugars with acid
- Enables denaturation of protein
- Acts as a medium for transfer of heat
- Acts as a solvent
- In collodial dispersions
How does water act as a medium for transfer of heat?
water absorbs heat which sets up convection currents
that equalise the temperature throughout preventing food from burning. Water transfers heat
to food in contact with it.
What is a colloidal dispersions?
a colloidal dispersion (colloid) is one in which the size of the
molecules are neither so large that they settle out nor so small that they form true solutions - substance is merely dispersed in water.
a suspension is a dispersion in which the molecules of the substance are so large and or complex that they can neither be dissolved nor dispersed in water.
For example: Starch granules in cold water
- Between 0-100 @ normal atmospheric pressure
- Flows due to H bonds
- H bonds are formed and broken continuously between molecules
-Temp lowered to 0
- Crystal lattice formed has unfilled spaces hexagonal in shape
- What water left exposed will eventually all evaporate
- molecules have kinetic energy due to random motion
- As long as the vapour pressure of the surrounding air is lower than the vapour pressure of water, evaporation will occur
- Rate of evaporation varies
When a solid melts or a liquid boils, energy (latent heat) is absorbed without a consequent rise in temperature.
Latent heat of fusion?
- The energy required to change unit mass from solid to liquid state without a change in temperature.
- 344kj/kg for water to steam
Implications of waters late heat of fusion?
- Causes steam to burn worse than boiling water
- Means water is used to keep foods cool
- Ice is a good coolant
Heat capacity (C') of a body is the ratio of the amount of heat Q supplied to a body in any process to its corresponding temperature change (delta T)
Heat capacity formula?
C' = Q/(delts)T
Q = Heat supplied
T = temp change
Specify heat capacity Equations
Is the heat capacity per init mass of body
c = C'/m
c = Q / (m x delta T)
Q = m x c x (Tf - Ti)
m = mass
C' = Heat capacity
Tf = temp final
Ti = temp initial
c = specific heat capacity
What is specific heat capacity?
Different liquids take in heat at different rates. this difference is called specific heat capacity and is the heat required to raise the temperature of 1 kg of the substance by 1 degrees.
Explaines why copper heats up quicker than Al and why milk boils quicker than water.
Foods with high water content will have a similar specific heat values to water.
Foods with lower water content like sugar and oil have much lower specific heat capacities.