food chem 21
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What are the common roles for water in food?
- A carrier of nutrients and waste
- As reactant and a reaction medium
- A stabiized of biopolymer configuration (proteins, carbohydrates)
What are changes in texture associated with?
Why do many of our food processing and preservation methods involve the maniplation and control of water content?
Water is a major reactant and serves as a reactio medium, many of our food- usually the manipulation and control of water content is to slow down deteriorative reactions
How do freezing, concentration and dehydration assist in preserving food?
Freezing, concentration and dehydration all reduce the moisture content and/or water activity of a system and assist in preserving food systems by removin or tying up water
What is the only compound that exists in all three states?
What other hydrates is water compared to across the periodic table?
CH4, NH3, OH2 and HF
What other hydrates is water compared to down the periodic table?
H2O, H2S, H2Se, HeTe
Why is water unlike many other hydrates?
It has a relatively low density and expands upo crystallization rather than contracting like the rest
How many lone electron pairs are left in the outer valence shell of the oxygen molecule?
What happens when a molecule has an electron rich and poor region?
There is an attempt to reduce the free energy difference by sharing that electron defficit
What is waters coordination number?
Since there are four regions of differential electron density per molecule, a water molecule has a coordination number of four
What happens when a water molecule is in the presence of other water molcules or other polar molecules?
These regions of differential electron density will lead to association to minimize the overall free energy state of the system
What is hydrogen bonding?
Associations due to differentials in electron density between hydrogen and other electronegative atoms is termed hydrogen bonding
What does odd numbered coordination number result in?
Odd numbered coordination number only allows two dimensional association (chaining), but not the formation of a three dimensional structure which water can have
Why is the boiling point of water so high?
Relative to other hydrates- the additional hydrogen bonding energy has to be overcome for it to turn into gas
Why is water considered to have a 'transitional' structure?
Hydrogen bonds can form, but are too weak to counteract the kinetic energy of the molecules and thus maintain any particular structure
What are 'flickering clusters'?
- Regions of three dimensionally hydrogen bonded water that exist in solution
- Flickering clusters continually form and break up in solution
What is the lifetime of a flickering structure?
about 10^-10 to 10^-11 seconds
How do flickering clusters change with temperature?
Number of clusters increases with temperature, but the number of molecules per cluster decreases- however, clusters still exist at the boiling point of water
What is the basis for water vapor accumulation in the atmosphere?
Hydrogen bonding is the basis for water vapor accumulation in the atmosphere leading to cloud formation and of course rain
What happens when ionic compounds are dissolved in water?
When ionic compound are dissolved in water, they ionize into their respective ions rather than stayingin their molecular form
Why do ionic compounds ionize in water?
They ionize because of the very polar nature of the water- this does not happen in hydrophobic solvents
How does ionic solvents disrupt the normal structure of water?
Formation of a hydratio shell forces the water into a specifric orientation around each ion, therefore disrupting the normal structure of water
What is the net result of ionic compounds in water?
- Inhibition of random flickering cluster formation due to a mismatch of the tetrahedral bond angles of free water
- The foration of bulky hydroates generally causes an increase in viscosity and tends to depress the freezing point as well as the vapor pressure of water
What happens when hydrogen bonding compounds are dissolved in water?
Overall effect is to tie up water locally in similar manner as ionic compounds
What happens to water molecules in macromolecules?
In the case of macromolecules that have a large number of hydrogen bonding sites (polysaccharides, proteins and polypeptides) the water can be completely or extensively immobilized by hydrogen bonding and in effect becomes part of the structure
What happens when non polar substances are added to water?
When low concentrations of non polar substances are dissolved or dispersed inw ater (hydrocarbons, fatty acids or non polar side chains of proteins) ice like inclusions are through to be induced arounda non polar group
How does water minimize their free energy state when no npolar substances are added?
To minimize their free energy state water is forced into a particular structure at the interface that differs from the bulk of the water
What is clathrate hydrate?
These 'ice-link' inclusions are not structurally identical to ice, but a semi0crystalline form of water is produced, termed 'clathrate hydrate'
What is a clathrate compound?
Compounds not formed by the action of valency bonds but by molecular imprisonment
Why are clathrate hydrate structures important?
Clathrate hydrate structures are important in stabilizing protein structure by localized imprisonment of water molecules around hdrophobic moieties
How do clathrate hydrates pertain to global warming?
The release of methane from clathrate hydrates in the ocean is an issue in relation to global warming
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