Water I Flashcards
Water can serve as a stabilizer of _________ configuration, composed of _______ and ________
biopolymer
proteins
carbohydrates
Which foods contain the highest water content?
- Tomatoes, lettuce (95%)
- Apple juice, milk (87%)
- Potato (78%)
- Meats (65-70%)
How much water does shortening contain?
0%
Texture is often associated with _______
moisture
Which three components are extremely important in terms of stability and acceptability?
- Amount of water
- Location of water
- Interactions with the chemical components
What are the three methods to reduce the water activity of a system?
- Freezing
- Concentration
- Dehydration
Is water the only compound that exists in all three states on Earth?
Yes
Water is a hydrate of _______
oygen
What are the hydrates of atoms (apart from water) across the periodic table? What are their properties?
- CH4, NH3, HF
- Gases
What are the hydrates of atoms (apart from water) down the periodic table? What are their properties?
- H2S, H2Se, H2Te
- Gases
Relative to the other hydrates, water is unusual in all of its physical properties. What are the 7 unusual physical properties?
- Melting point
- Boiling point
- Surface tension
- Dielectric constant
- Heat of fusion
- Heat of vaporization
- Heat of sublimation
Oxygen has ___ electrons, ___ of which are valence electrons, and the two hydrogens have ___ electrons each
8
6
1
The _____ 2p orbitals and the ____ 2s orbitals of the oxygen atom are combined to form _____ sp^3 hybrid orbitals
3
1
4
What combines to form four sp^3 hybrid orbitals?
- Three 2p orbitals
- One 2s orbital
Only ___ of oxygen’s ___ valence electrons are used up in the two covalent bonds that form between H and O
2
6
There are ___ oxygen lone electron pairs left in ____ of the sp3 orbitals
2
2
Why is oxygen a highly electronegative atom?
Because of its lone electron pairs
The electron from the hydrogen is drawn more towards the ______, which results in the (increase/decrease) of the electron shield around the hydrogen atom
oxygen
decrease
The hydrogen is slightly ______, while the oxygen is slightly _______
positive
negative
Water molecules can therefore be viewed as having ____ electron-rich regions and ____ electron-poor regions situated __________.
two
two
tetradehedrally
The ________ nature of the spatial arrangement of these regions of electron density promotes the ability to form a _____________ or network.
tetrahedral
3D association
Associations due to differential electron density between hydrogen and other electronegative atoms are termed ____________
hydrogen bonding
___________ is the basis for water vapor accumulation in the atmosphere, leading to formation of clouds.
Hydrogen bonding
Because water is capable of hydrogen bonding in three dimensions, ____________________ can form.
local regions of structure
In the liquid phase, water is considered to have “_________” structure, termed ”__________”
transitional
flickering clusters
The term “flickering clusters” is associated with which phase of water?
Liquid
Flickering clusters are regions of 3D H-bonded water molecules somewhat similar in structure to ________
ice
Flickering clusters continually _____ and ______
form
break up
The number of flickering clusters (increases/decreases) with temperature, and the number of molecules per cluster (increases/decreases)
increases
decreases
Flickering clusters (do/don’t) exist at the boiling point of water
do
_____________ have been postulated to explain the relatively normal (low) viscosity of water.
Flickering clusters
Ionic compounds ionize into ions in water due to the _________ of water
polar nature
Polarity is due to ____________
asymmetric electron density
Polarity is (sufficient/insufficient) to break covalent bonds
insufficient
Polarity is (sufficient/insufficient) to break ionic bonds
sufficient
Polarity can dissociate ionic bonds by allowing multiple water molecules to act as __________ for opposing charged ionic species
counterions
When NaCl is dissolved in water, ________ are attracted to chlorine, while _________ are attracted to sodium
hydrogen (+)
oxygen (-)
When an ion is dissolved in water, a __________ forms around each ion to ___________ the charge of the respective ions.
hydration shell
neutralize
When an ion is dissolved in water, the formation of a ______________ forces the water into a specific orientation around each ion, therefore disrupting the normal structure of water.
hydration shell
The formation of bulky hydrates generally causes an (increase/decrease) in viscosity and tends to (increase/decrease) the freezing point of the system.
increase
decrease
Hydrogen bonding compounds are also capable of disrupting the __________ of water.
tetrahedral geometry
The overall manner of hydrogen-bonding compounds is similar to ___________
ionic compounds
In the case of macromolecules that have a large number of hydrogen bonding sites (polypeptides, proteins, carbohydrates, and polysaccharides), the water can be completely or extensively _________ by hydrogen bonding if the amount of water present is limited.
immobilized
When low concentrations of nonpolar substances are dissolved in water, __________ are induced.
ice-like inclusions
The formation of an __________ around a nonpolar group or molecule is to (maximize/minimize) the free energy state of the system when symmetric electron-neutral constituents (hydrophobic) and asymmetric electron-rich hydrophilic materials are forced to associate.
ice-like structure minimize
Are ice-like inclusions structurally identical to ice?
No, it is a semi-crystalline form that is produced
The semi-crystalline form of ice-like inclusions are termed “______________”
clathrate hydrates
Define clathrate compounds (important).
Compounds which are not formed by the action of valence bonds, but by molecular imprisonment
_________ structures are important in stabilizing protein structure around hydrophobic moieties.
Clathrate hydrate
Clathrate compounds are formed by _________
molecular imprisonment
Does the presence of all solutes have a disruptive effect on the structure of water?
Yes
What is the activity of water?
Its freedom of movement
The activity of water is (increased/reduced) when a solute is added
reduced
In the freezing of water, the flickering cluster becomes a _________ or ________, due to a decrease in _______
minute crystal
nucleus
kinetic energy
In the freezing of water, what can happen to the nucleus?
Can grow in size as more water molecules fit into the 3D crystal lattice
At 0oC there is an equilibrium between __________ and liquid water.
crystal nucleation
In ultrapure water, _______ is difficult and does not readily take place
nucleation
In ultrapure water, _______ can occur when the water stays liquid well below 0oC
supercooling
Supercooled water will freeze (quickly/slowly) if disturbed, or if an ice crystal is thrown in
quickly
In normal water, ___________ often aid the start of the nucleation process
impurities (dust, container wall surface)
If the rate of freezing is slow, (many/few) nuclei are formed at any one time
few
If the rate of freezing is slow, the net result is the formation of _____________
few large ice crystals
If the rate of heat removal is rapid, (many/few) nuclei will form at the outset, leading to the formation of ___________
many
smaller ice crystals
Explain the differences between freezing slowly and quickly in tissue systems.
- Slowly: large ice crystals form, which can damage cell walls (reduces water holding ability)
- Rapidly: small ice crystals formed uniformly
______ interfere with the nucleation process and also slow down the rate of diffusion of water to the ice crystals
Solutes
Once nucleation has taken place and water diffuses to the surface of the crystal, the solutes become (more/less) concentrated as the freezing process takes place.
more
The increased concentration of solutes leads to the (increase/decrease) of the freezing point since a _____ temperature is required for ice to form
decrease
lower
At a low enough temperature, called the _______ point, the solution can become concentrated to a point at which the remaining water will co-crystallize with the solute crystals to start to form and grow
eutectic
The ______ point lies between -55oC and -70oC
eutectic
When the eutectic point has been reached, all the water in the
system is ________.
immobilized
Why is total stability not attained in a frozen food product?
Since not all the water is immobilized (at the eutectic point); temperatures required for this are too low
Free water freezes quite (quickly/slowly), and once crystal growth is induced, then the concentration of solutes (increases/decreases)
quickly
increases
Within a biological sample (cells), the _________ will be disrupted if large ice crystals form.
organelles
Which solutes will concentration in the freezing process?
Salts, ions, enzymes
Why would a highly concentrated solution not freeze totally?
Because the temperature is well above the eutectic point
If the highly concentrated solution is not totally frozen, water is still available to act as a ________ and the reaction medium is very _________
reactant
concentrated
Can reactions still take place at low temperatures?
Yes, especially enzymatic reactions because the substrate and enzyme may be mixed due to tissue disruption
Which two factors (that denature proteins) may affect water holding capacity?
- Ionic strength
- pH changes
Do frozen foods have a perpetual shelf-life?
No since they are not frozen to the eutectic point; enzymatic reactions may still take place
Because enzymatic reactions carry on under conditions of frozen storage, fruits and vegetables have to be ________ (mild heat treatment) before being frozen.
blanched
All enzymatic reactions speed up when the _________ process is started - much of the quality is lost in this step rather than during frozen storage.
thawing
______ water is relatively mobile, while _______ water is relatively immobile
Free
bound
What is a way to measure the mobility of water?
Water activity
The water activity is on a scale from ___ to ___, with ___ being pure water
0
1
1
What is the equation for water activity?
Aw = P/P0(T) P = partial pressure of the food matrix P0 = partial pressure of pure water T = temperature
A sealed container containing water comes to an equilibrium between ________ and its _________ at any constant temperature.
liquid water
vapor form
A 25% NaCl solution would lead to the partial pressure (or RH) being (increased/reduced) significantly.
reduced
Salt dissolved in water (increases/decreases) its freedom of movement
decreases
Any solute (increases/reduces) the activity of water
reduces
The ratio of the ____________ is a useful measure of the relative mobility, activity, or availability of water.
partial pressures
We can classify water in foods based on their ________ ranges.
water activity
Type ____ water has an Aw of 1
IV (pure)
Type ____ water has an Aw of 0.99 to 0.80
III
Water that is physically entrapped in the tissue matrix is type _____ water
III
Type ____ water contains some solutes but not enough to immobilize the water in any
major way.
III
In type ____ water, many microorganisms are capable of growth, with some bacteria and yeasts inhibited at the lower end of the range.
III
In type ___ water, most hydrolytic, oxidative, and enzymatic reactions proceed readily in this water activity range.
III
Is the freezing point of water depressed in type III water?
Not significantly
Type ___ water has an Aw of 0.80 to 0.25
II
Type ___ water is the water present in macrocapillaries
III
Type ___ water is broad-range water present in microcapillaries to the outer layers of “bound water”
II
In type ____ water
most microbial growth is halted, with the exception of some molds at the upper end of this range.
II
In type ____ water , the freezing point of water is significantly reduced and most enzymatic reactions requiring water are slowed significantly
II
Non-enzymatic browning takes place readily with heat at the upper end range of type ____ water
II
In type II water, non-enzymatic browning takes place with heat at the (upper/lower) range, and without heat at the (upper/lower)
upper
lower
Type ___ water has an Aw of 0.25 to 0
I
Type ___ water refers to bound monolayers of water directly hydrogen-bonded to molecules
I
Type ____ water is very strongly hydrogen-bonded to the molecules and an integral part of the molecular structure
I
Type ___ water is essentially immobile
I
Type ____ water is very difficult to remove and cannot be frozen
I
In type I water, most reactions are minimal with the exception of ________
autoxidation
______ occurs when a wet food is placed in a dry environment
Desorption
Desorption is analogous to _______, but not the same
dehydration
Differentiate desorption and dehydration.
Desorption: food is moving towards equilibrium with the environment
Dehydration: permanent loss of water from a food
Desorption results in an (increase/decrease) in Aw, while dehydration results in an (increase/decrease) in Aw
decrease
decrease
Desorption is generally a (fast/slow) process
slow
________ occurs when a dry food is placed in a wet environment
Adsorption
Adsorption results in an (increase/decrease) in Aw
increase
The term “________” is used to describe foods or chemicals that adsorb moisture
hygroscopic
Define supercooling. In what kind of water can it occur in?
- Water stays liquid well below 0oC
- Ultrapure water
