Water and Food Dispersions Flashcards
What are 3 common roles of water in food systems?
- A carrier of nutrients and wastes
- A reactant and a reaction medium
- A stabilizer of biopolymer configuration (poteins and COHs)
Water makes up what % of tomatoes and lettuce?
95
Water makes up what % of apple juice/milk?
87
Water makes up what % of potatoes?
78
Water makes up what % of meats?
65-70
Water makes up what % of bread?
35
Water makes up what % of honey?
20
Water makes up what % of rice/wheat flour?
12
Water makes up what % of shortening?
0
The amount of water, its location and its interaction with the chemical components in a food product is extremely important from the standpoint of _________and ________.
- acceptability
2. STABILITY
What is texture often associated with? Example?
Moisture
Ex: freeze-dried orange vs. fresh orange
Because water is a major reactant and serves as a reaction medium, many of our food processing and preservation methods involve the control of water content to:
slow down deteriorative reactions
What 3 factors all reduce the water activity (Aw) of a system?
- Freezing
- Concentration
- Dehydration
What is one of the most abundant molecular species on Earth?
Water
Water is the only _____ liquid and the most _______ solid.
- common
2. abundant
Comparing water (hydrate of oxygen) to the hydrates of other atoms:
a) Across the periodic table (CH4, NH3, HF - all ____ at room temp)
b) Down the periodic table (H2S, H2Se, H2Te - all _____)
Gases
How many electrons does H2O have? How many are valence? How many electrons does each Hydrogen have?
8, 6, 2
The __ __ orbital(s) and the __ __ orbital(s) of the oxygen atom are combined to form __ __ _____ orbitals.
- three 2p
- one 2s
- four sp3 hybrid
How many of oxygen’s 6 valence electrons are used up in the two covalent bonds that form between H and O? What is the net result?
2
Net result: there are 2 oxygen lone electron pairs left in two of the sp3 orbitals
What makes oxygen highly electronegative in a water molecule?
Its 2 lone electron pairs
Water molecules can be viewed as having two ________ regions and two ________ regions situated ________.
- electron-rich
- electron-poor
- tetrahedrally
In the presence of other water molecules, the electron-rich/poor regions of H2O will associate in order to do what?
To minimize the overall free energy of the system.
What is hydrogen bonding?
The associations due to differential electron density between hydrogen and other electronegative atoms
The tetrahedral nature of the spatial arrangement of these regions of electron density in H2O molecules promotes the ability to form a _________________.
3D association/network
How strong are hydrogen bonds? Is this strong?
~10 kcal/mol
Not very strong
Hydrogen bonding is the basis for what in the atmosphere? What does this lead to?
Water vapor accumulation. Leads to the accumulation of clouds
What is capable of forming due to waters ability to hydrogen bond in 3D?
local regions of structure
What structure does water have in the liquid phase? What is this termed?
“Transitional” structure
Termed: “flickering clusters”
What do flickering clusters do?
They continually form and break up
What is the lifetime of flickering clusters?
10^-10 to 10^-11 seconds
What happens to the # of flickering clusters as the tempurature of water increases? What is simultaneously happeing to the number of molecules per cluster?
Increases
Decreases
Do flickering clusters still exist at the boiling point of water?
yes
What have flickering clusters been postulated to explain?
The relatively normal (low) viscosity of water.
All ______ affect the “structure” of water to some exten
solutes
What happens when ionic compounds are dissolved in water? What is this due to?
The ionize into their respective ions.
Due to the very polar nature of water molecules
Is the polarity of water sufficient enough to break covalent bonds?
No
The polarity of water molecules can dissociate ____ bonds by allowing multiple water molecules to act as ______ for opposing charged ionic species
- ionic
2. counterions
What happens to the ions of NaCl as it is dissolved in water?
Each ion is hydrated by water
When NaCl dissolves in water, what is attracted to the chlorine anions? The sodium cations?
Slightly + hydrogen
Slightly negative oxygen
When NaCl is dissolved in water, what forms around each ion? What does this do?
A hydration shell.
This NEUTRALIZES the charge of the respective ions.
What does the formation of hydration shells around the solute do? What does this do to the normal structure of water?
It forces water into a specific orientation around each ion. This disrupts the normal structure of water.
How does the formation of bulky hydrates affect viscosity of the system?
Increases viscosity
How does the formation of bulky hydrates affect the freezing pt. of the system?
Depresses the freezing pt.
Hydrogen bonding compounds are capable of __________ the structure of water.
Disrupting
Water around a hydrogen bonding site on a molecule will associate in a manner that disrupts what?
the natural tetrahedral geometry of water.
Can hydrogen bonding solutes disrupt the structure of water?
yes
Which macromolecules have a large number of hydrogen bonding sites?
polypeptides, proteins, carbohydrates, and polysaccharide
What can happen to water when exposed to macromolecules that have a large number of H-bonding sites? Under what condition?
The water can be completely/extensively immobilized by hydrogen bonding if the amount of water present is limited.
When ____ concentrations of non-polar substances (hydrocarbons, FAs, or nonpolar side chains of proteins) are dissolved in water, ______ are thought to be formed.
- low
2. ice-like inclusions
How does the formation of ice-like structures around a non-polar group/molecule affect free energy? Why?
Minimizes the free energy state of the system. Because symmetric electron-neutral constituents (hydrophobic) and asymmetric electron-rich hydrophilic materials are forced to associate.
What form does an ice-like inclusion produce? What is another term for this? Is it structurally identical to ice?
Semicrystalline.
Clathrate hydrates
No
What is the definition of Clathrate compounds?
Compounds not formed by the action of valence bonds but by molecular imprisonment
Clathratehydrate structures are important in stabilizing ________ structure around ________ moieties
Protein
Hydrophobic
Overall, the presence of any solute will tend to have a ______ effect on the structure of water.
disruptive
In all cases, the activity of water (its freedom of movement) is ______ when a solute is added
Reduced
When does the freezing or crystallization of water take place?
When the kinetic energy of the molecule is reduced to such an extent that a flickering cluster is able to become large enough to form a MINUTE CRYSTAL OR NUCLEUS.
What happens to water at 0 degrees celcius?
There is an equilibrium between crystal nucleation and liquid water.
In ultrapure water, what is difficult and does not readily take place? What can occur?
Nucleation
SUPERCOOLING
What is supercooling?
When the water stays liquid well below 0 degrees C
Supercooled water will freeze almost instantly if _______ or if an ________ is thrown in.
- Disturbed
2. Ice crystal
In normal water, what aids the start of nucleation?
Impurities such as dust or container wall surface
The nucleation process is a function of _________.
the rate of cooling
What is the result of a slow freezing rate of water? A rapid freezing rate?
A few large ice crystals
Many small ice crystals
How does slow freezing affect meat?
Larger ice crystals form, which can become large enough to burst the cell walls, causing physical damage that reduces water holding capacity and activates enzymes previously contained
How does rapid freezing affect meat?
Small ice crystals form uniformly through the product
Solutes interfere with the ________ process and also _________ the rate of diffusion of water to the ice crysta
nucleation
slow down
What happens to the solutes as the freezing process takes place?
They become more concentrated as the water diffuses to the surface of the crystal.
As the solutes become more concentrated, a _______ temperature is required for the ice to form.
lower
What is the eutectic point? What temperature range does it occur at?
When the temperature is low enough that the solution can become concentrated to a point at which the remaining water co-crystallizes with the solute crystals to start to grow and form.
-55C to -70C
When the eutectic point has been reached, what happens to all the water in the system?
immobilized
What are the conventional frozen storage tempuratures?
0F or -18C
Is total stability attained in a frozen food product?
No, only ~98% is immobilized
What becomes disrupted in cells when large ice crystal form?
Organelles
What concentrates in cells when large ice crystal form? Why does this concentrated solution not freeze totally?
Salts, ions, enzymes, and other solutes.
Because the temperature is well below the eutectic point.
What is interesting about water when large ice crystals form in cells and salts, ions and enzymes all concentrate?
Water is still available to act as a REACTANT and the REACTION MEDIUM is very concentrated.
Can reactions still take place when large ice crystals form in cells?
Yes, all types of reactions can still take place. Enzyme and substrates that are usually kept apart are mixed due to tissue disruption.
What can denature proteins in cells that have formed large ice crystals? Net result?
Ionic strength and pH changes . Deterioration
Do frozen foods have an infinite shelf life?
No b/c of large crystal formation which leads to a high solute in water concentration where many reactions can take place which ultimatly leads to deterioration
Because enzymatic reactions carry on under conditions of frozen storage, fruits and vegetables have to be ______ before being frozen.
blanched
What is blanching/
mild heat treatment
All reactions speed up when the _______ process is started.
thawing
During what step is a lot of a food’s quality lost relative to frozen storage?
Thawing
In what two ways can water be present in foods?
Bound or free
Free water vs bound water
Free: relatively mobile
Bound: effectively immobile
One way of assessing the mobility of water is to measure its ________.
Water activity
What is the formula for water activity? (Aw) ...where... P= P0= T=
Aw = P/(P0 x T)
P= partial pressure of the solution (or food matrix)...or...the vapor pressure above the solution. P0= partial pressure of pure water T= temp
What is the scale used to measure water activity?
0 to 1.0
What is the water activity of pure water?
1.0
A sealed container containing water comes to an equilibrium between what at any constant temperature?
Liquid water and its vapor form
What would happen if we measured the partial pressure of water vapor in the headspace over water?
We would obtain some constant value at a constant temp
Would the partial pressure (or RH) of a 25% NaCl solution be greater or higher than the partial pressure of a solely water solution? Why?
It would be reduced significantly because the salt, by being solvated, ties up some of the water, restricting its freedom of movement (activity) or mobility
The ratio of the partial pressures is a useful measure of the relative _____, _____, or _____of water.
mobility, activity, availablility
Any solute will _____ the activity of water.
Reduce
We can classify the water in foods based on their _____________.
Water activity ranges
What is Type IV water?
Pure water
Aw = 1.0
Does pure water exist in food systems?
No
What is the water activity of Type III water?
Aw = 0.99 - 0.80
Where can Type III water be found in animals/meat? Present in what? Diameter?
Trapped in the tissue matrix. (the water present in macrocapillaries > 1um in diameter)
How many solutes does Type III water contain?
Contains some solutes but not enough to immobilize the water in any major way.
In Type 3 Water activity range, many _________ are capable of growth, with some ________ and _______ inhibited at the lower end of the range
Microorganisms
Bacteria & yeasts
What 3 type or reactions proceed readily in Type III water activity range?
Hydrolytic, oxidative, enzymatic
Is the freezing point much lower in Type III water than in normal water?
no its not really that much lower
What is the water activity of type II water?
Aw = 0.80 - 0.25
What is the broad range of Type II Water?
From water in microcapillaries (<1 um in diameter) to outer layers of “bound water” (water hydrogen-bounded to solutes)
What is the microbial growth in Type II Water? What about molds?
Most microbial growth is halted, with the exception of some molds at the upper end of this range.
What is the freezing point of Type II Water relative to normal water?
significantly reduced
What happens to most enzymatic reactions requiring water in Type II Water?
Slowed significantly
Discuss non-enzymatic browning at both ends of the Type II Water range.
Non-enzymatic browning takes place readily with heat at the upper end of this range – it also occurs without heat but over time at the lower end of this range.
