Water - Summary Flashcards
What are the three methods to reduce the water activity of a system?
- Freezing
- Concentration
- Dehydration
Oxygen has ___ electrons, ___ of which are valence electrons, and the two hydrogens have ___ electrons each
8
6
1
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
In the liquid phase, water is considered to have “_________” structure, termed ”__________”
transitional
flickering clusters
Flickering clusters (do/don’t) exist at the boiling point of water
do
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
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
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
- Host molecule: water
- Guest: non-polar gas or liquid
In the freezing of water, the flickering cluster becomes a _________ or ________, due to a decrease in _______
minute crystal
nucleus
kinetic energy
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
When the eutectic point has been reached, all the water in the system is ___________.
immobilized
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 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
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 is essentially immobile
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
Differentiate desorption and dehydration.
Desorption: food is moving towards equilibrium with the environment
Dehydration: permanent loss of
________ occurs when a dry food is placed in a wet environment
Adsorption
A plot of ___________ vs. ___ or _____ provides the moisture sorption isotherm
equilibrium moisture content
Aw
ERH
In moisture sorption isotherm, ______ commonly occurs when the curve is different dependent on whether the product starts out dry or moist
hysteresis
Which compound may be used to modify the Aw profile of a product?
Sorbitol
Aw has to be reduced down to about ________ to reach a minimum for most of the deteriorative reactions.
0.3-0.4
The use of multiple barriers based on Aw, pH, ionic strength, modification of atmosphere, etc. is called the __________ concept
hurdle
_________ refers to natural biological systems (plant and animal tissues), which are very complex
Intact edible tissue
