Water Flashcards
water is the primary determiner of
texture
What is the hybrid orbital theory of the water molecule?
Every orbital in the n=2 energy becomes hybridized, so that there is 1 1s-orbital from the oxygen alone that is filled, and four sp3-hybridized orbitals combined from 2 already filled orbitals from O and 2 half-filled together with two electrons from two hydrogens.
Why do the hydrogens of a water molecule have the bond angle 104,5 deg?
Because the lone pair electrons on the other side of the oxygen are very negatively charged and thus strongly repelling the hydrogens.
Why is water a dipole?
Oxygen is much more electronegative than hydrogen and attracts electrons stronger. Thus, the average electron density around the oxygen atom in a water molecule is about 10 times that around the hydrogen atoms.
what is an electric dipole?
a molecule with nonuniform distribution of + and - charges
what does the polarization of water molecules have to do with the properties of water?
everything! since this enables hydrogen bonding
what is the technical definition of surface tension?
the energy, or work, required to increase the surface area of a liquid due to intermolecular forces
why is the surface tension of water so high?
because of hydrogen bonding. water molecules on the surface of water are pulled downwards and to the side by hydrogen bonds, but not upwards (cannot hydrogen bond to the air), and are able to pack more densely at the surface than inside the “interior” of the water droplet. molecules are thus always drawn inwards
what is cohesion?
water molecules like to stay close together
what is adhesion?
water molecules are attracted to and stick to other substances/surfaces
why do water molecules in the interior of a water droplet have lower energy status?
since they are hydrogen bonded to a higher number of molecules
what is compressibility?
a measure of the volume change of a fluid (or solid) as a response to a pressure change
how is the specific heat of water?
it is high
what is specific heat?
the amount of heat (energy) needed to raise a one gram of a substance by one degree celsius
why does water have a high specific heat?
due to hydrogen bonds, a lot of heat/energy is required before bonds start to break and molecules start moving (at which point it starts to heat up) and so water can absorb a lot of heat energy before being heated itself.
why is the high specific heat of water useful?
our body temperature is more stable thanks to this
what is a solution?
a homogenous mixture of two or more substances
what is a solvent?
the component of a solution that is present in the greatest amount, or the substance in which the solute is dissolved
what is a solute?
the substance that is dissolved in a solvent
what is solubility?
maximum amount of a solute that can be dissolved in a solvent at a specific temperature and pressure
what factors affect nutrient solubility?
temperature (most of the vitamins and minerals we eat are soluble at body temperature) - concentration of solute in the solvent - solubility of the nutrients (molecular structure)
what are three types of water-compound interactions?
ion-dipole, dipole-dipole and hydrophobic interactions
why are salts soluble in water?
negative ions are attracted by partially positive hydrogen atoms AND the positive ion in the salt. positive ions are attracted by partially negative oxygen atoms AND the negative ion in the salt. whether the crystal dissolved is determined by which attractive force is stronger
what are some examples of dipole-dipole interactions with water?
ethanol and water, glucose and water (both soluble)
how does water behave in hydrophobic interactions?
water molecules shy away from the hydrophobic compounds ex. oil, forming a cage-like structure around the non-polar groups. this reduces entropy, but water and oil do not bind to each other.
what is the enthalpy change of solution?
the overall amount of heat which is released or absorbed during the dissolving process at constant pressure
why is the density of ice lower than the density of water?
when water freezes, it forms a rigid lattice that increases the space between each molecule as each binds to 4 other water molecules. in liquid form, water molecules move around closer to each other
what are some advantages of freezing foods?
lower temperature reduces spoilage as water is unavailable for microorganisms and chemical reactions. this increases shelf life. foods can also be frozen in a shorter time than they can be dried or canned.
what are some disadvantages of freezing foods?
energy intensive - texture may be affected negatively - storage space limited to capacity of freezer
foods freeze over a range of temperature. this range depends on
water content (and solutes) and cell composition
why is rapid freezing and storage without temperature fluctuations good?
small intracellular ice crystals form and minimum damage to cell membranes
what are 4 forms of water in foods?
free water - entrapped water - vicinal water - multilayer water
what is entrapped water?
immobilized within the lattice of larger molecules, capillaries and cells
what is vicinal water?
water held at hydrophilic siter on molecules like proteins to form water-ion and water-dipole or hydrogen bonds
what is multilayer water?
additional layers of water around the hydrophilic groups
how much is vicinal water in meat?
5-10 g/100 g protein
how much is multilayer water in meat?
50-60 g/100 g protein
properties of bound water
(vicinal) water that is not free to act as a solvent for salts and sugars because it has more structural bonding than liquid or free water. it can be frozen only at very low temperatures. it exhibits essentially no vapor pressure (does not evaporate). its density is high than that of free water since molecules are more tightly packed.
how is water binding to proteins affected by pH?
at pH below pI (ex. 5,1), proteins have positive net charges and mostly the oxygen in water will bind to proteins. at pI, net charge is neutral and little water will bind. at pH above pI, proteins have negative net charges and mostly the hydrogen in water will bind to proteins.
what is water activity?
a measure of the energy status of water in a system. vapour pressure resulting from evaporation of a liquid above a sample of the liquid in a closed container.
where is Aw=1 found?
in pure water
where is Aw=0 found?
in total absence of free water molecules
the water sorption isotherm describes..
thermodynamic relationship between water activity and the equilibrium of moisture content of a food product at constant temperature and pressure. also explain the level of water binding. at very low water contents it is tightly bound, midrange loosely bound, and at very high water content it is trapped or free.
what is the adsorption isotherm?
the curve describing water absorption
what is the desorption isotherm?
the curve describin water desorption
what is hysteresis?
the difference between the desorption and adsorption curves
why are isotherms valuable in product development?
by comparing the isotherms of different formulations, it is possible to determine if a product can be adjusted to allow higher moisture content at a given water activity or lower water activity at a given moisture content. the result can be a moister product that is still shelf stable
what are some limit points of m.o. growth concerning aw?
above 60% aw mold can grow. above 85% can majority of bacteria grow.
how are enzymes affected by aw?
most enzymes are inactivated when aw < 0,85 - lipases are still active at aw 0,3 or less. maillard reactions occur at aw 0,6-0,7
how can water activity be controlled?
by dehydration or addition of additives
what are some limitations in use of additives for control of water activity?
sensory changes, potential crystallization (sugar) or chemical reactivity like maillard reaction risk
