HSN104 Science of Food week 1 - water Flashcards
What can you say about water’s melting and boiling points compared to molecules with similar weights?
They are both very high.
What are the charges of protons, neutrons and electrons?
Protons: positive charge. Neutron: neutral. Electrons: negative charge.
How is a covalent bond formed?
Atoms share electrons between them. Each atom contributes (an) electron(s).
How is an ionic bond formed?
Formed when one or more electrons are transferred from one atom to another. The donor atom becomes positively charged (a cation); the receiving atom becomes negatively charged (an anion). The positive and negative atoms are attracted to each other and the ionic bond is formed.
How does a covalent bond become polar?
If one atom in the bond is more electro-negative than the other(s), it will draw the shared electrons to itself, causing it to gain a partial negative charge. The other atom(s) will gain a partial positive charge. This causes one side of the molecule to be differently charged from the other.
How many valance electrons does oxygen have?
6
What kind of molecular bond does sodium chloride have?
Ionic
What kind of molecular bond does the water molecule have?
Polar covalent
What kind of molecular bond does methane have?
Non-polar covalent
What is methane’s chemical formula?
CH4
What is table salt’s chemical formula?
NaCl
How many lone pairs of electrons does the oxygen in a water molecule have?
2
What is the shape of the water molecule?
Bent
What are Van der Waals forces?
Temporary attraction by an atom’s nucleus of another molecule’s electrons.
What is the bond angle of water?
104.5 degrees
What is the bond angle of methane?
109.5 degrees
What is the shape of the methane molecule?
Tetrahedral
What bond angle does a bent molecule have?
104.5 degrees
What bond angle does a tetrahedral shaped molecule have?
109.5 degrees
Of covalent bonds, Van der Waals forces, and ionic bonds, which is the strongest?
Ionic bonds
How many covalent electrons does sodium (Na) have?
1
How many covalent electrons does chlorine (Cl) have?
7
What is the bond angle on a pyramidal molecule?
107 degrees
What shape is a molecule of ammonia?
Pyramidal
How does the Van der Waals force change molecules’ chemistry?
Through bending and/or stretching.
What type of bond / force is hydrogen bonding?
Van der Waals force
What causes hydrogen bonding?
Partially negative atoms in a polar molecule being temporarily attracted to a partially positive atom in another polar molecule.
What causes Van der Waals forces?
Partially negative atoms in a polar molecule being temporarily attracted to a partially positive atom in another polar molecule.
What is it called when the hydrogen atom in a water molecule is temporarily attracted to the oxygen atom in another water molecule?
Hydrogen bonding
What causes water’s high surface tension?
The surface molecules being pulled towards the inside molecules by the hydrogen bonds (Van der Waal forces).
What is ‘viscosity’?
Resistance to flow
What happens to the boiling point if sugar is added to water?
It is raised (goes higher).
What happens to the freezing point if salt is added to water?
Is is lowered.
What is the term for the molecular organisation of ice?
Crystal lattice
Why does ice float on water?
Hydrogen bonds: It is less dense than water as the crystal lattice arrangement pushes the hydrogens as far away from each other as possible.
How much larger is the volume of ice compared to the volume of water with the same number of mole?
9%
What does a molecule’s coordination number mean?
The number of molecules arranged in an orderly fashion around it (e.g. the number of hydrogen bonds around a water molecule)
What is frozen water’s coordination number?
4
What is water’s coordination number at 1.5°C?
4.4
What is water’s coordination number at 83°C and above?
4.9
Why does increased temperature influence a molecule’s coordination number?
The molecules move faster, bump into each other more often, and form more connections.
What does the triple point of water mean?
The point where all three phases of water co-exist (liquid, water, gas)
What is the triple point of water?
0.006 atm, 0.01°C
What does the symbol P stand for?
Pressure (in atmosphere)
How do you calculate the dry basis moisture content of food?
Mass of water in sample / mass of (only the) solids in sample
How do you calculate the wet basis moisture content of food?
Mass of water in sample / total mass of sample
How do you calculate water activity in a food?
a(w)=(partial vapour pressure of water above food) / (pressure of vapour of pure water)
What are the minimum and maximum values of water activity?
0 and 1
Which figure denotes the higher amount of water activity, 0.1 or 0.9?
0.9 - a(w)=(partial vapour pressure of water above food) / (pressure of vapour of pure water)
What is the vapour pressure of pure water at temperature T in the equation a(w) = P / P0?
1, so a(w) = P / P0 = P / 1
In a moisture sorption isotherm, what are the types of water available?
Type I: tightly bound H2O (monolayer)
Type II: hard to move H2O (hydrogen bonded)
Type III: loosely bound H2O (available water)
Is water activity in food directly influenced by water content?
No. It is influenced by water availability in the food, i.e. how much water is available, how much is hard to move due to hydrogen bonds, and how much is tightly bound to other molecules.
What is water bound to polar groups such as sugar and protein in food called?
Vicinal water
What is water formed in additional layers around polar groups in food called?
Multilayer water
What is water held in cells or in gels (with polysaccharides) called?
Entrapped water
What is water available to vapourise out of food called?
Free water
What are the properties of viscinal water in food?
Bound to polar groups such as sugar and protein in food. Does not freeze at -40°C. Approx. 0.5% of water in foods. Hard to get rid of. Not available for reactions.
What are the properties of multilayer water in food?
Hydrogen bonded layers of water around polar groups. in food. Does not freeze at -40°C. Approx 3% of water in foods. Hard to get rid of. Not easily available for reactions.
What are the properties of entrapped water in food?
Held in cells or gels (with polysaccharides). Similar properties to dilute salt solutions (higher freezing and boiling points). Up to 96% of water in foods. Easy to get rid of.
What are the properties of free water in food?
Properties as in pure water. Up to 96% of water in foods. Easy to get rid of.
What is the minimum water activity rate for the survival of normal bacteria?
0.91
What is the minimum water activity rate for the survival of normal yeast?
0.88
What is the minimum water activity rate for the survival of normal moulds?
0.80
What is the minimum water activity rate for the survival of halophilic bacteria?
0.75
What is the minimum water activity rate for the survival of xerophilic moulds?
0.65
What is the minimum water activity rate for the survival of osmophilic yeasts?
0.60
What is halophilia (e.g. halophilic bacteria)?
Thriving in high salt conditions.
What is xerophilia (e.g. xerophilic moulds)
Thriving in low water activity conditions.
What is osmophilia (e.g. osmophilic yeast)
Thriving in high osmotic pressure conditions, e.g. high sugar concentrations (with low water activity).
What is a bacteria called that can thrive in salty conditions?
Halophilic bacteria
What is a mould called that can thrive in low a(w) conditions?
Xerophilic mould
What is a yeast called that can live in high osmotic pressure conditions?
Osmophilic yeast
At what water activity levels is lipid oxidation at its highest?
Both very low and very high a(w). It reduces to it’s lowest levels around 0.3 - 0.5 a(w)
Which of these grows under the lowest water activity level: yeast, mould, bacteria?
Mould (starts at approx 0.6 a(w)), then yeast (0.8), then bacteria (0.9).
What does water activity have to do with enzyme activity?
The higher the water activity, the higher the level of enzyme activity, which speeds up the rate of reactions
Which microbe needs a water activity level of 0.6 or above to grow?
Mould
Which microbe needs a water activity level of 0.8 or above to grow?
Yeast
Which microbe needs a water activity level of 0.9 or above to grow?
Bacteria
At what water activity level can enzyme reactions occur?
At any water activity level. The rate of reactions increases as water activity level increases.
At what water activity level does non-enzymatic browning peak?
0.6
At what water activity levels does non-enzymatic browning occur?
Between approx 0.2 and 0.75. Peaks at 0.6
Which bacteria is more likely to spoil food at lower water activity levels?
Halophilic bacteria
Which mould is more likely to spoil food at lower water activity levels?
Xerophilic mould
Which yeast is more likely to spoil food at lower water activity levels?
Osmophilic yeast
Which bacteria is more likely to spoil food at higher water activity levels?
Normal bacteria
Which mould is more likely to spoil food at higher water activity levels?
Normal mould
Which yeast is more likely to spoil food at higher water activity levels?
Normal yeast
