HSN104 Science of Food week 1 - water

Question 1

What can you say about water’s melting and boiling points compared to molecules with similar weights?

Answer 1

They are both very high.

Question 2

What are the charges of protons, neutrons and electrons?

Answer 2

Protons: positive charge. Neutron: neutral. Electrons: negative charge.

Question 3

How is a covalent bond formed?

Answer 3

Atoms share electrons between them. Each atom contributes (an) electron(s).

Question 4

How is an ionic bond formed?

Answer 4

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.

Question 5

How does a covalent bond become polar?

Answer 5

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.

Question 6

How many valance electrons does oxygen have?

Answer 6

6

Question 7

What kind of molecular bond does sodium chloride have?

Answer 7

Ionic

Question 8

What kind of molecular bond does the water molecule have?

Answer 8

Polar covalent

Question 9

What kind of molecular bond does methane have?

Answer 9

Non-polar covalent

Question 10

What is methane’s chemical formula?

Answer 10

CH4

Question 11

What is table salt’s chemical formula?

Answer 11

NaCl

Question 12

How many lone pairs of electrons does the oxygen in a water molecule have?

Answer 12

2

Question 13

What is the shape of the water molecule?

Answer 13

Bent

Question 14

What are Van der Waals forces?

Answer 14

Temporary attraction by an atom’s nucleus of another molecule’s electrons.

Question 15

What is the bond angle of water?

Answer 15

104.5 degrees

Question 16

What is the bond angle of methane?

Answer 16

109.5 degrees

Question 17

What is the shape of the methane molecule?

Answer 17

Tetrahedral

Question 18

What bond angle does a bent molecule have?

Answer 18

104.5 degrees

Question 19

What bond angle does a tetrahedral shaped molecule have?

Answer 19

109.5 degrees

Question 20

Of covalent bonds, Van der Waals forces, and ionic bonds, which is the strongest?

Answer 20

Ionic bonds

Question 21

How many covalent electrons does sodium (Na) have?

Answer 21

1

Question 22

How many covalent electrons does chlorine (Cl) have?

Answer 22

7

Question 23

What is the bond angle on a pyramidal molecule?

Answer 23

107 degrees

Question 24

What shape is a molecule of ammonia?

Answer 24

Pyramidal

Question 25

How does the Van der Waals force change molecules' chemistry?

Answer 25

Through bending and/or stretching.

Question 26

What type of bond / force is hydrogen bonding?

Answer 26

Van der Waals force

Question 27

What causes hydrogen bonding?

Answer 27

Partially negative atoms in a polar molecule being temporarily attracted to a partially positive atom in another polar molecule.

Question 28

What causes Van der Waals forces?

Answer 28

Partially negative atoms in a polar molecule being temporarily attracted to a partially positive atom in another polar molecule.

Question 29

What is it called when the hydrogen atom in a water molecule is temporarily attracted to the oxygen atom in another water molecule?

Answer 29

Hydrogen bonding

Question 30

What causes water's high surface tension?

Answer 30

The surface molecules being pulled towards the inside molecules by the hydrogen bonds (Van der Waal forces).

Question 31

What is 'viscosity'?

Answer 31

Resistance to flow

Question 32

What happens to the boiling point if sugar is added to water?

Answer 32

It is raised (goes higher).

Question 33

What happens to the freezing point if salt is added to water?

Answer 33

Is is lowered.

Question 34

What is the term for the molecular organisation of ice?

Answer 34

Crystal lattice

Question 35

Why does ice float on water?

Answer 35

Hydrogen bonds: It is less dense than water as the crystal lattice arrangement pushes the hydrogens as far away from each other as possible.

Question 36

How much larger is the volume of ice compared to the volume of water with the same number of mole?

Answer 36

9%

Question 37

What does a molecule's coordination number mean?

Answer 37

The number of molecules arranged in an orderly fashion around it (e.g. the number of hydrogen bonds around a water molecule)

Question 38

What is frozen water's coordination number?

Answer 38

4

Question 39

What is water's coordination number at 1.5°C?

Answer 39

4.4

Question 40

What is water's coordination number at 83°C and above?

Answer 40

4.9

Question 41

Why does increased temperature influence a molecule's coordination number?

Answer 41

The molecules move faster, bump into each other more often, and form more connections.

Question 42

What does the triple point of water mean?

Answer 42

The point where all three phases of water co-exist (liquid, water, gas)

Question 43

What is the triple point of water?

Answer 43

0.006 atm, 0.01°C

Question 44

What does the symbol P stand for?

Answer 44

Pressure (in atmosphere)

Question 45

How do you calculate the dry basis moisture content of food?

Answer 45

Mass of water in sample / mass of (only the) solids in sample

Question 46

How do you calculate the wet basis moisture content of food?

Answer 46

Mass of water in sample / total mass of sample

Question 47

How do you calculate water activity in a food?

Answer 47

a(w)=(partial vapour pressure of water above food) / (pressure of vapour of pure water)

Question 48

What are the minimum and maximum values of water activity?

Answer 48

0 and 1

Question 49

Which figure denotes the higher amount of water activity, 0.1 or 0.9?

Answer 49

0.9 - a(w)=(partial vapour pressure of water above food) / (pressure of vapour of pure water)

Question 50

What is the vapour pressure of pure water at temperature T in the equation a(w) = P / P0?

Answer 50

1, so a(w) = P / P0 = P / 1

Question 51

In a moisture sorption isotherm, what are the types of water available?

Answer 51

Type I: tightly bound H2O (monolayer) Type II: hard to move H2O (hydrogen bonded) Type III: loosely bound H2O (available water)

Question 52

Is water activity in food directly influenced by water content?

Answer 52

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.

Question 53

What is water bound to polar groups such as sugar and protein in food called?

Answer 53

Vicinal water

Question 54

What is water formed in additional layers around polar groups in food called?

Answer 54

Multilayer water

Question 55

What is water held in cells or in gels (with polysaccharides) called?

Answer 55

Entrapped water

Question 56

What is water available to vapourise out of food called?

Answer 56

Free water

Question 57

What are the properties of viscinal water in food?

Answer 57

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.

Question 58

What are the properties of multilayer water in food?

Answer 58

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.

Question 59

What are the properties of entrapped water in food?

Answer 59

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.

Question 60

What are the properties of free water in food?

Answer 60

Properties as in pure water. Up to 96% of water in foods. Easy to get rid of.

Question 61

What is the minimum water activity rate for the survival of normal bacteria?

Answer 61

0.91

Question 62

What is the minimum water activity rate for the survival of normal yeast?

Answer 62

0.88

Question 63

What is the minimum water activity rate for the survival of normal moulds?

Answer 63

0.80

Question 64

What is the minimum water activity rate for the survival of halophilic bacteria?

Answer 64

0.75

Question 65

What is the minimum water activity rate for the survival of xerophilic moulds?

Answer 65

0.65

Question 66

What is the minimum water activity rate for the survival of osmophilic yeasts?

Answer 66

0.60

Question 67

What is halophilia (e.g. halophilic bacteria)?

Answer 67

Thriving in high salt conditions.

Question 68

What is xerophilia (e.g. xerophilic moulds)

Answer 68

Thriving in low water activity conditions.

Question 69

What is osmophilia (e.g. osmophilic yeast)

Answer 69

Thriving in high osmotic pressure conditions, e.g. high sugar concentrations (with low water activity).

Question 70

What is a bacteria called that can thrive in salty conditions?

Answer 70

Halophilic bacteria

Question 71

What is a mould called that can thrive in low a(w) conditions?

Answer 71

Xerophilic mould

Question 72

What is a yeast called that can live in high osmotic pressure conditions?

Answer 72

Osmophilic yeast

Question 73

At what water activity levels is lipid oxidation at its highest?

Answer 73

Both very low and very high a(w). It reduces to it's lowest levels around 0.3 - 0.5 a(w)

Question 74

Which of these grows under the lowest water activity level: yeast, mould, bacteria?

Answer 74

Mould (starts at approx 0.6 a(w)), then yeast (0.8), then bacteria (0.9).

Question 75

What does water activity have to do with enzyme activity?

Answer 75

The higher the water activity, the higher the level of enzyme activity, which speeds up the rate of reactions

Question 76

Which microbe needs a water activity level of 0.6 or above to grow?

Answer 76

Mould

Question 77

Which microbe needs a water activity level of 0.8 or above to grow?

Answer 77

Yeast

Question 78

Which microbe needs a water activity level of 0.9 or above to grow?

Answer 78

Bacteria

Question 79

At what water activity level can enzyme reactions occur?

Answer 79

At any water activity level. The rate of reactions increases as water activity level increases.

Question 80

At what water activity level does non-enzymatic browning peak?

Answer 80

0.6

Question 81

At what water activity levels does non-enzymatic browning occur?

Answer 81

Between approx 0.2 and 0.75. Peaks at 0.6

Question 82

Which bacteria is more likely to spoil food at lower water activity levels?

Answer 82

Halophilic bacteria

Question 83

Which mould is more likely to spoil food at lower water activity levels?

Answer 83

Xerophilic mould

Question 84

Which yeast is more likely to spoil food at lower water activity levels?

Answer 84

Osmophilic yeast

Question 85

Which bacteria is more likely to spoil food at higher water activity levels?

Answer 85

Normal bacteria

Question 86

Which mould is more likely to spoil food at higher water activity levels?

Answer 86

Normal mould

Question 87

Which yeast is more likely to spoil food at higher water activity levels?

Answer 87

Normal yeast