Water in Food

Question 1

Describe the role of water in food. [6]

Answer 1

• Ubiquitous!
  
  • Media for temperature
  • Important in flavour/taste
  • Important in safety
  • Solvent, reaction medium and reactant
  • Carrier of nutrients and waste products
  • Lubricant, plasticizer (water-soluble materials are physically transformed), stabilizer

Question 2

What is a plasticizer?

Answer 2

Materials that are water-soluble are physically transformed (usually using differences in solubility)

Question 3

Describe water in food systems. [4]

Answer 3

• Continuous phase (aqueous) in dispersions and solutions
• Water entrapment in 3D continuous phases
• Entrapment in cells
• Attraction to hydrophilic compounds

Question 4

What is free water? [3]

Answer 4

• Lightly entrapped
• Acts as dispersing agent and solvent
• Can be removed by drying

Question 5

What is adsorbed water? [3]

Answer 5

• Associates in layer through intermolecular hydrogen bonds around hydrophilic food particles.
• Water is held tightly in cell walls (protoplasm) and is bound to proteins
• Not free; not heavily bound

Question 6

What is bound water? [4]

Answer 6

• Unavailable as a solvent
• Tightly bound, remains unfrozen below 0C.
• ~8-10% of total water in animal tissues is bound
• Unfreezable water is based on protein content; varies

Question 7

How does water influence the physical state of materials?

Answer 7

Dehydration is the rapid removal of water and transformation of the solutes back to an amorphous state, leaving large internal surface areas.

Water is a plasticizer whereby materials that are water-soluble are physically transformed.

Question 8

What are the three forms of water in food?

Answer 8

Free water; dispersing agent & solvent; can be removed by drying

Adsorbed water; intermolecular hydrogen bonds form around hydrophilic molecules (i.e., hydration shell); or capillary entrapment; tightly held water unavailable for ice formation or microbe use

Bound water; remains unfrozen below 0C; bound to monolayer; unavailable for freezing

Question 9

Describe the structure of a water molecule.

Answer 9

• Water molecule is composed of a single atom of oxygen covalently bound to two atoms of hydrogen.
• Unequal distribution of positive and negative charge makes the water molecule bipolar.
• The charges cause a bond angle of 104.5; giving approximate tetrahedral structure.
• Each hydrogen atom shares a pair of electrons with the oxygen atom (= covalent bond)
• Water molecules are held together by hydrogen bonding.

Question 10

Describe the water molecule in ice.

Answer 10

• Each water molecule is associated with 4 other water molecules in a hexagonal fashion, with an open network structure
• The volume of water expands as temperature drops and as heat is withdrawn
• If you apply heat, ice melts, due to molecular motion that puts stress on the strength of the hydrogen bonds (affects density and viscosity of water)

Question 11

Describe the 3 phases of water.

Answer 11

Question 12

What happens to entropy when solutes are dissolved in water? What does this cause? [2]

Answer 12

Entropy decreases since water is oriented with respect to solute.

Water molecules are less free to escape from liquid in vapour phase; thus, vapour pressure is lowered.

This causes (1) depression in freezing point and (2) elevation in boiling point.

Question 13

Compare the physical properties of water and ice.

Answer 13

Heat capacity of water is greater than ice.

Question 14

What is the difference between latent heat and sensible heat.

Answer 14

Latent heat: energy needed to induce a phase change without changing temperature

Sensible heat: change in temperature of a substance without phase change

Question 15

How is freeze drying accomplished?

Answer 15

A vacuum reduces pressure then heated slightly resulting in dehydration by sublimation

Question 16

Discuss supercooled liquid water.

Answer 16

• Can be as cold as -48C.
• Ice from liquid water requires a ‘seed crystal’ to become the nucleus around which other crystals form
• Hard to achieve in pure water with no contaminants

Question 17

How many crystalline polymorphic structures can ice form?

Answer 17

8 crystalline

Non-crystalline structures also exist

Only relevant polymorph is Ice-Ih (ordinary hexagonal ice)

Ice holds shape as atoms or molecules are not free to move about but merely vibrate in fixed crystal lattices.

Question 18

Describe the initial events of ice crystallization. [4]

Answer 18

1. The first thermal event that occurs is super-cooling below the freezing point before the induction of crystallization.
2. This is a non-equilibrium, metastable state which is analogous to an activation energy necessary to initiate nucleation.
3. Pure water can be under-cooled by several degrees before nucleation
4. Water, with a tight tetrahedral structure, crystallizes to open hexagonal system.

Question 19

What are the events of ice crystal formation? [4]

Answer 19

1. Undercooling
2. Nucleation (generates latent heat of crystallization)
3. Crystal growth
4. Sensible heat

Question 20

Describe ice formation in a sugar solution as compared to water.

Answer 20

Faster nucleation in presence of sugar; latent heat of crystallization is not as great because it starts to freeze and crystallize as a faster rate; freezing point depression to Tf.

Solutes depress the freezing point of water because they disrupt the hydrogen bonding that occurs between water as it begins to freeze into ice; solutes lower free energy of the system, therefore facilitating faster nucleation and enhanced crystallization.

Sugar solution doesn’t need to undercool as far, nucleation is faster, and nuclei that are formed are preserved resulting in better quality ice. Results in smaller, more homogenous, and greater in number (= more stable ice phase)

Question 21

How is freezing time defined?

Answer 21

Onset of nucleation to end of crystal growth

Question 22

Describe how freezing can affect the structure of foods.

Answer 22

Disruption of cell walls and membranes, for example.

Question 23

Why is faster freezing preferable?

Answer 23

Produces small ice crystals; greater nucleation (results in more uniform/dense ice crystals and better organoleptic properties)

Minimizes concentration effects by increasing the time solutes contact other food constituents during transition from unfrozen to frozen.

Question 24

Describe water interactions with ions and ionic groups that have polarizing power/electrical field strength.

Answer 24

Water’s tetrahedral arrangement is disrupted.

Entropy decreases.

Question 25

Describe water interactions with nonpolar groups.

Answer 25

Leads to 'hydrophobic hydration' due to hydrophobic effect.

Question 26

Describe water interactions with neutral groups. (hydrophilic and nonionic)

Answer 26

Hydrogen bonding occurs with hydroxyl, amino, carbonyl, amide, and imino groups; may lead to ‘water bridges’

Question 27

Why does sucrose dissolve more slowly than salt?

Answer 27

The van der waals interactions that hold sucrose molecules together are not as strong as the ionic bonding of salt. ## Footnote NaCl crystals are made of Na and Cl ions, which are charged entities. Water is also a polar molecule, and multiple water molecules can stabilize ions by forming a shield of sorts. A crystal of sugar, on the other hand, is of sugar molecules, which are single, non charged, but polar units. Simply put, NaCl(aq) is a much more tightly held state with water than sugar(aq), because ion water bonds are stronger and easier to form the sugar's dipole bonds.

Question 28

Describe the importance of the hydrophobic effect for protein spatial configuration and stability.

Answer 28

A nonpolar moiety imposes an increase in order of water; water forces nonpolar groups to aggregate; facilitated by release of water from low-energy hydration shells to high-energy free state.

Question 29

What is water activity?

Answer 29

Quantifies water available for microbial or enzymatic activity. The tendency of water molecules to escape the food from liquid to vapour. Aw = p/po p=partial vapour pressure of water in food po = partial vapour pressure of pure water at similar temperature

Question 30

Why is water activity important for product formulation?

Answer 30

For reasons like: * Inhibition of microbial growth * Shelf-life extension * Moisture migration inhibition (e.g., from raisins to bran; higher aw to lower aw)

Question 31

What is Raoult's law?

Answer 31

Relationship between concentration of solutes and vapour pressure. An ideal solute; relative lowering of vapour pressure is equal to the mole fraction of solute.

Question 32

Estimate a_w of 10% mass of sucrose solution. MW (water) = 18 kg/kgmol MW (sucrose) = 342 kg/kgmol

Answer 32

mole fraction = mass fraction / molecular weight mole fraction of water = 90/18 mole fraction of sucrose = 10/342 a_w = moles water / (moles water + solute) **a_w = 0.99**

Question 33

What is hysteresis? What is it an indicator of? What is it defined by?

Answer 33

The difference between desorption and adsorption isotherms. Indicator of food stability; defined by chemical composition of food as well as T and P.

Question 34

What is a moisture sorption isotherm?

Answer 34

Water activity relates to the moisture content of the food and can be measured using a moisture sorption isotherm.

Question 35

What is the BET-point?

Answer 35

The amount of water needed to form the monolayer. The point where type I water becomes type II.

Question 36

Describe type I water.

Answer 36

Bound water No chemical reactions can occur; no freezing can occur Strongly bound monolayer; not sufficient to coat solutes Water is adsorbed on the surface of macromolecular colloids **Water of hydration** Forces include hydrogen bonds and van der waals forces

Question 36

Describe type I water.

Answer 36

Bound water No chemical reactions can occur; no freezing can occur Strongly bound monolayer; not sufficient to coat solutes Water is adsorbed on the surface of macromolecular colloids **Water of hydration** Forces include hydrogen bonds and van der waals forces

Question 37

Describe type II water.

Answer 37

Monolayer Moisture content Restricted mobility Less strongly bound water layers and capillary adsorbed water Monolayer is adsorbed First multilayers are added

Question 38

Describe type III water.

Answer 38

Solvent and free water Freezable; free for use by microbial life Water fills intergranular spaces and spaces in pores Mobile dispersing agent and solvent Food in this zone requires refrigeration More water than binding sites

Question 39

# Define: Sorption Adsorption Desorption

Answer 39

Sorption: process wherein food solids reversibly combine with water molecules Adsorption: humidification Desorption: drying

Question 40

How does water activity relate to temperature at constant moisture content?

Answer 40

Food water activity increases with increasing temperature (for any given moisture content)

Question 41

Why is water content not a reliable predictor of safe and stable food systems?

Answer 41

Moisture content can vary from product to product and does not inform on ‘available’ water.

Question 42

Describe the effect of water content on water activity. What forces control this? What reactions are facilitated?

Answer 42

Water activity will be higher when a certain water content is achieved by absorption of water from a dry food. Controlled by three forces: (1) osmotic, (2) matrix, and (3) capillary. This facilitates many reactions: non enzymatic browning fat oxidation protein denaturation starch retrogradation vitamin degradation enzymatic reactions starch gelatinization

Question 43

Describe the effect of solute concentration on water activity.

Answer 43

Higher solute concentration decreases water activity. Very solute specific in terms of effect.

Question 44

Discuss water activity and the stability of food.

Answer 44

* Higher water activity leads to lower stability * Lipid oxidation reaction rate is greatest at lower aw (and higher aw) * aw\>0.45 leads to increased reaction rates for non-enzymatic browning, water-soluble nutrient loss, enzymatic activity, mold, yeast, and bacterial growth * Higher aw solubilizes substrates required for chemical and enzymatic activity

Question 45

Describe water activity and texture.

Answer 45

High aw foods = juicy, tender, chewy (decrease leads to toughness/staleness) Low aw foods = crunchy (increase leads to soggy)

Question 46

Discuss MR, vitamin B1 degradation, and microbial growth in regards to water activity.

Answer 46

Curves for MR, vitamin B1 degradation and microbial growth exhibit maximal rates at intermediate to high aw

Question 47

Why are peanuts salted and roasted

Answer 47

Salted to prevent rancidity of the oils; desorption is 3-6x faster in peanuts; catalysts are no longer soluble; toasted to denature enzymes.

Question 48

How may water activity be reduced?

Answer 48

Drying Evaporating Increasing solute concentration Freezing (as fast as possible to promote small ice crystals) Kill animals quickly since anaerobic conditions release water from proteins

Question 49

Describe how water activity predicts shelf-life.

Answer 49

Food packaging considerations: the rate of exchange of moisture through a package barrier can change the water activity of the food and influence its shelf life. Phosphates are added to meat to entrap water and prevent water loss, for example.

Question 50

What is moisture migration?

Answer 50

Multi-component products; different components = different energy status of the water = driving force for moisture migration to come to an equilibrium Moisture migrates from high aw to low aw.

Question 51

What water activity will microbial proliferation occur?

Answer 51

a_w \> 0.5 Pathogenic bacteria have the highest a_w requirements.

Question 52

Discuss water in oil.

Answer 52

Even oil is capable of holding some water; max amount = saturation point (further water separates and forms distinct layer) Water activity in oil can be measured; takes a long time to equilibrate

Question 53

How is water activity measured?

Answer 53

Capcitance hygrometer: consists of two charged plates separated by a dielectric membrane which when water is absorbed, a change in capacity to hold charge increases and the capacitance is measured. Roughly proportional to the water activity as determined by sensor-specific calibration. Requires equilibrium with vapour pressure of water in the sample with vapour pressure of the headspace.

Question 54

What are potentially hazardous foods?

Answer 54

Need to know pH and a_w to ensure safety of a product. PHF require time/temperature controls for safety to limit pathogenic organism growth or toxin formation. Non-pathogenic foods have a_w = 0.85 below a_w for *S.aureus*, and pH = 4.6 below pH for *C.botulinum* growth and toxin production.

Question 55

Explain the lipid oxidation curve.

Answer 55

* At very low water activity the reducing capacity of water does not exist; lipids oxidize in an auto-oxidation propagation reaction * As water activity increases, neutralize the peroxy-radicals that are produced, less lipids are oxidized * At higher water activity, enzymes that accelerate the reaction are solubilized * At extremely high water activity, enzymes and substrates are diluted.