Physical properties and food dispersions

Binding forces

• Van der Waals

• Ionic

• Covalent

• Hydrogen bonding

Van der Waals bonding

+ +

Involves the attraction of one nucleus for theouter shell electrons around a different nucleus

1-3 kcal/mole

Van der Waals bonds• Van der Waals bonds (and hydrogen bonds)

are quite weak individually but when there are many of them they are collectively very strong. Much like the strings that bound Gulliver…

Ionic bonding

M+ X-

Opposite charges attract each other (Coulomb’s law)

20-30 kcal/mole

Covalent bonding

H3C. .CH3

A sharing of electrons between two atoms.The sharing may be equal or unequal, dependingon the atoms involved (their electronegativity).

Covalent bonding

H3C CH3

30-100 kcal/mole

Effect of electronegativity on electron distribution

Image courtesy of edie.cprost.sfu.ca/~rhlogan/covalent.html

Electronegativity: C = 2.55, Cl = 3.96

Hydrogen bonding

H

O

H

2-10 kcal/mole

Types of associations

• Hydrophilic– Water loving (polar)

• Salts, sugars, alcohols, etc.

• Hydrophobic– Water hating

• Fats, oils, waxes, hydrocarbons

Functions of water in foods (APK)

• A solvent for chemicals• A medium for heat transfer• A reaction medium• A chemical reactant or product• A texturizer or plasticizer• Influences food perishability

Water activity

• Water activity = aw• aw = p/po

– where p = water vapor pressure over a food and po = water vapor pressure over pure water

• Or aw = ERH/100– where ERH = equilibrium relative humidity

Water activity• Old concept

– Water activity was thought of as the ratio of “free” and “bound” water

• New concept– It is now regarded as a measure of the

energy status of water in the system– The closer the aw of a system is to 1 the

nearer the water in that system is to behaving like pure water

• This ratio affects many food properties, especially shelf life and texture

Look at water content

Water content =15%

Water content =7%

Which way does water move and what is the resultof that movement?

Based on water content

• We would predict movement of water from cake to icing until each was at about 11% water content

• The cake would dry out and the icing would become more soggy

Look at water activity

Aw = 0.61

Aw = 0.79

Which way does water move and what is the resultof that movement?

What actually happens?

• Water will move between the frosting and cake until their water activities are equal

• This results in water moving from the frosting into the cake. The cake gets more soggy and the frosting dries out.

• This is the opposite of what you would predict based on water content

Water activity

• Most chemical reactions stop at aw < 0.8• Bacterial growth stops at aw < 0.9• Molds and yeasts stop growing at aw <

0.8-0.88• Enzymes can’t act at aw less than 0.85

More on the importance of water activity

Water activity

Image courtesy of www.dfst.csiro.au/water_fs.htm

Moisture sorption isotherms

Zone Imonolayerwater, verystronglyassociated

Zone IImultilayerwater, less stronglyassociated

Zone IIIbulk phasewater, notassociated

Water zones analogy

Zone I = water in your skin(very tightly associated)

Zone II = water in yourswimming suit (less tightly

associated)

Zone III = water in the pool(not associated)

Aw is also important because it influences the rates of chemical reactions in foods

Figure from http://aqualab.decagon.com.br/educacao/measurement-of-water-activity-for-product-quality/

Properties of water

• Freezing point (0o C) and boiling point (100o C) are much higher than other molecules of similar molecular weight

• Due to hydrogen bonding

Water shape and hydrogen bonding

WaterH-bonds

Water phase diagram

At the triple point,all three phasesexist at once.

Properties of water (cont.)

• Density -- water is most dense at 4o C• Dipole moment -- Water has a relatively

large dipole moment, 1.82 debye. This means it interacts well with other highly polar molecules. Also, microwave cooking depends on the polarity of water.

Dipole moment of water

H

O

H

1.82 debye

Dielectric constant

• Water has a relatively high dielectric constant and is very effective at solvating highly polar things like salts

Na+ Cl-

Specific heat

• Water has a high specific heat, 1 cal/gram/degree C

• This is the amount of heat needed to raise the temperature of 1 gram of water 1o C

• Easier to think of as “resistance to heat flow”– The bigger the specific heat number, the

more resistance to heat flow

How do we tell we are heating something?

Temp

Time of heating

0

100

Latent heats (APK)

• Water has two important latent heats– Heat of fusion (Hfus)=80cal/gram

• Amount of heat needed to melt one gram of ice at 0o C to one gram of water at 0o C

– Heat of vaporization (Hvap) = 540 cal/gram• Amount of heat needed to vaporize one gram of

water at 100o C to one gram of steam at 100o C

Heat transfer problem

• Specific heat of water = 1 cal/gram/oC• Specific heat of steam = 0.5 cal/gram/oC• Specific heat of ice = 0.5 cal/gram/oC• For in-phase heat flows

– Q = mcT • For phase transitions

– Q = mH(vap or fus)

Heat transfer problem

• Given the information on the previous two slides, how much heat is required to convert 100 grams of ice at -30o C to 100 grams of steam at 110o C?

Heat transfer problem

Temp

Time of heating

0

100

1

1 = -30 to 0oC

2

2= melting at 0o C33 = 0o to 100o C

4

4 = vaporization at 100o C

5

5 = 100 to 110o C

Heat transfer problem

• 1 = (100g)(0.5cal/g/C)(30oC) = 1500 cal• 2 = (100)(80 cal/g) = 8000 cal• 3 = (100g)(1cal/g/C)(100oC) = 10,000 cal• 4 = (100g)(540 cal/g) = 54,000 cal• 5 = (100g)(0.5cal/g/C)(10oC) = 500 cal• Total = 74,000cal = 74 kcal

Heat transfer problem

Temp

Time of heating

0

100

1

1 = 1500 cal

2

2= 8000 cal33 = 10,000 cal

4

4 = 54,000 cal

5

5 = 500 calTotal = 74,000 cal

Dispersion of matter

• Classification systems– Particle size

– Physical state of the phases

• Any dispersion has at least two parts: the dispersed (discontinuous) phase and the continuous component (usually water or oil)

Particle size classification

• True solutions– Solutes are small molecules

• Colloidal dispersions– Solutes are large (macromolecular)

molecules• Suspensions

– Large pieces of material– Subject to gravitational settling

Solution colligative properties

• Vapor pressure– Lowered by solute

• Boiling point– Raised by 0.52o C/mole of dissolved solute

Solution colligative properties

• Freezing point– Lowered by -1.86o C/mole of dissolved solute

• Osmotic pressure– Relates to the movement of water across a

semi-permeable membrane. Movement of water (pressure) is always toward the higher concentration of solute.

Osmosis

Colloidal dispersion stabilizing factors• Brownian movement -- minor factor

• Electrical charge

• Water of hydration

Electrical charge

Electrical charge

Like charges repel

Water of hydration

Water of hydration physically interferes with molecular interaction

Water of hydration

Physical state of phase classification

Solid Liquid Gas

Solid - Sol Solidaerosol

Liquid - Emulsion -

Gas Solidfoam

Foam -

Examples of physical state classification foods• Sol -- protein in skim milk• Emulsion -- vinegar and oil dressing• Foam -- meringue• Solid foam -- bread, marshmallows, cakes• Solid aerosol -- smoke flavoring

Emulsions (Oil in water, O/W)

oil

oil

Oil droplets wantto coalesce--reducessurface tension

Emulsions (Oil in water, O/W)

oil

Emulsions (Oil in water, O/W)

oil

Separates and floats to top because oildensity is less than that of water

Emulsions (Oil in water, O/W)

oil

oil

To prevent phaseseparation we needan emulsifier

Emulsifiers

• Emulsifiers have the following general molecular features

head Polar head, likes water

Non-polar tail, likes oil

Emulsions (Oil in water, O/W)

oil

oil

What happens to thispicture when we addemulsifiers?

Emulsions (Oil in water, O/W)

oil

oil

This system is stabilizeddue to a lowering ofsurface tension by the emulsifier.

Emulsifier analogyProblem here? No.

Problem here? No.

Problem here? Yes!

Emulsifier analogy

GoPurdue!

Nuts!!

Mr.Emulsifier

Emulsifiers and HLB value• Deciding which emulsifier to use is

important and depends on whether you have an o/w emulsion or a w/o emulsion.

• Generally, hydrophile-lipophilebalance values will give some guidance– HLB > 7 useful for o/w

emulsions– HLB < 7 useful for w/o

emulsions

O/W

W/O

pH and foods

• pH influences– Pigment colors in meats, fruits, and

vegetables– Extent of Maillard browning– Texture in meats, fruits, and vegetables– Flavor– Enzymatic and microbial action

pH definition and food rangepH = -log[H+]

Limes Egg whites

1.8 9.5

NeutralAcid Alkaline (basic)

Buffers

• Buffers are mixtures of a weak acid and its salt or a weak base and its salt– These systems resist pH change– Most biological systems are buffered– Proteins are common food buffering agents

as they contain both carboxyl groups (capable of reacting with bases) and amino groups (capable of reacting with acids)

High acid/low acid foods

• This is extremely important in food processing (canning)– It is more difficult to kill bacteria in low acid

foods than in high acid foods– A low acid food is defined as having a pH of

greater than 4.6 and an aw of greater than 0.85

– An exception to this rule is tomatoes (pH < 4.7 are not low acid foods)