Physical properties and food dispersions

Transcription

1 Physical properties and food dispersions

2 Binding forces Van der Waals Ionic Covalent Hydrogen bonding

3 Van der Waals bonding + + Involves the attraction of one nucleus for the outer shell electrons around a different nucleus 1-3 kcal/mole

4 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

5 Ionic bonding M + X - Opposite charges attract each other (Coulomb s law) kcal/mole

6 Covalent bonding H 3 C.. CH 3 A sharing of electrons between two atoms. The sharing may be equal or unequal, depending on the atoms involved (their electronegativity).

7 Covalent bonding H 3 C CH kcal/mole

8 Effect of electronegativity on electron distribution Electronegativity: C = 2.55, Cl = 3.96 Image courtesy of edie.cprost.sfu.ca/~rhlogan/covalent.html

9 Hydrogen bonding O H H 2-10 kcal/mole

10 Types of associations Hydrophilic Water loving (polar) Salts, sugars, alcohols, etc. Hydrophobic Water hating Fats, oils, waxes, hydrocarbons

11 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

12 Water activity Water activity = a w a w = p/p o where p = water vapor pressure over a food and p o = water vapor pressure over pure water Or a w = ERH/100 where ERH = equilibrium relative humidity

13 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 a w 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

14 Look at water content Water content = 7% Water content = 15% Which way does water move and what is the result of that movement?

15 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

16 Look at water activity A w = 0.79 A w = 0.61 Which way does water move and what is the result of that movement?

17 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

18 Water activity Most chemical reactions stop at a w < 0.8 Bacterial growth stops at a w < 0.9 Molds and yeasts stop growing at a w < Enzymes can t act at a w less than 0.85 More on the importance of water activity

19 Water activity Image courtesy of

20 Moisture sorption isotherms Zone I monolayer water, very strongly associated Zone II multilayer water, less strongly associated Zone III bulk phase water, not associated

21 Water zones analogy Zone I = water in your skin (very tightly associated) Zone II = water in your swimming suit (less tightly associated) Zone III = water in the pool (not associated)

22 A w is also important because it influences the rates of chemical reactions in foods Figure from

23 Properties of water Freezing point (0 o C) and boiling point (100 o C) are much higher than other molecules of similar molecular weight Due to hydrogen bonding

24 Water shape and hydrogen bonding Water H-bonds

25 Water phase diagram At the triple point, all three phases exist at once.

26 Properties of water (cont.) Density -- water is most dense at 4 o 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.

27 Dipole moment of water O H H 1.82 debye

28 Dielectric constant Water has a relatively high dielectric constant and is very effective at solvating highly polar things like salts Na+ Cl-

29 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 1 o C Easier to think of as resistance to heat flow The bigger the specific heat number, the more resistance to heat flow

30 How do we tell we are heating something? 100 Temp 0 Time of heating

31 Latent heats (APK) Water has two important latent heats Heat of fusion ( H fus )=80cal/gram Amount of heat needed to melt one gram of ice at 0 o C to one gram of water at 0 o C Heat of vaporization ( H vap ) = 540 cal/gram Amount of heat needed to vaporize one gram of water at 100 o C to one gram of steam at 100 o C

32 Heat transfer problem Specific heat of water = 1 cal/gram/ o C Specific heat of steam = 0.5 cal/gram/ o C Specific heat of ice = 0.5 cal/gram/ o C For in-phase heat flows Q = mc T For phase transitions Q = m H (vap or fus)

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

34 Heat transfer problem Temp = -30 to 0 o C 2= melting at 0 o C 3 = 0 o to 100 o C 4 = vaporization at 100 o C 5 = 100 to 110 o C Time of heating

35 Heat transfer problem 1=(100g)(0.5cal/g/C)(30 o C) = 1500 cal 2=(100)(80 cal/g) = 8000 cal 3=(100g)(1cal/g/C)(100 o C) = 10,000 cal 4 = (100g)(540 cal/g) = 54,000 cal 5=(100g)(0.5cal/g/C)(10 o C) = 500 cal Total= 74,000cal = 74 kcal

36 Heat transfer problem Temp = 1500 cal 2= 8000 cal 3 = 10,000 cal 4 = 54,000 cal 5 = 500 cal Total = 74,000 cal Time of heating

37 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)

38 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

39 Solution colligative properties Vapor pressure Lowered by solute Boiling point Raised by 0.52 o C/mole of dissolved solute

40 Solution colligative properties Freezing point Lowered by o 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.

41 Osmosis

42 Colloidal dispersion stabilizing factors Brownian movement -- minor factor Electrical charge Water of hydration

43 Electrical charge

44 Electrical charge Like charges repel

45 Water of hydration Water of hydration Water of hydration physically interferes with molecular interaction

46 Physical state of phase classification Solid Liquid Gas Solid - Sol Solid aerosol Liquid - Emulsion - Gas Solid foam Foam -

47 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

48 Emulsions (Oil in water, O/W) oil oil Oil droplets want to coalesce--reduces surface tension

49 Emulsions (Oil in water, O/W) oil

50 Emulsions (Oil in water, O/W) oil Separates and floats to top because oil density is less than that of water

51 Emulsions (Oil in water, O/W) oil oil To prevent phase separation we need an emulsifier

52 Emulsifiers Emulsifiers have the following general molecular features head Polar head, likes water Non-polar tail, likes oil

53 Emulsions (Oil in water, O/W) oil oil What happens to this picture when we add emulsifiers?

54 Emulsions (Oil in water, O/W) oil oil This system is stabilized due to a lowering of surface tension by the emulsifier.

55 Emulsifier analogy Problem here? Yes! Problem here? No. Problem here? No.

56 Emulsifier analogy Go Purdue! Nuts!! Mr. Emulsifier

57 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-lipophile balance values will give some guidance HLB > 7 useful for o/w emulsions HLB < 7 useful for w/o emulsions W/O O/W

58 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

59 ph definition and food range ph = -log[h+] Limes Egg whites Acid Neutral Alkaline (basic)

60 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)

61 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 a w of greater than 0.85 An exception to this rule is tomatoes (ph < 4.7 are not low acid foods)