Water Quality use, function, water activity

Water Quality use, function, water activity

1. WATERWATER
1

2. WATER'S IMPORTANCEWATER'S IMPORTANCE
2
1.Solvent
◦Most molecules dissolved in water
2.Reactant
◦Water's involvement in hydrolysis reactions
3.Product
◦Water's involvement in condensation reactions
4.Heat transfer medium
◦boiling, steaming, cooling

3. WATER'S IMPORTANCEWATER'S IMPORTANCE
5.Texture
◦Juiciness, mouthfeel
Snack foods
Vegetables
Meat
6.Preservation
◦Highly perishable foods usually have high water activity
E.g. bread vs. cracker or cereal
7.Economics
◦More water added = more $
UNDERSTANDING THE PHYSICAL AND
CHEMICAL PROPERTIES OF WATER IS
IMPORTANT IN THE STUDY OF FOOD AND
PROCESSING
3

4. PHYSICAL & CHEMICAL PROPERTIES OF PHYSICAL & CHEMICAL PROPERTIES OF
WATERWATER
Compound Melting point Boiling point
H
2O 0ºC 100ºC
H
2
S -83ºC -60ºC
NH
3
-78ºC -33ºC
Methanol -98ºC 65ºC
4
Water has very unique properties not shared by other
similar hydrogen compounds or compounds of similar
weight
Why? – this is explained by the unique structure of H
2O

5. STRUCTURE OF WATERSTRUCTURE OF WATER
5
Tetrahedral arrangement
Two free electrons of O
act as H-bond acceptors
while H acts as donor
Highly electronegative O
pulls electrons from H,
making H behave like a
bare proton
Forms a dipole because
of the electronegative O

6. STRUCTURE OF WATERSTRUCTURE OF WATER
6
Because of the DIPOLE
and TETRAHEDRAL
structure we can get
strong H-bonding
Water capable of
bonding to 4 other water
molecules
Unique properties of
water from other
hydrides
H-bond NOT a static
phenomenon
◦T dependent

7. PHASE CHANGES OF WATERPHASE CHANGES OF WATER
7
P
r
e
s
s
u
r
e
Temperature

8. WATER VAPORWATER VAPOR
Water is “free” and devoid of any H-bonds
◦Large input of energy needed
endothermic process
◦Large dissipation of same energy needed to make
water lose kinetic energy
exothermic process
Waters latent heat of vaporization is
unusually high
◦to change 1 L from liquid to vapor need 539.4
kcal
8

9. LIQUID WATERLIQUID WATER
T (ºC) Density (kg/m
3
) Viscosity (m
2
/s)
0 999.9 1.7895
5 1000.0 1.535
25 997.1 0.884
100 958.4 0.294
9
Extensive H-bonding
H-bond formation dependent on T
◦With increasing T get more mobility and increased fluidity

10. ICEICE
10
Forms when exactly 4 H-bonds are
formed between water molecules
◦2.78 A vs. 2.85 A in liquid
◦To get this order a lot of energy
needs to be adsorbed by the
environment
The strong H-bonding in ice forms
an orderly hexagonal crystal lattice
◦6 H
2O molecules
Has 4X more thermal conductivity
than water at same temperature

11. 11
Can go from ICE to GASCan go from ICE to GAS
P
r
e
s
s
u
r
e
Temperature
Basis for Freeze Drying
Sublimation

12. PROPERTIES OF ICEPROPERTIES OF ICE
Crystallization
◦Crystal growth occurs at freezing point
◦Rate of crystal growth decreases with decreasing temperature
◦Solutes slow ice crystal growth
Nucleation - affects ice crystal size.
◦Slow freezing results in few nucleation sites and large, coarse
crystals
◦Fast freezing results in many nucleation sites and small, fine
crystals
◦Heterogeneous nucleation
usually caused by a foreign particle, such as salt, protein, fat, etc.
◦Homogeneous nucleation
very rare, mainly occurs in pure systems
12

13. PROPERTIES OF ICEPROPERTIES OF ICE
SUPERCOOLING
◦Water can be cooled to temperatures below
its freezing point without crystallization
◦When an ice crystal is added to supercooled
water, temperature increases and ice
formation occurs
13
1.http://www.youtube.com/watch?v=czmQ2_ymaOo
2.http://www.youtube.com/watch?v=gGpNhBPYNfs&feature=related
3.http://www.youtube.com/watch?v=DpiUZI_3o8s

14. PROPERTIES OF ICEPROPERTIES OF ICE
Freezing induced changes
in foods (examples)
Destabilization of emulsions
Flocculation of proteins
Increased lipid oxidation
Meat toughening
Cellular damage
Loss of water holding
capacity
14
Example: Effect of freezing on seafoods

15. WATER SOLUTE INTERACTIONSWATER SOLUTE INTERACTIONS
Association of water to hydrophilic
substances
◦Bound water - occurs in vicinity of solutes
Water with highly reduced mobility
Water that usually won't freeze even at -40ºC
Water that is unavailable as a solvent
◦“Trapped” water
Water holding capacity
Hydrophilic substances are able to entrap large
amounts of water
Jellies, jams, yogurt, jello, meat
Yogurt - often see loss of water holding as whey is
released at the top of the yogurt
15

16. WATER SOLUTE INTERACTIONSWATER SOLUTE INTERACTIONS
16
Ionic polar solutes
◦React readily with water and most are usually
soluble in water
◦Water HYDRATES the ions
◦Charge interactions due to waters high
DIELECTRIC CONSTANT
Can easily neutralize charges due to its high dipole
moment
Large ions can break water structure
◦Have weak electric fields
Small ions can induce more structure in water
◦Have strong electric fields

17. WATER SOLUTE INTERACTIONSWATER SOLUTE INTERACTIONS
17
Nonionic polar solutes
◦Weaker than water-ion bonds
◦Major factor here is H-bonding to the polar site
◦Example: SUCROSE
4-6 H
2O per sucrose
Concentration dependent
>30-40% sucrose all H
2O is bound
T dependent solubility
◦C=O, OH, NH
2 can also interact with each other and therefore water
can compete with these groups
◦H-bond disrupters
urea - disrupts water
Water bridge

18. WATER SOLUTE INTERACTIONSWATER SOLUTE INTERACTIONS
18
Nonpolar
◦Unfavorable interaction with water
◦Water around non-polar substance
is forced into an ordered state
Water affinity for water high
compared to non-polar compound
Water forms a shell
Tries to minimize contact
◦Hydrophobic interactions
Caused because water interacts with
other water molecules while
hydrophobic groups interact with
other hydrophobic groups

19. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
19
Boiling point
Vapor pressure is equal to
atmospheric pressure
Strongly influenced by water -
solute interaction
◦Solutes decrease vapor pressure and
thus increase boiling point
Sucrose  +0.52ºC/mol
NaCl  +1.04ºC/mol
ATMOSPHERIC PRESSURE
VAPOR PRESSURE

20. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
20
Freezing point lowering
Freezing point can get extensive
depression via solutes
Alter ability of water to form
crystals due to H-bond disruption
Sucrose  -1.86ºC/mol
NaCl  -3.72ºC/mol
◦Eutectic pt - temp.
Where “all” water is frozen - usually
around -50ºC
◦In most cases small amounts of
water remains unfrozen (-20ºC)
These small patches of water can
promote chemical reactions and
damage

21. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
What explains all this?
Raoult's law
P = P
*
/X
1
or
P
*
-P/P
*
= x/55.5M
P = vapor pressure of solution; P
*
= vapor pressure
pure solvent; X
1
= mole fraction of solute; x = grams
solutes in solution; 55.5M = moles of water per liter
This relationship is not only important for explaining
the concepts of depressing freezing point and elevating
boiling point
◦Also explains the concept of water activity
21

22. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Osmotic pressure of solutions
There is a tendency for a system containing water and a
solution separated with a membrane to be at equilibrium
The pressure needed to bring the two solutions at
equilibrium is called OSMOTIC PRESSURE
The more the solution has of dissolved solutes (e.g. salt) the
higher its osmotic pressure
Can use this in food processing and preparation
◦E.g. Crisping salad items
Increase turgor
22

23. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
23
Surface tension
Water surface behaves
differently than bulk
phase
◦Like an elastic film
◦Due to unequal inward
force
◦Resist formation of a new
surface thus forming
surface tension
1.http://www.youtube.com/watch?v=45yabrnryXk&feature=fvw
2.http://www.youtube.com/watch?v=76CNkxizQuc&feature=related

24. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Water has high surface tension
◦72.75 dynes/cm (20ºC)
Because of the high surface tension
special considerations are needed in food
processing
To affect it one can:
◦Increase T (more energy)  reduces surface
tension
◦Add solutes
NaCl and sugars  increase surface tension
Amphipathic molecules  reduce surface tension
24

25. 25
PhotoFrost®

26. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Ionization of water
Water can ionize into hydronium (H
3O
+
) and hydroxyl (OH
-
) ions
◦Transfer of one proton to the unshared sp
3
orbital of another
water molecule
Pure water: K
eq = Equilibrium (or ionization) constant
K
eq = [H
3O]
+
[OH]
-
[H
2
O]
[H
3O]
+
[OH]
-
= K
eq = K
w (Water dissociation constant)
[10
-7
] [10
-7
] = [10
-14
]
26

27. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Acids and bases in food systems
◦Acid - proton donor
NH
3 + H
2O  NH
4
+
+ OH
-
◦Base - proton acceptor
CH
3COOH + H
2O  CH
3COO
-
+ H
3O
+
◦Weak acids and bases
Most foods are weak acids
These constituents are responsible for buffering of
food systems
◦Some examples
Acetic, citric, lactic, phosphoric, etc.
27

28. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Acids and bases in food systems
◦Is there a difference between weak and strong acids?
Strong acids
When placed in solution, 100% ionized
Weak acids
When placed in solutions weak acids form an equilibrium
28
HCl = H
+
+ Cl
-
pH = -log [acid] = -log [H]
+
HOAC H
+
+ OAC
-
pKa = -log Ka
Keq = [H]
+
[OAC]
-
[HOAC]

29. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Weak acids and bases
◦One cannot relate pH to concentration for weak acids
and bases because of this equilibrium
◦One must understand how the acid behaves in solution
◦Knowing the dissociation constant of the acid is
important to determine the effect on the pH of the
system
◦The relationship of pH for weak acids and bases relies
on the Henderson - Hasselbach equation:
29
pH = pKa + log [salt]
[acid]

30. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
30
Weak acids
◦Graphically behave
like the figure when
titrated with a
strong base. The
reverse holds true
for weak bases

31. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
31
Buffering
◦Buffers resist
changes in pH
when acids and
bases are added
◦Characteristics of
a buffer
Maximum when
pH = pKa or
when [acid] =
[salt]
Rule of thumb:
pH =
pKa ± 1
What is this point and its
significance to food systems?

32. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
32
Let’s return to
Henderson-Hasselbach
pH = pKa + log [salt]
[acid]
K
1
= 4.6 x 10
-3
; K
2
= 2.04 x 10
-10
pH
Equivalents OH
-
O
NH
2
CH
3
OH

33. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
33
Let’s return to
Henderson-Hasselbach
pH = pKa + log [salt]
[acid]
K
1
= 4.6 x 10
-3
; K
2
= 2.04 x 10
-10
pH
Equivalents OH
-
O
NH
2
CH
3
OH
O
NH
3
+
CH
3
OH
2.5 meq
O
NH
3
+
CH
3
O
-
O
NH
3
+
CH
3
OH
1.25:1.25 meq
O
NH
3
+
CH
3
O
-
2.5 meq
O
NH
2
CH
3
O
-
O
NH
3
+
CH
3
O
-
1.25:1.25 meq
O
NH
2
CH
3
O
-
2.5 meq
O
NH
3
+
CH
3
OH
O
NH
3
+
CH
3
O
-
O
NH
2
CH
3
O
-
pK
1 pK
2
OH
-
OH
-

34. 34

35. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
35
Examples of natural pH control
◦Fruits - citric, malic, acetic, etc
Microbial control
Flavoring
◦Milk – pH around 6.5
Controlled by three components
Phosphate, citrate, carbonate
◦Eggs
Fresh eggs - pH = 7.6
After storage for several weeks - pH = 9-9.7
Due to loss of CO
2
Problem - Loss of carbohydrate groups on proteins. Loss of
protein functionality, causing decreased viscosity and poor
foaming properties

36. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Examples of “man made” pH control
◦Food additives - ACIDULANTS
Citric acid - pectin jellies
pH must be around 2.9-3.0
Also provides balance between tartness and sweetness
Yogurt and cottage cheese
Fermentation - glucose or lactose to lactic acid
pH reduction to around 4.6 will cause the gelation
Can add acidulants to imitate dairy yogurts - lactic, citric,
phosphoric, HCl
Cheese
Alkaline salts of phosphoric acid to get good protein dispersion
Thermal process control
pH below 4.5 usually hinders C. botulinum growth
Less severe heat treatment required for these
Acidulants used to lower pH below 4.5 for some fruit and
tomato products
36

37. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Examples of “man made” pH control
◦Acidulants - leavening agents
Used in the baking industry to give rise (release of
CO
2
) - alternative to yeast
When HCO
3
-
becomes acidic (pH < 6), CO
2
forms,
CO
2
not very soluble so released as a gas
Overall eq: H
+
+ HCO
3
-
H
2O + CO
2
37

38. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Examples of “man made” pH control
◦Leavening systems
Bicarbonate (NaHCO
3
) - source of HCO
3
and CO
2
Leavening acids
Drive bicarbonate (HCO
3) to CO
2
Rate of acid release varies and therefore CO
2 release
Phosphate - rapid release of CO
2
Sulfate – slow release of CO
2
Pyrophosphate - can be cleaved by phosphatases
becoming more soluble - used in refrigerated doughs
-Glucono-lactone - used in refrigerated doughs
38

39. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
Examples of “man made” pH control
◦Acidulants - antimicrobials
pH is important for two reasons: 1. Solubility and 2. Activity
The salt is more soluble in aqueous systems
The acid is more active in its antimicrobial efficiency
Benzoic acid (0.05-0.1%)
Found naturally in prunes, cranberries, cinnamon and cloves
Active below pH 4 (active acidic form of the salt)
Highly soluble in the form of sodium salt
Effective - yeasts and bacteria, less for molds
Uses in acid foods - soft drinks, juices, pickles, dressings etc.
Parabens or r-hydroxybenzoate esters (0.05-0.1%)
Broader pH range (active at higher pH)
Mainly use methyl and propyl esters
Uses in baked goods, wines, pickles, jams, syrups, etc.
39

40. EFFECT OF SOLUTES ON WATEREFFECT OF SOLUTES ON WATER
◦Acidulants - antimicrobials
Sorbic acid (Na
+
and K
+
salt forms) (0.02-0.3%)
Max activity at pH 6.5; active at acid pH values
Most effective for yeast and molds
Inhibit, not inactivate
Uses in cheese, juices, wines, baked goods, etc.
Proprionic acid (proprionate) Ca
2+
salt
Active up to pH 5
Uses in breads (retards Bacillus) which causes ropiness in breads
Ropiness - thick yellow patches that can be formed into a rope-like
structure making the bread inedible
Acetic acid
Nitrites and Nitrates
Sulfites
40

41. WATER ACTIVITYWATER ACTIVITY
What is meant by water activity?
◦Water has different levels of binding and thus
activity or availability in a food sample
◦Simply put, Water activity (a
w
) helps to explain
the relationship between perishability and
moisture content
Greater moisture content  faster spoilage
(normally)
Why are there some perishable foods at the same
moisture content that don't spoil at the same rate?
There is a correlation found between a
w and various
different spoilage and safety patterns
41

42. WATER ACTIVITYWATER ACTIVITY
42
Water has different levels of binding and thus activity or availability in a food
sample
Food companies and regulatory agencies (e.g. FDA) rely on a
w as an indicator of
how fast and in what fashion a food product will deteriorate or become unsafe,
and it also helps them set regulatory levels of a
w for different foods
Highly perishable foods a
w
> 0.9
Intermediate moist foods a
w
= 0.6-0.9
Shelf stable foods a
w
< 0.6

43. WATER ACTIVITYWATER ACTIVITY
Thermodynamic definition of a
w
◦The tendency of water molecules to escape the
food product from liquid to vapor defines the a
w
a
w
= p/p
O
=%RH/100
◦Water activity is a measure of relative vapor
pressure of water molecules in the head space
above a food vs. vapor pressure above pure water
◦Scale is from 0 (no water) to 1 (pure water)
43

44. WATER ACTIVITYWATER ACTIVITY
44
Sorption isotherms
◦Help relate moisture content to
a
w
◦Each food has their own
sorption isotherm
◦It is interesting that when water
is added to a dry product, the
adsorption is not identical to
desorption
◦Some reasons
Metastable local domains
Diffusion barriers
Capillary phenomena
Time dependent equilibrium
Temp. dependent

45. WATER ACTIVITYWATER ACTIVITY
Water sorption of a mixture
◦A mixture of two different food components with different a
w

leads to moisture migration from one food to another which can
create problems
◦This is one reason why it is important to know the a
w
of a food
product or ingredient
◦Examples:
Caramel, marshmallows and mints – all similar %moisture but very
different a
w
Fudge (a
w = 0.65-0.75) covered with caramel (a
w = 0.4-0.5) – what
happens?
Granola bar with soft chewy matrix (a
w = 0.6) and sugar coat (a
w =
0.3)?
Hard candy (a
w
= 0.2-0.35) on a humid day?
45

46. WATER ACTIVITYWATER ACTIVITY
46
So, knowing the a
w of a
food component one
can select the proper
ingredients for a
particular food product
For example, it is
possible to create a
multi-textured food
product if components
are added at the same
a
w

47. WATER ACTIVITYWATER ACTIVITY
47
Temperature dependency of the sorption isotherm can be a
major problem and often overlooked
Example:
Crackers that experience a
temperature rise during
transportation
At the same moisture content
which
would spoil faster?

48. WATER ACTIVITYWATER ACTIVITY
48
Sorption isotherms also explain the
level of water binding in a food (i.e.
types of water)
◦Type I: Tightly “bound” water
(monolayer)
Unavailable/Unfreezable (at -40C)
Water - ion; water - dipole interactions
◦Type II: additional water layer (Vicinal
water)
Slightly more mobility
Some solvent capacity
◦Type III: Water condensating in
capillaries and pores (Multilayer 
Bulk-phase water)
More available (like dilute salt solution)
Can be entrapped in gels
Supports biological and chemical
rections
Freezable
Monolayer
True monolayer

49. WATER ACTIVITYWATER ACTIVITY
49
Importance of a
w
in foods
◦Food stability
directly related to
a
w
◦Influences storage,
microbial growth,
chemical &
enzymatic
deteriorations, etc.

50. WATER ACTIVITYWATER ACTIVITY
50
A.Microbial stability
◦Foods with a
w
> 0.9 require refrigeration because of bacteria spoilage
Exception: Very low pH Foods
◦Can control by making intermediate moisture foods (IMF)
Food with low a
w to prevent microbial spoilage at room temp. But which
can be eaten w/o hydration
Aw = 0.7 - 0.9 (20 -50% water) - achieved by drying or using solutes (sugar,
salt)
dried fruits, jelly and jam, pet foods, fruity cakes, dry sausage, marshmallow, bread,
country style hams
Minimal processing however preferred over IMF
Special problems
May need mold inhibitor
Lipid oxidation - may need antioxidant or inert packaging
◦Important in grains to prevent mold growth & possibly mycotoxin
development
Must be below 0.8

51. WATER ACTIVITYWATER ACTIVITY
B.Chemical stability
◦Maillard browning
Doesn't occur below type II water
Increases in type II water - water becomes a better
solvent while reactants become more mobile
Reduced in type III - dilution or water is an
inhibitor
Depends on food product (a
w 0.53-0.55 in apple
juice vs. 0.93 in anchovy)
51

52. WATER ACTIVITYWATER ACTIVITY
B.Chemical stability
◦Lipid oxidation
Low a
w
, lipid oxidation high - due to instability of
hydroperoxides (HP)
- unstable w/o water, no H-bonding
Slightly more addition of water stabilizes the HP
and catalysts
Above type II water, water promotes the lipid
oxidation rate because it helps to dissolve the
catalysts for the reaction
52

53. WATER ACTIVITYWATER ACTIVITY
B.Chemical stability
◦Vitamin and pigment stability
Ascorbic acid very unstable at high a
w
Stability best in dehydrated foods - type II water
Problem with intermediate to high moisture foods
Must consider packaging for these foods
53

54. WATER ACTIVITYWATER ACTIVITY
C.Enzyme stability
◦Hydration of enzyme
◦Diffusion of substrate (solubility)
◦Not significant in dehydrated foods
◦Little enzyme activity below type II water
◦Exceptions: in some cases we get activity at a
↓
w
Frozen foods
Lipases (work in a lipid environment)
54