WATER
1
WATER'S IMPORTANCE
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
2
WATER'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
PHYSICAL & CHEMICAL PROPERTIES OF
WATER


Water has very unique properties not shared by other
similar hydrogen compounds or compounds of similar
weight
Compound
Melting point
Boiling point
H 2O
0ºC
100ºC
H2S
-83ºC
-60ºC
NH3
-78ºC
-33ºC
Methanol
-98ºC
65ºC
Why? – this is explained by the unique structure of H2O
4
STRUCTURE OF WATER




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
5
STRUCTURE OF WATER




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
6
Pressure
PHASE CHANGES OF WATER
Temperature
7
WATER 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
LIQUID WATER
Extensive H-bonding
H-bond formation dependent on T


◦ With increasing T get more mobility and increased fluidity
Density (kg/m3)
T (ºC)
Viscosity (m2/s)
0
999.9
1.7895
5
1000.0
1.535
25
997.1
0.884
100
958.4
0.294
9
ICE

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 H2O molecules

Has 4X more thermal conductivity
than water at same temperature
10
Can go from ICE to GAS
Pressure
Basis for Freeze Drying
Sublimation
Temperature
11
PROPERTIES 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
PROPERTIES 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
1.
2.
3.
http://www.youtube.com/watch?v=czmQ2_ymaOo
http://www.youtube.com/watch?v=gGpNhBPYNfs&feature=related
http://www.youtube.com/watch?v=DpiUZI_3o8s
13
PROPERTIES OF ICE
Freezing induced changes
in foods (examples)






Example: Effect of freezing on seafoods
Destabilization of emulsions
Flocculation of proteins
Increased lipid oxidation
Meat toughening
Cellular damage
Loss of water holding
capacity
14
WATER 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
WATER SOLUTE INTERACTIONS

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
16
WATER SOLUTE INTERACTIONS

Nonionic polar solutes
◦ Weaker than water-ion bonds
◦ Major factor here is H-bonding to the polar site
◦ Example: SUCROSE
 4-6 H2O per sucrose
 Concentration dependent
 >30-40% sucrose all H2O is bound
 T dependent solubility
◦ C=O, OH, NH2 can also interact with each other and therefore water
can compete with these groups
◦ H-bond disrupters
 urea - disrupts water
Water bridge
17
WATER SOLUTE INTERACTIONS

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
18
EFFECT OF SOLUTES ON WATER
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
19
20
21
EFFECT OF SOLUTES ON WATER
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
22
EFFECT OF SOLUTES ON WATER
What explains all this?

Raoult's law
P = P*/X1
or
P*-P/P*= x/55.5M
P = vapor pressure of solution; P* = vapor pressure
pure solvent; X1 = 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
23
EFFECT 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
24
EFFECT OF SOLUTES ON WATER
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.
2.
http://www.youtube.com/watch?v=45yabrnryXk&feature=fvw
http://www.youtube.com/watch?v=76CNkxizQuc&feature=related
25
EFFECT 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
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PhotoFrost®
27
EFFECT OF SOLUTES ON WATER
Ionization of water

Water can ionize into hydronium (H3O+) and hydroxyl (OH-) ions
◦ Transfer of one proton to the unshared sp3 orbital of another
water molecule

Pure water: Keq = Equilibrium (or ionization) constant
Keq = [H3O]+ [OH][H2O]
[H3O]+ [OH]- = Keq = Kw (Water dissociation constant)
[10-7] [10-7] = [10-14]
28
EFFECT OF SOLUTES ON WATER

Acids and bases in food systems
◦ Acid - proton donor
 NH3 + H2O  NH4+ + OH-
◦ Base - proton acceptor
 CH3COOH + H2O  CH3COO- + H3O+
◦ 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.
29
EFFECT 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
HCl = H+ + ClpH = -log [acid] = -log [H]+
 Weak acids
 When placed in solutions weak acids form an equilibrium
HOAC
pKa = -log Ka
H+ + OAC-
Keq = [H]+ [OAC][HOAC]
30
EFFECT 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:
pH = pKa + log [salt]
[acid]
31
EFFECT OF SOLUTES ON WATER

Weak acids
◦ Graphically behave
like the figure when
titrated with a
strong base. The
reverse holds true
for weak bases
32
33
34
EFFECT OF SOLUTES ON WATER

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?
35
EFFECT OF SOLUTES ON WATER
Let’s return to
Henderson-Hasselbach
pH = pKa + log [salt]
[acid]
K1 = 4.6 x 10-3 ; K2 = 2.04 x 10-10
O
H3C
pH
OH
NH2
Equivalents OH-
36
EFFECT OF SOLUTES ON WATER
Let’s return to
Henderson-Hasselbach
O
H3C
OH
NH2
pH
pH = pKa + log [salt]
[acid]
K1 = 4.6 x 10-3 ; K2 = 2.04 x 10-10
O
O
pK1
H3C
Equivalents OH-
OH
H3C
-
HO
+
NH3
O
-
O
+
pK2
-
HO
NH3
O
H3C
O
O
NH3
O
+
NH3
2.5 meq
H3 C
H3 C
-
O
+
OH
-
O
NH2
O
H3 C
H3C
H3 C
O
OH
+
NH3
1.25:1.25 meq
2.5 meq
-
NH2
-
H3 C
O
O
H3 C
O
+
NH3
O
-
+
NH3
1.25:1.25 meq
O
-
NH2
2.5 meq
37
38
EFFECT OF SOLUTES ON WATER

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 CO2
 Problem - Loss of carbohydrate groups on proteins. Loss of
protein functionality, causing decreased viscosity and poor
foaming properties
39
EFFECT 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
40
EFFECT OF SOLUTES ON WATER

Examples of “man made” pH control
◦ Acidulants - leavening agents
 Used in the baking industry to give rise (release of
CO2) - alternative to yeast
 When HCO3- becomes acidic (pH < 6), CO2 forms,
CO2 not very soluble so released as a gas
Overall eq: H+ + HCO3- H2O + CO2
41
EFFECT OF SOLUTES ON WATER

Examples of “man made” pH control
◦ Leavening systems
 Bicarbonate (NaHCO3) - source of HCO3 and CO2
 Leavening acids
 Drive bicarbonate (HCO3) to CO2
 Rate of acid release varies and therefore CO2 release
 Phosphate - rapid release of CO2
 Sulfate – slow release of CO2
 Pyrophosphate - can be cleaved by phosphatases
becoming more soluble - used in refrigerated doughs
 -Glucono-lactone - used in refrigerated doughs
42
EFFECT 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.
43
EFFECT 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) Ca2+ 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
44
WATER 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 (aw) 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 aw and
various different spoilage and safety patterns
45
WATER ACTIVITY
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 aw 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 aw for different foods

Highly perishable foods aw > 0.9
Intermediate moist foods aw = 0.6-0.9
Shelf stable foods aw < 0.6
46
WATER ACTIVITY

Thermodynamic definition of aw
◦ The tendency of water molecules to escape the
food product from liquid to vapor defines the aw
aw = p/pO=%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)
47
WATER ACTIVITY

Sorption isotherms
◦ Help relate moisture content to
aw
◦ 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




Temp. dependent
Metastable local domains
Diffusion barriers
Capillary phenomena
Time dependent equilibrium
48
WATER ACTIVITY

Water sorption of a mixture
◦ A mixture of two different food components with different aw
leads to moisture migration from one food to another which can
create problems
◦ This is one reason why it is important to know the aw of a food
product or ingredient
◦ Examples:
 Caramel, marshmallows and mints – all similar %moisture but very
different aw
 Fudge (aw = 0.65-0.75) covered with caramel (aw = 0.4-0.5) – what
happens?
 Granola bar with soft chewy matrix (aw = 0.6) and sugar coat (aw =
0.3)?
 Hard candy (aw = 0.2-0.35) on a humid day?
49
WATER ACTIVITY


So, knowing the aw 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
aw
50
WATER ACTIVITY

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?
51
WATER ACTIVITY

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)
True monolayer
Monolayer
 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
52
WATER ACTIVITY

Importance of aw
in foods
◦ Food stability
directly related to
aw
◦ Influences storage,
microbial growth,
chemical &
enzymatic
deteriorations, etc.
53
WATER ACTIVITY
A.
Microbial stability
◦ Foods with aw > 0.9 require refrigeration because of bacteria spoilage
 Exception: Very low pH Foods
◦ Can control by making intermediate moisture foods (IMF)
 Food with low aw 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
54
WATER 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 (aw 0.53-0.55 in apple
juice vs. 0.93 in anchovy)
55
WATER ACTIVITY
B.
Chemical stability
◦ Lipid oxidation
 Low aw, 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
56
WATER ACTIVITY
B.
Chemical stability
◦ Vitamin and pigment stability




Ascorbic acid very unstable at high aw
Stability best in dehydrated foods - type II water
Problem with intermediate to high moisture foods
Must consider packaging for these foods
57
WATER 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 ↓aw
 Frozen foods
 Lipases (work in a lipid environment)
58