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WATER
1
WATER'S IMPORTANCE
Solvent
Reactant
Water's involvement in hydrolysis reactions
Product
Most molecules dissolved in water
Water's involvement in condensation reactions
Heat transfer medium
E.g. boiling, steaming, cooling
2
WATER'S IMPORTANCE
Texture
Juiciness, mouthfeel
Preservation
Highly perishable foods usually have high water activity
Snack foods
Vegetables
Meat
E.g. bread vs. cracker or cereal
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
H 2S
-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
PHASE CHANGES OF WATER
7
8
WATER VAPOR
Water is “free” and devoid of any H-bonds
Large input of energy needed
Large dissipation of same energy needed to make
water lose kinetic energy
an endothermic process
an exothermic process
Waters latent heat of vaporization is unusually
high
to change 1 L from liquid to vapor need 539.4 kcal
10
LIQUID WATER
Extensively H-bonded
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
11
ICE
Forms when exactly 4 H-bonds
are formed between water
molecules
The strong H-bonding in ice
forms an orderly hexagonal
crystal lattice
2.78 A vs. 2.85 A in liquid
To get this order a lot of energy
needs to be adsorbed by the
environment
6 H2O molecules
Has 4X more thermal
conductivity than water at same
temperature
12
Can go from ICE to GAS
BASIS FOR
FREEZE DRYING
• SUBLIMATION
13
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
14
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
15
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
16
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
17
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
Large ions can break water
structure
Can easily neutralize charges due
to its high dipole moment
Have weak electric fields
Small ions can induce more
structure in water
Have strong electric fields
18
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
19
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
20
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
21
EFFECT OF SOLUTES ON WATER
1 atm (sea level)
mountains
90C 100C
So does it take longer or
shorter to boil an egg in
the Rocky Mountains?
Why?
22
EFFECT OF SOLUTES ON WATER
Let's go back to our egg,
what would happen if you
added salt?
Raoult's
Law
Recommended that you add salt to water at high altitudes
23
EFFECT OF SOLUTES ON WATER
Freezing point lowering
Freezing point can get extensive
depression via solutes
Eutectic pt - temp.
Alter ability of water to form crystals
due to H-bond disruption
Sucrose -1.86ºC/mol
NaCl -3.72ºC/mol
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
24
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
25
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
26
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
27
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
28
PhotoFrost®
29
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]
30
EFFECT OF SOLUTES ON WATER
Acids and bases in food systems
Acid - proton donor
Base - proton acceptor
CH3COOH + H2O CH3COO- + H3O+
Weak acids and bases
NH3 + H2O NH4+ + OH-
Most foods are weak acids
These constituents are responsible for buffering of food
systems
Some examples
Acetic, citric, lactic, phosphoric, etc.
31
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]
32
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 - Hasselback equation:
pH = pKa + log [salt]
[acid]
33
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
What do we
call this point?
34
EFFECT OF SOLUTES ON WATER
Buffering
Buffers resist
changes in pH
when acids and
bases are added
Characteristics of a
buffer
What is this
point and its
significance to
food systems?
Maximum when
pH = pKa or when
[acid] = [salt]
Rule of thumb:
pH = pKa ± 1
35
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
36
EFFECT OF SOLUTES ON WATER
Examples of “man made” pH control
Food additives - ACIDULANTS
Citric acid - pectin jellies
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
pH must be around 2.9-3.0
Also provides balance between tartness and sweetness
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
37
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
38
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
d-Glucono-lactone - used in refrigerated doughs
39
EFFECT OF SOLUTES ON WATER
Examples of “man made” pH control
Acidulants - antimicrobials
pH is important for two reasons: 1. Solubility and 2. Activity
Benzoic acid (0.05-0.1%)
The salt is more soluble in aqueous systems
The acid is more active in its antimicrobial efficiency
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.
40
EFFECT OF SOLUTES ON WATER
Acidulants - antimicrobials
Sorbic acid (Na+ and K+ salt forms) (0.02-0.3%)
Proprionic acid (proprionate) Ca2+ salt
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.
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
41
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
42
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
43
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)
44
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
45
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?
46
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 multitextured food product if
components are added at
the same aw
47
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?
48
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)
Type II: additional water layer (Vicinal
water)
Unavailable/Unfreezable (at -40C)
Water - ion; water - dipole interactions
Slightly more mobility
Some solvent capacity
True monolayer
Monolayer
Type III: Water condensating in
capillaries and pores (multilayer bulkphase water)
More available (like dilute salt solution)
Can be entrapped in gels
Supports biological and chemical rections
Freezable
49
WATER ACTIVITY
Importance of aw in foods
Food stability directly
related to aw
Influences storage,
microbial growth, chemical
& enzymatic deteriorations,
etc.
Vit C loss
50
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)
Minimal processing however preferred over
IMF
Special problems
dried fruits, jelly and jam, pet foods, fruity
cakes, dry sausage, marshmallow, bread,
country style hams
May need mold inhibitor
Lipid oxidation - may need antioxidant or
inert packaging
1.00
0.80
0.60
0.40
0.20
0.00
OSMOPHILIC YEAST
YEAST & MOLDS
aw is a major HURDLE
for microorganisms but
not the only one
BACTERIA
Important in grains to prevent mold growth
& possibly mycotoxin development
Must be below 0.8
51
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)
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
52
WATER ACTIVITY
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
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)
53