Transcript water - WordPress.com

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