Transcript WATER - Food Science & Human Nutrition

WATER
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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
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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
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PHYSICAL & CHEMICAL PROPERTIES OF
WATER

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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
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STRUCTURE OF WATER
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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
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STRUCTURE OF WATER
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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
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Pressure
PHASE CHANGES OF WATER
Temperature
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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
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LIQUID WATER
Extensive H-bonding
H-bond formation dependent on T
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◦ 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
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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
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Can go from ICE to GAS
Pressure
Basis for Freeze Drying
Sublimation
Temperature
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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
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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
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PROPERTIES OF ICE
Freezing induced changes
in foods (examples)
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Example: Effect of freezing on seafoods
Destabilization of emulsions
Flocculation of proteins
Increased lipid oxidation
Meat toughening
Cellular damage
Loss of water holding
capacity
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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
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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
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Small ions can induce more structure in water
◦ Have strong electric fields
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WATER SOLUTE INTERACTIONS
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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
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WATER SOLUTE INTERACTIONS
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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
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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
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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
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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
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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
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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
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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
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EFFECT OF SOLUTES ON WATER
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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:
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◦ 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®
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EFFECT OF SOLUTES ON WATER
Ionization of water
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Water can ionize into hydronium (H3O+) and hydroxyl (OH-) ions
◦ Transfer of one proton to the unshared sp3 orbital of another
water molecule
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Pure water: Keq = Equilibrium (or ionization) constant
Keq = [H3O]+ [OH][H2O]
[H3O]+ [OH]- = Keq = Kw (Water dissociation constant)
[10-7] [10-7] = [10-14]
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EFFECT OF SOLUTES ON WATER
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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.
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EFFECT OF SOLUTES ON WATER
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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]
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EFFECT OF SOLUTES ON WATER
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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]
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EFFECT OF SOLUTES ON WATER
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Weak acids
◦ Graphically behave
like the figure when
titrated with a
strong base. The
reverse holds true
for weak bases
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EFFECT OF SOLUTES ON WATER
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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?
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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-
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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
H3 C
O
O
NH3
O
+
NH3
2.5 meq
H3C
H3C
-
O
+
OH
-
O
NH2
O
H3C
H3C
H3C
O
OH
NH3
1.25:1.25 meq
2.5 meq
O
H3C
O
O
+
+
-
NH2
-
NH3
O
H3C
O
-
+
NH3
1.25:1.25 meq
-
NH2
2.5 meq
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EFFECT OF SOLUTES ON WATER
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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
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EFFECT OF SOLUTES ON WATER
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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
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EFFECT OF SOLUTES ON WATER
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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
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EFFECT OF SOLUTES ON WATER
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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
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EFFECT OF SOLUTES ON WATER
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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%)
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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.
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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
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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
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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
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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)
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WATER ACTIVITY
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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
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Temp. dependent
Metastable local domains
Diffusion barriers
Capillary phenomena
Time dependent equilibrium
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WATER ACTIVITY
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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?
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WATER ACTIVITY
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
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
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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?
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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
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WATER ACTIVITY

Importance of aw
in foods
◦ Food stability
directly related to
aw
◦ Influences storage,
microbial growth,
chemical &
enzymatic
deteriorations, etc.
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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
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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)
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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
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WATER ACTIVITY
B.
Chemical stability
◦ Vitamin and pigment stability
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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
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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)
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