WATER

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WATER
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WATER

• Outline
• Importance to food systems
• General properties
• Structure
• Water phases
• Water/solute interactions
• Water activity

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WATER IS THE MAJOR CONSTITUENT IN MOST FOODS
Sugar
0 Salt 0
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WATER'S IMPORTANCE

• Solvent
• Most molecules dissolved in water
• Reactant
• Water's involvement in hydrolysis reactions
• Product
• Water's involvement in condensation reactions
• Heat transfer medium
• E.g. boiling, steaming, cooling

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WATER'S IMPORTANCE

• Texture
• Juiciness, mouthfeel
• Snack foods
• Vegetables
• Meat
• Preservation
• Highly perishable foods usually have high water
  activity
• 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

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PHYSICAL CHEMICAL PROPERTIES OF WATER

• 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 H2O

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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

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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

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PHASE CHANGES OF WATER
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WATER VAPOR

• Water is free and devoid of any H-bonds
• Large input of energy needed
• an endothermic process
• Large dissipation of same energy needed to make
  water lose kinetic energy
• an 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

• Extensively H-bonded
• H-bond formation dependent on T
• With increasing T get more mobility and increased
  fluidity

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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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ICE

• Ice can exist in 9 different polymorphic forms,
  due to
• Variants of H2O
• Vibrations of H2O
• Small molecules
• Ice is not static
• When ice melts H-bonds are broken liquid water
  becomes more tightly packed together
• T influences distance between H-bonds

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Can go from ICE to GAS
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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

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PROPERTIES 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

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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
• Small ions can induce more structure in water
• Have strong electric fields

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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
• gt30-40 sucrose all H2O is bound
• T dependent solubility
• CO, OH, NH2 can also interact with each other
  and therefore water can compete with these groups
• H-bond disrupters
• urea - disrupts water

Water bridge
More sucrose can go into solution as it is
heated - Sucrose adsorbs heat when it dissolves
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WATER SOLUTE INTERACTIONS

• Nonpolar
• Unfavorable interaction with water
• Water around nonpolar substance is forced into an
  ordered state
• Water affinity for water high compared to
  nonpolar 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
ATMOSPHERIC PRESSURE

• 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

VAPOR PRESSURE
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EFFECT OF SOLUTES ON WATER
1 atm (sea level)
mountains
So does it take longer or shorter to boil an egg
in the Rocky Mountains? Why?
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EFFECT OF SOLUTES ON WATER
Let's go back to our egg, what would happen if
you added salt?
Recommended that you add salt to water at high
altitudes
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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
• 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

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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
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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

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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.

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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
• Weak acids
• When placed in solutions weak acids form an
  equilibrium

HCl H Cl- pH -log acid -log H
HOAC H OAC- pKa -log Ka
Keq H OAC- HOAC
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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
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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?
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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?
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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

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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

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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 lt 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

• 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

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(sodium aluminum sulfate)
PYROPHOSPHATE
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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.

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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 is a measure of
  relative vapor pressure of water molecules in the
  head space above a food vs. vapor pressure above
  pure water
• 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

• 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 gt 0.9 Intermediate
moist foods aw 0.6-0.9 Shelf stable foods aw lt
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/pORH/100
• When moisture content exceeds that of solids, aw
  is near or approaches 1.0
• All water molecule escape into vapor phase
• When moisture content lower than solids, aw is
  below 1.0
• Some water molecules are left in the food (bound)
  if no escapes or none is present then aw is 0
• Therefore, scale range from 0 - 1.0

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WATER ACTIVITY

• Sorption isotherms
• aw can be measured by measuring the humidity in
  the head space above the food sample in a closed
  system
• aw can also be measured by equilibrating a food
  sample against a reference material to generate a
  SORPTION ISOTHERM
• This can give very useful information on the
  hygroscopic nature of a food or food component

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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
• Metastable local domains
• Diffusion barriers
• Capillary phenomena
• Time dependent equilibrium

Temp. dependent
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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?

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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

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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
• Type II additional water layer (Vicinal water)
• Slightly more mobility
• Type III Water condensing in capillaries and
  pores (multilayer ? bulk-phase water)
• More available

True monolayer
Monolayer
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WATER ACTIVITY

• Type I water bound/monolayer
• Most strongly absorbed water (very low of water
  in foods)
• Water - ion water - dipole interactions
• Creates a monolayer over highly polar groups
  (junction of Type I and II)
• Controversial
• May just be physically trapped and thus have very
  slow diffusion rate
• Most immobile water not available for reactions
• Low solvent capacity of the water
• Unfreezable at -40C
• Relative range
• 0 -7 moisture 0 - 0.25 aw

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WATER ACTIVITY

• Type II water - vicinal
• Forms remaining layers of water in the tissue
• Hydrophilic group interactions through water -
  water and water - solute H-bonding
• Creates true monolayer close to Type II and III
  junction (i.e. completes the monolayer)
• Some solvent capacity
• More water becomes frozen at -40C
• Most still unfreezable
• 7 - 20 moisture 0.25 - 0.75 aw

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WATER ACTIVITY

• Type III water - multilayer
• Least strongly bound and most mobile water -
  "bulk phase water
• In gel type foods or cellular systems, this water
  is entrapped so flow impeded but still very
  active
• Properties similar to dilute salt solutions
• Freezable, very little unfrozen
• Solvent capacity very high
• Supports chemical reactions microbial growth
• Causes most problems in foods
• 80 - 99 moisture 0.75 - 0.99 aw

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WATER ACTIVITY

• Free vs. Bound water
• Definition not very good for both of these
• Bound water - mostly defined by regions I II
• Free water - mostly defined by regions II III
• Major point
• The boundaries between zones are not accurately
  established - thus interchanges do occur between
  zones
• One thing is for sure though
• As ? solute, water vapor pressure ?
• As ? solute, water is less free to react and be
  a solvent
• Therefore, free water is more reactive than
  bound water
• Thus, it is the amount of free rather than bound
  water in foods that govern food stability

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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.

Vit C loss
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WATER ACTIVITY

• A) Microbial stability
• Foods with aw gt 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

aw is a major HURDLE for microorganisms but not
the only one
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WATER ACTIVITY

• B) Texture
• Increasing aw of a solid material and thus water
  mobility leads to a change from hard to brittle
  mass (glassy) to softened mass (rubbery)
• This has a large impact on the rheological, i.e.
  textural, properties of the material

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WATER ACTIVITY

• C) 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

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WATER ACTIVITY

• 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)
• 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