Water Activity, Drying

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Water Activity,Drying

• FSN 661
• April 2009

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Reducing water

• Third major method of food preservation
• Makes water less available to microorganisms
• Reduces mobility of water slows reactions
• Reduces the weight and volume of foods

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History

• Drying is one of the oldest methods of
  preservation
• Sun and wind drying
• Smoking, and salting
• Drying herbs, meats, fruits and vegetables.
• Historians have credited the ability to store
  foods with the start of civilization.
• Today, it is still the simplest, cheapest, and
  most widely used method in the World.

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History

• Sun drying around the Mediterranean
• Figs, raisins, dates, apricots
• Andes potatoes Chuno
• Native Americans Pemmican
• Central Asia pastrami

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Simple drying system
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Fish drying on beach
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Rack systems
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Drying Fish Heads
Geothermal energy is used (Iceland)
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Water in foods

• Constituents in foods bind water in varying
  degrees
• Hygroscopic (salt, sugars, proteins.)
• Non-hygroscopic (lipids)

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Water activity

• In the equilibrium state
• mo RT ln(f / fo)
• where m (J mol-1) is the chemical potential of
  the system i.e. thermodynamic activity or energy
  per mole of substance
• mo is the chemical potential of the pure
  material at the temperature T(K)
• R is the gas constant (8.314 J mol-1 K-1)
• f is the fugacity or the escaping tendency of a
  substance
• fo is escaping tendency of pure material

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Water activity

• The activity of a species is defined as
• a f/fo
• fugacity is closely approximated by the vapor
  pressure (f p) so
• aw f/fo _at_ p/po
• partial pressure of water over
  food
• Water activity ---------------------------------
  -------------
• vapor pressure of water
  at the same T

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Partial pressure
Pure water
Solution
P, T
P, T
When there is a solute in water, it lowers the
vapor pressure of water over the solution
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Partial Pressure

• For ideal, very dilute solutions, Raoults law

pwater po,water xwater
p partial pressure of water po vapor pressure
of water (from the steam tables) X mole
fraction of water
pwater / po,water xwater
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Raoults law
1.0
Pure water
Ideal dilute
X Mole fraction water
Non-ideal concentrated
0.0
1.0
0.0
p/po
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State of water

• Free water (available to microorganisms,
  available to chemical reactions either as a
  reactant, or as the transport medium for the
  reactants to move together, and the products to
  move away).
• Bound water (Tightly bound to individual sites
  hydroxyl groups of polysaccharides, carbonyl and
  amino groups of proteins, hydrogen bonding sites,
  ion-dipole sites, and other strong interactions
  not available as a solvent).

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Isotherms

• (Constant temperature)
• Correlation between water activity of food and
  its water content.
• Water content is generally expressed in dry
  weight basis.

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Dry basis / wet basis

• M wet basis moisture content
  (water/total100)
• X dry basis moisture content (water/solid100)

solid
total
water
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Hysteresis

• When a dry food is re-hydrated, its aw is higher,
  at the same moisture level, than a wet food.
• Drying causes irreversible changes in food, so
  adding water does not bring it back to its
  original state.
• Ink bottle theory

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Effect of Temperature

• For most foods, when T increases, then the aw
  increases at the same moisture level.
• For some food with much sugar, increase in T may
  increase the solubility of sugar, and therefore
  decrease aw.

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Isotherm Equations

• Not possible to theoretically predict aw of any
  food for a given water content
• Measure the aw vs moisture content
• Fit isotherm equations, select the best.
• Isotherm equations are empirical

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Halseys equation
X d.b. moisture content B1, B2 experimental
parameters
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Bradleys equation
X d.b. moisture content B1, B2 experimental
parameters
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Hendersons equation
X d.b. moisture content B1, B2 experimental
parameters
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Iglesias/Chirife equation
X d.b. moisture content B1, B2 experimental
parameters X0.5 X at aw0.5
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Kuhns equation
X d.b. moisture content B1, B2 experimental
parameters
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Oswins equation
X d.b. moisture content B1, B2 experimental
parameters
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Smiths equation
X d.b. moisture content B1, B2 experimental
parameters
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BET equation
X d.b. moisture content Xm monolayer
moisture C experimental parameter BET should
not be used for awgt0.5
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GAB equation
k correction factor for multilayers Xm
monolayer moisture content C Guggenheim constant
Hl Heat of condensation of pure water vapor, at
T, kJ/Kg mol Hq Total heat of sorption of the
multilayers, kJ/Kg mol Hm Total heat of sorption
of the first layer on primary sites, kJ/Kg mol R
gas constant, 8.314 kJ/Kg-mol K T
temperature, must be in K
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Changes with temperature

• Most isotherm equations do not have a parameter
  that involves temperature.
• GAB has temperature parameters
• Other isotherm equations can be modified
  (extended) to include temperature

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Extended Equations
Modified Chung-Pfost Modified
Halsey Modified Oswin Modified Henderson
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aw and microorganisms

• The lowest aw at which the majority of food
  spoilage bacteria will grow is about 0.90.
• Staphylococcus aureus under anaerobic conditions
  is inhibited at an aw of 0.91, but aerobically
  the aw level is 0.86.
• The aw for mold and yeast growth is about 0.61
  with the lower limit for growth of mycotoxigenic
  molds at 0.78 aw

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Effects of aw

• Enzymatic reactions slow down and stop as aw is
  reduced mobility, reactant (hydrolysis)
• Non-enzymatic reactions as aw increases,
  reaction rates increase mobility then decrease
  (dilution effects)

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Effects of aw

• Lipid oxidation
• In dry food as water increases O2 absorption
  increases
• -Peroxide formation decreases as water increases
  below monolayer. H bonds to hydroperoxides reduce
  their rate of decomposition
• Trace metal are hydrated
• Metals form insoluble hydroxides
• At monolayer, oxidation rate is minimum.
• As moisture increases, oxidation rate increases
• Mobility, reactant, swelling of matrices

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Drying

• The removal of water from fish muscle to extend
  shelf life
• Requires input of thermal energy
• Sun, wind, flame, electrical etc.

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Drying mechanisms

• Drying is a sequence of events that take place in
  two main phases
• Phase 1 (constant rate period)
• Water is being removed from the fish surface
• Phase 2 (falling rate period)
• Water is removed from the fish muscle
• Water migrates to the fish surface
• Water is removed from the fish surface
• Water is removed from the atmosphere surrounding
  the fish surface
• If too rapid we can get case hardening

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Constant Drying Rate Period

• There must be free, contiguous water at the
  surface.
• Then, the air sees water, and not the food.
  Rate of drying depends on air properties
• This period may or may not exist in foods

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Falling drying rate period

• After the free water layer is partially
  evaporated, there are dry patches at the
  surface
• The water molecules are more tightly bound to the
  food, and are more difficult to remove
• Drying rate falls

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Co-current / counter-current

• The air and the food exchange
• Heat (the air supplies heat)
• Moisture (the food loses moisture)
• Contact can happen co-currently, or
  counter-currently.

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Types of driers

• Belt driers
• Tunnel driers
• Spray driers
• Drum driers
• Fluidized bed driers
• Freeze driers

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Advanced automated driers
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Tray Drier
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Solar Driers
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Tray drier
Vortex drier
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Quality

• Irreversible changes in dried foods
• Vitamin C can be oxidized easily
• Lipids can be oxidized
• Case hardening (too fast a drying rate may dry
  the outer layers that become impermeable to
  water, and further drying stops)

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Problems with dried fish

• If environmental T is too high and fish is too
  wet
• Molds (Awgt0.75)
• Grow very well when drying in tropical areas
• Black spots, which can be scraped off (fish not
  unfit for consumption)
• Aerobic spoilage bacteria (if non-salted)
• If salted (solar salt) can get halophiles growing
• Pink colonies
• Major problem with salted and dried cod
• Insects
• Insect parts and eggs a major quality problem
• Flies (Dipteria) lay eggs and can spread disease
• Larvae feeds on the fish
• Less problem with fish pre-drying
• Salting can deter flies
• Some use insecticides on fish!
• Also beetles (India) and roaches (post-drying)
• Rodents and birds are a constant problem

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Problems with dried fish (cont.)

• Lipid oxidation (rancidity)
• Protein denaturation
• Get muscle fragmentation
• Case hardening
• Flesh soft on the inside and spoiled by bacteria
  (extreme putrid odor)

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Pilot plant / lab

• See the water activity meter
• Observe the air drier
• Observe the drum drier
• Observe the spray drier