Actividad De Agua

IFT Short Course Water Activity and Sorption Isotherms

Dr Ted P. Labuza

Department of Food Science and Nutrition University of Minnesota St Paul 55108 USA 612-624-9701 fax 625-5272 tplabuza@umn.edu
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Thermodynamics and Kinetics

KINETIC A SUB W NTROP

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EQUILIBRIUM DEFINITION Chemical Potential = µ

p ∴ µv = µo + RT ln aw = µo + RT ln po
µ definessystem at equilibrium Related to activity (a) of species a related to relative vapor pressure
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Equilibrium @ T ¡C

p %RH = aw • 100 = • 100 po
p = vapor pressure of substance (water) in equilibrium with system po = equilibrium vapor pressure of pure substance (water) a = activity of species For water a = aw %RH = % relative humidity
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tplabuza © 2002definitions
u Water activity u Water activity
Ð equilibrium property of water at a given Ð equilibrium property of water at a given temperature and moisture content temperature and moisture content

u Water Holding Capacity (WHC) u Water Holding Capacity (WHC)
Ð moisture content at some equilibrium Ð moisture content at some equilibrium condition condition

u Water Binding Capacity (WBC) uWater Binding Capacity (WBC)
Ð water left in system after applying some Ð water left in system after applying some external stress (mechanical, osmotic, vapor) external stress (mechanical, osmotic, vapor)
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Moisture vs water activity

1

aw

0

0

moisture g /g solids 1 2 3 ---> 19

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Factors influencing Water activity
u solute interactions usolute interactions u capillary suction forces u capillary suction forces u surface force interactions u surface force interactions

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Equilibration process

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Salt Solutions

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cautions
u u u u u u u u u u u u u u

need pure salt solutions problem of volatile contamination large surface area to food chamber volume eg 1 L air holds 23 mgwater at saturation must be slurry - need excess salt discrete values so not continuous temperature and solubility - prepare at desired temperature takes time 7 to 21 days rains in desiccator when opened to cold humid air time proportional to thickness squared mixed type samples vacuum fluxing entrainment of salts in vapor toxicity of salts - eg lithium , nitrite, iodide, bromide

tplabuza © 2002EU COST-91 study
u u u u u u u u u u

based on 32 labs with 5 replicates use micro-crystalline cellulose standard equilibrium 10 days - aw < 0.75 at 25¡C 21 days - aw > 0.75 at 25¡C 10 salts 0.1 to 0.9 aw no vacuum and/or stirring Weigh at start and end Need initial moisture content use GAB equation to describe curve sd = + 0.25 g H2O/100 g solids (0.25% error) variance ± 0.17 g water/100g solids
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Moisture content
u dry u wet

basis (g water/100 g dry solids)

basis (g water/100 g total) = % of total wt by solids non-fat ( g water per 100 g non-fat solids
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u Moisture

Sorption Isotherm Types
Type I Type 3 Type 2

amount
water activity
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Typical Type 2 Moisture Adsorption Isotherm

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Automatic devices
u Autosorp
Ð French $100,000 Ð 32 simultaneous sample Ð ~ 50 g sample Ð ± 2 % RH

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email message
We actually propose the third version of this system but unfortunately we are not able to sell it outside of Europe because we do not have thefacilities to install and service it abroad. Thanks again for contacting us. Best Regards, Francois Sorrentino. Biosystemes 9 Rue des Mardors 21560 Couternon France tel : 00 33 380475762 fax : 00 33 380475071 email: fsorrentino@biosystemes.com

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http://www.smsuk.co.uk/dynamiv_vapour_sorption.htm

DyaSorb DVS-1

u Single

sample 1.5 g max

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What do we do...