FST 141: Moisture Sorption Isotherm

Moisture Sorption Isotherm

describes the relationship between MC and Aw in food at constant temperature and pressure

Significance of MSI

• for concentration and dehydration processes

• for formulating food mixes

• To determine what MC will limit the growth of the microorganism of interest

• To predict the chemical and physical stability

Adsorption

• surface phenomenon

• MSI

Absorption

permeates the system

Physiosorption

adsorbate adheres to the surface only through can der waals force

Chemisorption

molecule adheres to a surface through the formation of a chemical bond

Type 1 (monolayer)

• Dry, crystalline foods

• High sugar or salt products

Type II (sigmoidal)

• mostly hygroscopic

• easily absorbs H2O

Type III (J-type)

• low polymetric substances

• sugary foods

Region I

monolayer value

0 - 0.25

Region II

0.25 - 0.80

corresponds to the adsorption of additional layer of water over the monolayer

Region III

0.80 - 1.0

water molecules as much less bonded

free water

Desorption

• exposing the initially wet material to atmospheres of excessively decreasing RH

  • involves removal of water
    

Resoprtion

• adsoprtion; exposing dry material to atmospheres of successively increasing RH

Sorption Hysteresis

The difference in the amount of adsorption and desorption for food at equal VP.

Reasons of hysteresis

• changes in folding and orientation of polymers including proteins

• phase changes

• effects of capillarity

Brunauer–Emmett–Teller model

• applicable fow Aw values between 0 - 0.50

• one of the OLDEST and most POPULAR model

Guggenheim–Anderson–de Boer model

• adequate to describe experimental data for Aw values

• extension of BET equation but with additional parameter